Compile by :Abdul hye
st Helen's school and college

Notes of CHEMISTRY Chapter 1 For 9th Class

FUNDAMENTALS OF CHEMISTRY

Basic Concepts of chemeistry

Chemistry

      Chemistry is the study of matter. In this branch of science we deal with the composition, properties and chemical changes take placein matter.

Matter

     Anything which occupies space and having mass is called matter e.g. Air, Water, Wood etc. Matter exists in three states called Liquid, Solid, and Gas.

Branches of Chemistry

     The field of chemistry is so vast that it is further divided into many branches. The main braches of chemistry are:

Physical chemistry

     The branch of chemistry which deals with the study of relations between the physical properties of substances, laws and principals involved in it is called physical chemistry.

Organic chemistry

     The branch of chemistry which deals with the study of hydrocarbons and their derivatives is called organic chemistry. Those compounds which contain carbon and hydrogen are called hydrocarbons. Carbonates, bicarbonates, CO, CO2, H2CO3 and HCN are not included in organic chemistry.

Inorganic chemistry

     The branch of chemistry which deals with the study of all elements and their compound except hydrocarbons and their derivatives.

Analytical chemistry

     The branch of chemistry which deals with the kinds and types of various components present in matter is called analytical chemistry.

Biochemistry

     The branch of chemistry which deals with the study of chemical changes occur in living organisms.

Nuclear chemistry

     The branch of chemistry which deals with the study of changes occur in the nuclei of atoms and nuclear radiations.

Industrial chemistry

     The branch of chemistry which deals with the study of different techniques and chemical process involved in industries for the preparation of different products such as cement, glass etc is called industrial chemistry.

Environmental chemistry

     The branch of chemistry which deals with the study of interaction of chemical substances with the environment and their impact on it is called environmental chemistry.

Elements

      An element is a pure substance that cannot be broken down into simpler substances by chemical or physical means. The fundamental unit of an element is the atom.Elements are the pure substances which are made up of only one type of atoms e. g. Gold, copper, mercury, oxygen etc. Elements are represented by chemical symbols which are abbreviation for their names.

Compounds

      Compounds are the pure substances which are made up of two or more elements e.g. water, ammonia, methane etc. Compounds possess different physical and chemical properties from those its parent atoms.

Mixtures

      A mixture is a simple combination of two or more substances in which the constituent substances retain their separate identities. The compositionof mixture is not fixede.g. Mixture of ethyl alcohol and water, salt in water, mixture of gases, etc.Mixtures are of two types:

Homogeneous mixture

      A mixture in which the concentration of the constituents remain uniform throughout the mixture and all the constituents are present in one phase, is called a homogeneous mixture e.g.  Mixture of salt and water.

Heterogeneous mixture

If two or more phases are present in a mixture, it is called a heterogeneous mixture e.g. Phenol - water system, iron fillings-sand system.

Atomic number

The number of protons in the nucleus or number of electrons revolving around the nucleus is called atomic number. It is denoted by Z.

Mass Number

The sum of the number of protons and neutrons present in the nucleus of an atom is called mass number. Mass number is represented by ‘A’. Mathematically,

Mass number (A) = Number of protons (Z) + number of neutrons

From above equation

Number of neutrons = A-Z

The information about an atom is normally given by writing mass number above left hand side and atomic number below left hand side of an atom symbol.

Relative atomic mass

Relative atomic mass is the mass of an atom of an element as compared to the mass of an atom of carbon taken as 12. The unit used to express the relative atomic mass is called atomic mass unit (amu) .One amu is the mass exactly equal to of the mass of one C-12 atom. The mass of an atom is extremely small quantity, we cannot measure the mass of a single atom, and therefore we use the relative atomic mass unit scale.

Average Atomic Mass

Average atomic mass of an element depends upon the number of possible isotopes and their natural abundance. For example the atomic mass of carbon is 12.01 amu not 12.00 amu. The reason for the different is existence of isotopes.

Naturally carbon has two isotopes C-12 and C-13, their natural abundance is 98.89% and 1.11% respectively .The atomic mass of C-12 is 12.00 amu, and C-13 has 13.00335amu. The average atomic mass is calculated as follow.

