Mole Concepts

Question 1

What is law of definite proportion?

Answer 1

The ratio of weights of different elements in a compound always remain constant.

Question 2

Law of multiple proportion (john Dalton)

Answer 2

According to this law, when two elements A and B combine to form more than one chemical compound
then different weights of A , which combine with a fixed weight of B , are in a proportion of simple whole
number

Question 3

Law of reciprocal proportions [Ritche, 1792-94]

Answer 3

When two elements combines separately with third element and form different types of molecules, their
combining ratio is directly reciprocated if they combine directly
Example :
C with H form methane and with O form CO2. In CH4 , 12 grams of C reacts with 4 grams of H whereas
in CO2 12 gram of C reacts with 32 grams of O. Therefore when H combines with O they should combine
in the ratio of 4 : 32 (i.e. = 1 : 8) or in simple multiple of it. The same is found to be true in H2O molecule.
The ratio of weights of H and O in Water is 1 : 8

Question 4

Gay-Lussac’s [1808] law of combining volumes :

Answer 4

This law states that under similar conditions of pressure and temperature, volume ratio of gases is always
in terms of simple integers.
Ex.
N2(g) + 3H2(g) = 2NH3(g)
vol. ratio 1 : 3 : 2

Question 5

AVOGADRO’S HYPOTHESIS :

Answer 5

Vapour density =
Molecular weight/2

Question 6

On hydrogen scale :

Relative atomic mass (R.A.M)

Answer 6

Relative atomic mass (R.A.M) = Mass of one atom of an element/ mass of one hydrogen atom

Relative atomic mass (R.A.M) =
mass of one C 12 atom/12*1 Mass of one atom of an element

Question 7

Atomicmass unit (or amu) :

Answer 7

The atomic mass unit (amu) is equal to (1/12)th
mass of one atom of carbon-12 isotope.

therefore 1 amu = 1/12 × mass of one C-12 atom
~ mass of one nucleon in C-12 atom.

Atomic mass = R.A.M × 1 amu

Question 8

Y-map : Interconversion of mole - volume, mass and number of particles :

Answer 8

Question 9

Relative density or Vapour density

Answer 9

Vapour density is defined as the density of the gas with respect to hydrogen gas at the same temperature
and pressure.
Vapour density = dgas/dH2

V.D. =Mgas /MH2=Mgas/2

Mgas = 2 V.D.

Question 10

Mole-mole analysis :

Answer 10

This analysis is very much important for quantitative analysis point of view. Students are advised to
clearly understand this analysis.
Now consider again the decomposition of KClO3 .
2KClO3 ———- 2KCl + 3O2
In very first step of mole-mole analysis you should read the balanced chemical equation like
2moles KClO3 on decomposition gives you 2moles KCl and 3moles O2. and from the stoichiometry
of reaction we can write
Moles of KClO3/2 = Moles of KCl/2 = Moles of O2/3
Now forr any general balance chemical equation like
a A + b B ————–c C + d D
you can write
Moles of A reacted/a = moles of B reacted/b = moles of C produced/c = moles of D produced/d

Question 11

Molarity (M) :

Answer 11

The number of moles of a solute dissolved in 1 L (1000 ml) of the solution is known as the
molarity of the solution.
i.e., Molarity of solution = number of moles of solute/ volume of solution in litre

Question 12

Molality (m)

Answer 12

The number of moles of solute dissolved in1000 gm (1 kg) of a solvent is known as the molality of
the solution.

=number of moles of solute/ mass of solvent in gram*1000

Question 13

Mole fraction (x) :

Answer 13

The ratio of number of moles of the solute or solvent present in the solution and the total number
of moles present in the solution is known as the mole fraction of substances concerned.

Question 14

% calculation :

weight by weight (w/w)

Answer 14

The concentration of a solution may also expressed in terms of percentage in the following way.
— % weight by weight (w/w) : It is given as mass of solute present in per 100 gm of solution.

Question 15

AVERAGE/ MEAN ATOMIC MASS :

Answer 15

The weighted average of the isotopic masses of the element’s naturally occuring isotopes.
Mathematically, average atomic mass of X (Ax) =

a1x1 + a2x2+…. +anxn/100
Where :
a1, a2, a3 ……….. atomic mass of isotopes.
and x1, x2, x3 ……….. mole % of isotopes

Question 17

% calculation :

– % weight by volume (w/v) :

Answer 17

The concentration of a solution may also expressed in terms of percentage in the following way.

– % weight by volume (w/v) : It is given as mass of solute present in per 100 ml of solution

Question 18

% calculation :

–% volume by volume (v/v) :

Answer 18

The concentration of a solution may also expressed in terms of percentage in the following way.

–% volume by volume (v/v) : It is given as volume of solute present in per 100 ml solution.