Class-XI Chemistry

SOME BASIC CONCEPTS OF CHEMISTRY

ATOMIC AND MOLECULAR MASSES

 ATOMIC AND MOLECULAR MASS

One of the most important concepts derived from Dalton's atomic theory is that of atomic mass,
i.e.,” each element has a characteristic atomic mass. As atoms are very tiny particles, their
absolute masses are difficult to measure. However, it is possible to determine the relative masses
of different atoms if a small unit of mass is taken as a standard. For this purpose, mass of one
atom
of hydrogen was assumed as unity and was accepted as standard. The be atomic mass of an
element can be defined as the be number wbich indicates how many times the be mass of one
atom of the element is heavier in comparison to the mass of one atom: of hydrogen.
A = Atomic mass of an element
Mass of an element
=
Mass of one atom of hydrogen

In 1858, oxygen atom was adopted as a standard on account of the following reasons:
(i) It is much easier to obtain compounds of elements with oxygen than with hydrogen as oxygen
is more reactive than hydrogen.
(ii) The atomic masses of most of the elements. Become approximately whole numbers but with
hydrogen as standard the atomic masses of most of the elements are fractional. The mass of
one atom of natural oxygen was taken to be 16.0. Thus, atomic mass of an element

Mass of one atom of the element

= 1
th part of the mass of one atom of oxygen
16

Mas of one atom of the element

=
mass of one atom of oxygen

By accepting oxygen as a standard, the atomic mass of hydro 6gen comes as 1.008, sodium 22.991
and sulphur 32.066.
In 1961, the International Union of Chemists selected a new unit for expressing the atomic
masses. They accepted the stable isotope of carbon (12C) with mass number of 12 as the standard.
Atomic mass of an element can be defined as the number which indicates how many times the

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mass of one atom of the element is heavier in comparison to 1/12 the part of the mass of one
atom of carbon-l2 (12C).

A= Atomic mass of an element

Mass of one atom of the element

= 1
12
th part of the mass of one atom of carbon−12

Mass of one atom of the element

= × 12
Mass of one atom of carbon−12

[The quantity' A' was formerly known as atomic weight. However, this term is no longer used as
the word 'weight' means gravitational force.]
Atomic mass unit: The quantity ~ mass of an atom of 12 carbon-12 (12 C) is known as the atotinc
mass unit and is abbreviated as amu. The actual mass of one atom of carbon-l2 is 1.9924 x 10-23
g or 1.99.24 x 10-26 kg.

Thus,

1.9924 ×10−23
1 amu = = 1.66 ×10−24 g or 1.66 × 10−27 kg
12

A= atomic mass of an element

Mass of one atom of the element

= 1 amu

The atomic masses of some elements on the basis ofcarbon-l2 are given below:

Hydrogen 1.008 amu Iron 55.847 amu

Oxygen 16.00 amu Sodium 22.989 amu

Chlorine 35.453 amu Zinc 65.38 amu

Magnesium 24.305 amu Silver 107.868 amu

Copper 63.546 amu

The actual mass of an atom of an element the atomic mass of an element in amu x 1.66 x 10-24 g
So, the actual mass of hydrogen atom 1.008 x 1.66 x 10-24 = 1.6736 X 10-24 g Similarly,
the actual mass of oxygen atom 16 x 1.66 x 10-24 = 2.656 X 10-23 g
It is clear from the above list of atomic masses that atomic masses of a number of elements are

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not nearly whole numbers. Actually, the above values are average relative masses. Most of
the elements occur in nature as a mixture of isotopes. (Isotopes-the atoms. of the same element
having different atomic masses). With very few exceptions, however, elements have constant
mixtures of isotopes. Chlorine is found in nature as a mixture containing two isotopes CI-35
(34.969 amu) and CI-37 (36.966amu). These are found in the ratio of 75.53% (CI-35) and
24.47% (CI-37).
Therefore, the average relative mass of chlorine is calculated as:
(34.969 x 0.7553) + (3'6.966 x 0.2447) = 35.46 amu
Based on the average mass, the atomic mass of chlorine is 35.46 or 35.5 amu but it is never
possible to have-an atom having a relative mass 35.5 amu. It can have relative mass of about
35.0 or 37.0 amu depending on the particular isotope. Thus, average relative mass of any
naturally occurring sample of chlorine is 35.46 or 35.5 amu as it is a mixture of two isotopes
present in definite proportion. The same reasoning applies to all other
elements. The average atomic masses of various elements are det~ed by multiplying the atomic
mass of each is. tope by its fractional abundance and adding the values thus obtained.