Average atomic mass of carbon= (98.89%)(12.00amu)+(1.11%)(13.0033amu)

= (0.9889)(12) + (0.0111)(13.00335)

= 11.8668+0.1443

= 12.01amu

Relative molecular mass

Relative molecular mass can be obtained by summing up the relative atomic masses of the elements present in a compound. The relative molecular mass of glucose C6H12O6 is given by relative atomic masses of carbon, hydrogen and oxygen is 12.01amu, 1.008amu, and 16.00amu, respectively. The relative molecular mass of C6H12O6is

C à 12.01x6 = 72.06

H à 1.008x12 = 12.096

O à 16.00x6 = 96.00

Relative Molecular Mass = 180.156g

Chemical Formula

Compounds are represented by chemical formula. A chemical formula of the contain symbols of the elements. A compound can be represented by empirical formula and molecular formula.

Empirical formula


It is the simplest formula of a chemical compound which represents the element present in the compound and also represent the simplest ratio between the elements of the compound.
Examples
The empirical formula of benzene is "CH". It indicates that the benzene

molecule is composed of two elements carbon and hydrogen and the ratio between these two elements is 1:1.
The empirical formula of glucose is "CH2O". This formula represents that glucose molecule is composed of three elements carbon, hydrogen and oxygen. The ratio between carbon and oxygen is equal but hydrogen is double.

Formula unit

All the ionic compounds are represented by formula unit which shows the simplest ratio between the atoms which are present in the structure of ionic compounds.

Determination of Empirical Formula
following steps are requiredto determine the empirical formula of a compound.
1. To detect the elements present in the compound.
2. To determine the masses of each element.
3. To calculate the percentage of each element.
4. Determination of mole composition of each element.
5. Determination of simplest ratio between the elements of the compound.
Note
If the number obtained in the simplest ratio is not a whole number then multiply this number with a smallest number such that it becomes a whole number maintain their proportion.


Molecular Formula


The formula which shows the actual number of atoms of each element present in a molecule is called molecular formula or it is a formula which represents the element ratio between the elements present in the compound for examples the molecular formula of benzene is "C6H6". It indicates that
1. Benzene molecule is composed of two elements carbon and hydrogen.
2. The ratio between carbon and hydrogen is 1:1.
3. The number of atoms present in molecule of benzene are 6 carbon and 6 hydrogen atoms.
The molecular formula of glucose is "C6H12O6". The formula represents that
1. Glucose molecule is composed ofcarbon, hydrogen and oxygen.
2. The ratio between the atoms of carbon, hydrogen and oxygen is 1:2:1.
3. The number of atoms in glucose are 6 carbon, 12 hydrogen and 6 oxygen.


Determination of Molecular Formula


The molecular formula of a compound is an integral multiple of its empirical formula.
Molecular formula = n x (Empirical formula)
Where n is a digit = 1, 2, 3…………
The first step in the determination of molecular formula is to calculate its empirical formula. To calculate the value of n
n = Molecular Mass / Empirical Formula Mass
Example
The empirical formula of a compound is CH2O and its molecular mass is 180.
To calculate the molecular formula of the compound first of all we will calculate its empirical formula mass
Empirical formula mass of CH2O = 12 + 1 x 2 + 16
= 30
n = Molecular Mass / Empirical Formula Mass
= 180 / 30= 6
Molecular formula = n x (Empirical formula)
= (CH2O) 6
= C6H12O6


Molecular Mass


The sum of masses of the atoms present in a molecule is called as molecular mass Or It is the comparison that how much a molecule of a substance is heavier than 1/12th weight or mass of carbon atom. For example the molecular mass of CO2 may be calculated as
Molecular mass of CO2 = Mass of Carbon + 2 (Mass of Oxygen)
= 12 + 2 x 16
= 44 a.m.u
Molecular mass of H2O = (Mass of Hydrogen) x 2 + Mass of Oxygen
= 1 x 2 + 16
= 18 a.m.u
Molecular mass of HCl = Mass of Hydrogen + Mass of Chlorine
= 1 + 35.5
= 36.5 a.m.u


Gram Molecular Mass


The molecular mass of a compound expressed in gram is called gram molecular mass or mole.
Examples
1.The molecular mass of H2O is 18. If we take 18g H2O then it is called 1g molecular mass of H2O or 1 mole of water.
2. The molecular mass of HCl is 36.5. If we take 36.5g of HCl then it is called as 1 gm molecular mass of HCl or 1 mole of HCl.

Formula mass

For an ionic solid, such as NaCl, which do not have molecules, we calculate formula mass based on the empirical formula of the compound. The formula mass calculated in the same manner as we did for molecular formula mass. Thus for NaCl

Formula mass = (atomic mass of Na) + (atomic mass of Cl)

= 23 a.m.u+ 35.5 amu = 58.5a.m.u

Gram formula mass

 

It is a terminology used for ionic compounds to express the maths on the formula unit in grams. For example formula mass of NaCl is 58.5g and that of CaCl2 is 111g.

 

Chemical Species

 

Atom

Atom is the smallest particle of an element, which can take part in a chemical reaction. An atom is further composed of sub-atomic particles like electron, proton, and neutron. Electron, proton and neutron are fundamental particles of atoms.

Ion

Ion is that specie which carry either positive or negative charge. When an atom loses one or more electrons, positive ion is formed. The formation of positive ion is an endothermic process. A sufficient amount of energy is provided to a neutral atom to ionize it.

A ­­­­­­­­­ A+1 +1e-

B B+2 +2e-

C C+3+3e-

A positive ion is called cation. The most common positive ions are Na+1, K+1, Ca+2, Mg+2, Al+3, Fe+3, etc. When a neutral atom picks up one or more electrons a negative ion is produced.

X+e- X-1

Y+2e- Y-2

Z+3e- Z-3

A negative ion is called an anion. Energy is released when a uni negative ion is formed. So formation of anion is exothermic process. The most common negative ions are F-1, Cl-1, Br-1, S-2, O-2 etc.

 

Molecular Ion

When a molecule loses or gains electron form a molecular ion e.g CH4+, CO+, N+2, NH+4. Cationic molecular ions are abundant than anionic ones.

Free radicals

Free radical highly reactive species formed by the breaking of more stable molecule, in such a manner that the resulting reactive species gets separated with unpaired electrons. For example, during the reaction between chlorine molecule and methane, CH4 in the presence of diffused sunlight, the chlorine molecule first form chlorine free radicals.

Cl2→2Cl˚

The dot on the symbol on chlorine atom, represent be single unpaired electron. Such a species with the unpaired electron are known as free radicals.

The ˚Cl thus formed doesn't for long and reacts with CH4 to for methyl free radical as:

CH4 →˚CH3+HCl

The˚CH3 being very reactive, reacts with another Cl2 molecule to form chloro methane and Cl again.

˚CH3+Cl2→CH3-Cl + ˚Cl

In this way chain of reactions is propagated and goes on till the final product he is formed. Free radicals for highly reactive species and do not exist independently.

Molecule

 

Molecule is the smallest particle of the compound. When two or more than two atoms comes close together and a force of attraction produces which hold them tighter, a molecule is formed. For example water molecule has hydrogen and oxygen atoms, which are held by a force of attraction, called chemical bond.

Monatomic molecules

Monatomic molecules this type of molecules is made up of only one atom. Examples Of such molecules are the molecules of noble gases such as He,Ne, Ar, Kr, Xe and Rn.

Polyatomic molecules

The molecules made up of more than one atom are called polyatomic molecules. Diatomic which is made up of two atoms, tri atomic made up of three atoms, tetraatomic that is made up of four atoms.

Homoatomic molecules

When two or more atoms of the same element combine to form molecules are called homoatomic molecules. It consists of atoms of the same element for examples, N2, F2, P4 etc.

Hetero atomic molecules

When two or more atoms of the different elements combine to form molecules are called heteroatomic molecules. It consists of atoms of the same element. It consists of atoms of the different element. For Examples,HNO3, C12H22O11 etc.

Mole

Atom is an extremely small particle. The mass of an individual atom is extremely small quantity. It is impossible to measure the mass of individual atoms. Therefore we use the atomic mass unit (amu) to express the atomic masses.

The atomic mass of an element expressed in grams is called one gram atom. It is also called one gram mole or simply mole of that element. Similarly when mass molecular mass of a compound is expressed in grams is called gram molecule or simply the mole of that compound.

The atomic mass of an element or molecular mass of a compound is always equal to one mole. Therefore atomic mass or molecular mass is also called molar mass.

For example

1.008g of Hydrogen atoms = 1 mole

12.01g of carbon atoms = 1 mole

23.00g of sodium atoms = 1 mole

18.00g of water molecules = 1 mole

98.00g of sulphuric acid molecules = 1 mole

180g of glucose molecules = 1 mole

Number of moles of a substance can be determined by

Number of moles of a substance =

 

Avogadro’s number

The number of atoms, molecules and ions in one mole of a substance is called Avogadro’s number. To understand the Avogadro’s number considers the following.

1.008g of Hydrogen = 1 mole of hydrogen = 6.023x1023 atoms of Hydrogen

23g of Sodium = 1 mole of sodium = 6.023x1023atoms of Sodium

238g of Uranium = 1 mole of uranium = 6.023x1023 atoms of Uranium

18g of water = 1 mole of water = 6.023x1023molecules of water

342g of sucrose =1 mole of sucrose = 6.023x1023 molecules of sucrose

96g of SO-24 = 1mole of SO-24 = 6.023x1023 ions of SO-24

6.023x1023 is called Avogadro’s number denoted by NA.

It is interesting to know that different masses of element have the same number of atoms. Similarly one mole of different compounds have different masses but have same number of molecules.

Exercise

 Answer the following short questions. Give example where needed.

(i) Given the atomic masses of C, H, and O to be 12, 1 and 16 respectively. Work out mathematical operation to calculate the molecular mass of glucose (C6H12O6).

Solution

Glucose molecular formula = C6H12O6.

The molecular mass of C6H12O6

= 6(Atomic mass of C)+12(Atomic mass of H)+6(Atomic mass of O)

= 6(12) + 12(1) + 6(16)

= 72 + 12 + 96

= 180amu

(ii) Define the atomic number of an element. Give two examples.

Answer

Atomic Number

The number of protons in the nucleus of an atom is called its atomic number. OR The number of electrons present around the nucleus of neutral atom is called its atomic number.

Atoms are made up of small subatomic particles such as electron, proton and neutron. The number of electrons or protons is called atomic number .The atomic number is also known as the proton number. It determines the identity of an element as different elements have different atomic number. This is the reason that elements are arranged in the periodic table on the basis of its atomic number.Atomic number is represented by capital letter “Z”.Where,Z = Number of proton or Number of electron of an atom For examples the atomic number of hydrogen is 1 and the atomic number of carbon is 6.

(iii) Define atomic mass unit(amu).

Answer

Atomic mass unit (amu)

One twelfth (1/12) of the mass of an atom of carbon-12 isotope is called atomic mass unit. It is represented by ‘amu’

1 amu =

The numerical value of atomic mass unit is 1.67x10-24g or 1.67x10-27Kg.

It is a unit of mass used to express atomic masses and molecular masses.

iv. How would you differentiate between a substance and a mixture?

Substance

The matter which has definite properties and Composition is called substance.

• Substance is pure

• Pure substances are composed of one kind of particles

• Components of mixture can be separated by special methods

• Pure substances are represented by symbol or formula

Examples:Pure iron is a substance because made from only iron atoms (Fe) which are chemically identical.Similarly, pure water (H2O), Oxygen, Hydrogen, Carbon dioxide etc are examples of substances. Types of substance: Elements and compounds

Mixture

The physical combination of two or more substances is called mixture.

  • Mixture is impure
  • In mixture each substance retains their original properties
  • Components of mixture can be separated by physical methods
  • Mixture is not represented by chemical formula

Examples: Salt solution is a mixture because salt can be separated from water by physical method (evaporation).Similarly, Air, Coke, alloys, sugar solution, soil etc are examples of mixture. Types of Mixtures are Homogeneous and Heterogeneous

(V) How does a molecule differ from an atom?

Atom

The smallest particles of an element which may or may not have independent existence but take part in a chemical reaction is called atom

Characteristics:

  • It is the fundamental unit of an element
  • Atom of an element gives atomic weight
  • Atom is composed of subatomic particles called electron, proton and neutron etc
  • Atom is denoted by symbol

Examples: Carbon (C), Nitrogen(N), Oxygen (O), Zinc (Zn) etc

Molecule

The smallest particle of an element or compound which have independent existence is called molecule

Characteristics:

  • It is formed by the chemical combinations to two or more atoms
  • Molecules of a substance give molecular weight
  • Molecule is composed of atoms
  • Molecule is denoted by formula

Examples: H2O, N2, CO2, HCl etc

  1. Give the formula of a compound that contains:
  2. One atom of nitrogen and two atoms of oxygen
  3. One atom of phosphorus and five atoms of bromine

Answers

(a). As we know that nitrogen symbol is ‘N’ and oxygen symbol is ‘O’ while formula is formed by the combination of symbols. Therefore the formula formed by the combination of one atom of nitrogen and two atoms of oxygen is ‘NO2’ and called ‘nitrogen dioxide’

(b). As we know that phosphorus symbol is ‘P’ and Bromine symbol is ‘Br’ while formula is formed by the combination of symbols. Therefore the formula formed by the combination of one atom of phosphorus and five atoms of Bromine is ‘PBr5’ and called phosphorus penta bromide

vii Calcium loses two electrons, when it is ionized. Give the symbol for the calcium ion.

Answer

When atom loses electrons it forms positive ion. When one electron is removed from valence shell of an atom, one positive charge is produced. Similarly when two electrons are removed from valence shell of an atom, double positive charge is produced. For example Calcium ion symbol is Ca2+

viii. Define free radical with an example.

Answer

An atom or group of atoms that has at least one unpaired electron is called a free radical Characteristics:

  • Free radical is highly unstable
  • Very reactive
  • Free radical has no charge (Neutral)

Example: Free radical of chlorine is produced by breaking of a bond between chlorine atoms in chlorine molecule

 

  1. Why the atomic masses of the elements (atoms) are compared with a standard mass of C-12?

Ans. Atom is the smallest particle of matter and cannot be seen with naked eyes. So it is impossible and impractical to measure the mass of single atom. Therefore the atomic masses of elements are compared with the standard mass of C-12. The ‘carbon-12’ isotope is chosen as a standard because carbon is stable, most common isotope of carbon and mass is exactly 12amu.

  1. What do you mean by the molar mass of an element or a compound? Give examples.

Molar mass. The mass in grams of one mole of a substance is called molar mass. The unit of molar mass is gram/mol. For example the mass of one mole CO2 is 44 gram so molar mass will be 44g/mol

3 Define chemistry, state and explain the main branches of chemistry.

Answer

Chemistry

Chemistry is the study of matter. In this branch of science we deal with the composition, properties and chemical changes take place in matter.

Branches of Chemistry

The field of chemistry is so vast that it is further divided into many branches. The main braches of chemistry are:

Physical chemistry

The branch of chemistry which deals with the study of relations between the physical properties of substances and laws and principals involved in it is called physical chemistry.

Organic chemistry

The branch of chemistry which deals with the study of hydrocarbons and their derivatives is called organic chemistry. Where hydrocarbons are those compounds which are made from hydrogen and carbon except CO, CO2, HCO3 and HCN.

 

 

Inorganic chemistry

The branch of chemistry which deals with the study of all elements and their compound except hydrocarbons and their derivatives.

Analytical chemistry

The branch of chemistry which deals with the study of methods and techniques used to determine the kind and types of various components present in matter is called analytical chemistry.

Biochemistry

The branch of chemistry which deals with the study of chemical changes occur in living organisms.

Nuclear chemistry

The branch of chemistry which deals with the study of changes occur in the nuclei of atoms and resulted nuclear radiations.

Industrial chemistry

The branch of chemistry which deals with the study of different techniques and chemical process involved in industries for the preparation of different products such as cement, glass etc is called industrial chemistry.

Environmental chemistry

The branch of chemistry which deals with the study of interaction of chemical substances with the environment and their impact on it is called environmental chemistry.

4(a) differentiate by giving examples between element, a compound and mixture.

Answer

Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical or physical means. The fundamental unit of an element is the atom. Elements are the pure substances which are made up of only one component e. g. Gold, silver, copper, mercury, bromine, oxygen etc. element are represented by chemical symbols which are simply abbreviation for their names.

 

 

Compounds

Compounds are the pure substances which are made up of two or more components e.g. Water, ammonia, methane etc. compounds possess different physical and chemical properties from those its parent atoms.

Mixtures

A mixture is a simple combination of two or more substances in which the constituent substances retain their separate identities. The composition of mixture can be varied to any extent. Therefore, mixtures do not have fixed composition e.g. Mixture of ethyl alcohol and water, salt in water, mixture of gases, etc. Mixtures are of two types:

Homogeneous mixture

A mixture in which the concentration of the constituents remain uniform throughout the mixture and all the constituents are present in one phase, is called a homogeneous mixture e.g. Mixture of salt and water.

Heterogeneous mixture

If two or more phases are present in a mixture, it is called a heterogeneous mixture. e.g. Phenol - water system, silver chloride-water system, iron fillings-sand system, etc.

4(b) Classify the following as elements, a compounds and mixtures.

Platinum(Pt), air, sodium chloride, ethyl alcohol, zinc(Zn), rock, ice cream, hydrochloric acid, mineral water, distilled water, solution, nitrogen(N2), muddy water, phosphorous(P)

Answer

Element:Platinum (Pt), zinc(Zn), nitrogen (N2), phosphorous (P).

Compound: Sodium chloride, ethyl alcohol, hydrochloric acid,

distilled water.

Mixture: Rock, ice cream, mineral water, muddy water, air, solution.

 

5(a) What is atomic number of an element? How it differ from the massnumber?

Answer

Atomic number

  • Atomic Number is the number of protons or the number of electrons present in an atom.
  • Atomic number is represented by capital letter “Z”.

Mass number

  • It is the sum of number of protons and neutrons present in the nucleus.
  • Mass number is represented by ‘A’’

5(b) Differentiate between mass number and relative atomic massof an element.

Answer

Mass number

  • The sum of the number of protons and neutrons present in the nucleus of an atom is called mass number.
  • Mass number is represented by ‘A’.
  • Mass number (A) = Number of protons (Z) + number of neutrons

Relative atomic mass

  • It is the mass of an atom of an element as compared to the mass of an atom of carbon-12.
  • The unit used to express the relative atomic mass is called atomic mass unit (amu).
  • 1 amu = the mass C-12 atom.

5(c) find out the number of electrons, protons and neutrons in the following

Element

Atomic Number(Z)

Mass Number(A)

Electron

Proton

Neutron A-Z

107Ag47

47

107

47

47

107-47=60

23Na11

11

23

11

11

23-11=12

56Fe26

26

56

26

26

56-26=30

40Ar18

18

40

18

18

40-18=22

16O8

8

16

8

8

16-8=8

24Mg12

12

24

12

12

24-12=12

31P15

15

31

15

15

31-15=16

207Pb82

82

207

82

82

207-82=125

238U92

92

238

92

92

238-92=146

6 Calculate the average atomic mass of silver (Ag) which exists naturally as a mixture of two isotopes 42 107Ag (57%0 and 42 109Ag (43%).

Solution

Atomic mass of first isotopes 47107Ag = 107

% age composition of 47107Ag = 57%

Atomic mass of 2nd isotopes47109Ag = 109

% age composition of 2nd isotope 47109Ag = 43%

Average atomic mass of silver=?

Average atomic mass of silver = (107 x 57%) + (109 x 43%)

= (107 x0.57) + (109 x 0.43)

= 60.99 + 46.8 = 107.86 amu

7 State and explain with examples:

(a) The empirical formula of a compound

Empirical formula

The formula which shows the simplest ratio of atoms of elements present in the molecule of a compound is called empirical formula

Examples. The empirical formula of benzene (C6H6) is ‘CH’ which shows simplest ratio (1:1). Similarly the empirical formula of glucose (C6H12O6) is CH2O (1:2:1).

(b) The molecular formula of compound

Molecular formula

The formula which shows the actual number of atoms of elements present in the molecule of a compound is called molecular formula

Examples. The molecular formula of benzene is C6H6. Similarly the molecular formula of glucose is C6H12O6.

8(a) what is molecular mass of a compound? How will you differentiate from formula mass of a compound?

Answer

Molecular mass

Molecular mass is calculated for covalent compounds. The sum of masses of the atoms present in a molecule is called as molecular mass.

Formula mass

For an ionic solid, such as NaCl, which do not have molecules, we calculate formula mass based on the empirical formula of the compound.

8 (b). Calculate the molecular mass or formula mass of the following compounds.

Benzene (C6H6) ii. Ethan gas, C2H6 iii. Aluminium chloride (AlCl3) IV. Iron Oxide (Fe2O3)

Answer

  1. Molecular mass of Benzene (C6H6)= 6(12amu) + 6(1amu)=72+6= 78amu
  2. Molecular mass of ethane (C2H6)= 2(12 amu) + 6(1amu)= 24 + 6 = 30amu
  • Molecular mass of (AlCl3) =(27amu)+3(35.5amu)=5amu
  1. Molecular mass of Iron Oxide (Fe2O3) = 2(56amu) + 3(16amu) =160 amu

9 define gram atomic mass, gram molecular mass and gram formula mass of the elements and compounds. Give at least two examples in each case.

Gram Atomic Mass

When the relative atomic mass is expressed in grams we call it the gram atomic mass. One gram-atom of any element is the relative atomic mass of that element expressed in grams. For example, one atom of chlorine is equal to 35.5 g.

Gram Molecular Mass
The molecular mass of a compound expressed in gram is called gram molecular mass or mole.
Examples
1. The molecular mass of H2O is 18. If we take 18g H2O then it is called 1g molecular mass of H2O or 1 mole of water.
2. The molecular mass of HCl is 36.5. If we take 36.5g of HCl then it is called as 1 gm molecular mass of HCl or 1 mole of HCl.

Gram formula mass

It is a terminology used for ionic compounds to express the maths on the formula unit in grams. For example formula mass of NaCl is 58.5g and that of CaCl2 is 111g.

  1. What are homoatomic and heteroatomic molecules? Classify the following molecules as either homoatomic or heteroatomic.HNO3, N2, F2, C12H22O11, P4

Answer

Homoatomic molecules

When two or more atoms of the same element combine to form molecules are called homoatomic molecules. It consists of atoms of the same element.

For examples, N2, F2, P4 etc

Hetero atomic molecules

When two or more atoms of the different elements combine to form molecules are called heteroatomic molecules. It consists of atoms of the same element. It consists of atoms of the different element. For Examples, HNO3, C12H22O11 etc

11 what do you understand the terms mole and Avogadro’s number? Give examples

Answer

Mole

The atomic mass or molecular mass or formula mass of a substance expressed in grams is called mole or the gram atomic mass or gram molecular mass or gram formula mass of a substance that contains 6.02 x 1023 particles is called mole it is the mass in grams of the atoms or molecules or ions which contains Avogadro's number of particles i.e. 6.02 x 1023 particles. Mathematically

Examples. 1gram of hydrogen = 1 mole of hydrogen

18 gram H2O = 1 mole of water

Avogadro’s number

The number of particles (atoms, ions or molecules) present in one mole of a substance is called Avogadro’s number or One mole of any substance contains equal number of particles (atoms or molecules or ions) which is called Avogadro’s number.

The numerical value is 6.02 x 1023and denoted by "NA".

Examples: One mole of hydrogen = 6.02 x 1023 molecule of hydrogen.

One mole of sodium = 6.02 x 1023 sodium of hydrogen.

One mole of Ca+2 = 6.02 x 1023 ions of Ca+2.

12 Calculate the number of moles in 550 g of NaI.

Solution

Given weight of NaI = 550g

Atomic mass of Na = 23 amu 

Atomic mass of I= 127 amu

Number of moles in 550g NaI =?

Formula weight of NaI = 23 + 127 = 150g 

Mole of NaI= 550/150 = 3.67 moles

13 How many molecules are there in 40.5g of HBr?

Solution

Mass of HBr in grams = 40.5g

Molar mass of HBr = 80gmol-1

Number of moles = 40.5/80 = 0.506mol

Number of molecules = number of moles x NA

= 0.506 x 6.023 x 1023= 3.01 x 1023

14 what is the molar mass each of the following?

(i) HF (ii) KNO3 (iii) C2H2

Solution

  • H = 1, F = 19

1+19= 20g

  • K=39, N= 14, O=16

39+14+3(16) =101g

  • C=12, H=1

2(12) +2(1) =26

15 what is the mass of 5 moles of ice?

Solution

Number of moles of ice (water) = 5moles

Molar mass of water = 18gmol-1

Mass of ice =?

Mass of ice in grams = number of moles x molar mass

= 5moles x 18gmol-1= 90g

16 how many moles of CO2 are there in 7.5 x 1024molecules of the gas?

Solution

Number of molecules = 7.5 x 1024

Numberof moles =

Number of moles = = 12.45moles

17 calculate the molar mass of the compound if 10 moles of that compound weigh 260.

Solution

Number of molecules = 10moles

Mass in grams = 260g

Molar mass of the compound =?

Molar mass of the compound = = = 26gmol-1

 

 

 

 

 

Explain with reasons.

  1. Alloys are mixture but not compound.

The solid mixture of two or more metals are called alloy. For example brass is alloy of zinc and copper. Alloy is a mixture not a compound because alloy is formed by the physical combination of elements and there is no chemical bond between alloy components. Additionally, alloy can be formed by mixing any proportion of their components due to which it is not a compound.

  1. Elements and compounds are called pure substances but mixtures are not

Pure substances are those substances which are made of only one kind of particles and have definite properties. Now these particles can be atoms or molecules. Elements are considered to be pure because it has same kind of atoms, same atomic number and same chemical properties. For example, a pure copper metal has only copper atoms (Cu) in it. Similarly, compounds have the same kind of molecules (one kind of particles) throughout which possess same structure and properties.

For example, water is a pure substance as pure water only contains H2O molecules (one kind of particle). In contrast, mixture is not a pure substance because does not contain one kind of particles. For example, salt solution (mixture) contains salt and water (two components).

  • NaCl is not a molecule but a formula unit.

Ionic compounds form crystal lattices, not molecules. The term formula unit is used to indicate the simplest whole-number ratio of ions in the compound. NaCl is an ionic compound due to which does not exist as individual molecule. In NaCl we have a repetitive crystal structure in which each Na+ ion is surrounded by Cl- ions, and vice-versa. The ratio between sodium and chlorine ions is 1:1 which only represents empirical ratio not the actual ratio. Therefore we cannot call NaCl a molecule but a formula unit.

  1. Ion is a charged particle but free radical is not.

Ion is a charged particle but free radical is not because when neutral atoms loss or gain electron ions are formed. For example

Na Na+1 + i.e.

Cl +1e Cl-1

Therefore ions are charged particles. In contrast, free radicals are formed when bond in a stable molecule is broken down and each atom gets their own single electron back. Therefore free radicals have no charge because number of protons becomes equal to number of electron.

  1. Mole is a definite quantity and definite number as well but Avogadro number is only a number.

Ans. Mole is the mass in grams of the atoms or molecules or ions which contains

6.02 x 1023 particles.

One mole of nitrogen = 14 gram of nitrogen (definite quantity)

One mole of nitrogen = 6.023x1023 atoms of nitrogen (definite number) From above example it is clear that mole represents definite quantity and definite number of particles of a substance but Avogadro number is just number 6.023x1023 not a quantity of substance.




View and Download More Tutorials !

Chemistry Chapter 1

Chemistry Chapter 2