The fractional abundance is determined by· dividing percentage abundance by hundred.

Example 11. Boron has two isotopes boron-1O and boron-II whose percentage abundances are
19.6% and 80.4% respectively. What is the average atomic mass of boron?
Solution:
Contribution of boron-1O 10.0 X 0.196 = 1.96amu
Contribution of boron-II =ll. Ox 0.804 = 8.844amu
Adding both =1.96+8.844=10.804amu
Thus, the average atomic mass of boron is 10.804 amu. Gram-atomic Mass or Gram Atom
When numerical value of atomic mass of an element is expressed in grams, the value becomes
gram-atomic mass or gram atom. The atomic mass of oxygen is 16 while gram-atomic mass or
gram atom of oxygen is 16 g. Similarly, the gram-atomic masses of hydrogen, chlorine and
nitrogen are 1.008 g, 35.5 g and 14.0 g respectively; Gram atomic. mass or gram. Atom of every
element consists of same number of atoms. This number is called Avogadro's number. The value
of Avogadro's number is 6.02 x 1023. Absolute mass of one oxygen atom
= 16amu =16x 1.66 x 10-24 g
Therefore, the mass of 6.02 x 1023 atoms of oxygen will be
= 16x 1.66 x 10-24 x 6.02 X 1023

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Class-XI Chemistry

16g (gram-: atomic mass)

Thus, gram-atomic mass can be defined as the absolute mass in grams of 6.02 x 1023 atoms of
any element. Number of gram atoms of any element can be calculated with the help of the
following formula:

Mass of the element in grams

No. of gram atoms =
Atomiv mass of the element in grams

Molecular Mass:
Like an atom, a molecule of substance is also a very small particle possessing a mass of the order
of lO-24 to 10-22 g. Similar to atomic mass, molecular mass is also expressed as a relative
mass with respect to the mass of the standard substance which is an atom of hydrogen or an atom
of oxygen or an atom of carbon-12. The molecular mass of a substance may be defined as the
mass of a molecule of a substance relative to the mass of an atom of hydrogen as 1.008 or of
oxygen taken as 16.00 or the mass of one atom of carbon taken as 12. Molecular mass is a number
which indicates how many times one molecule of a substance is heavier in comparison to 1/16th
of the mass of oxygen atom or 1/12th of the mass of one atom of carbon-12.
M = Molecular mass
Mass of one molecule of the substance
= 1
th mass of one atom of carbon−12
12

The mass of a molecule is equal to sum of the masses of the atoms present in a molecule. One
molecule of water consists of 2 atoms of hydrogen and one atom of oxygen. Thus, molecular. mass
of water (2 x 1.008) + 16.06 18.016amu. One molecule of H2 S04 (sulphuric acid) consists of 2
atoms of hydrogen, one atom of sulphur and four atoms of oxygen. Thus, the molecular mass of
sulphuric acid is
= (2x 1.008) + 32.00 + (4 x 16.00)
= 98.0160r 98.016amu
Gram-molecular Mass or Gram Molecule A quantity. of substance whose. mass. in grams is
numerically equal to its molecular mass' is called gram molecular mass. In other words;
molecular mass of a substance
expressed in grams is called gram-molecular mass to gram molecule. For example, the molecular
mass of chlorine is 71 and, therefore, its gram-molecular mass or gram molecule is 71 g.
Similarly, molecular mass of oxygen (02) is 32, i. e.,

2x 16= 32amu.

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Gram-molecular mass of oxygen 32g Molecular mass of nitric acid (HN03) is 63, i. e.,
= 1+ 14 + 3 x 16 = 63 amu

Gram-molecular mass of nitric acid = 63 g

Gram-molecular mass should not be confused with the mass of one molecule of the substance in
grams. The mass of one molecule of a substance is known as its actual mass.
For example, the actual mass of one molecule of oxygen is equal to.
32x 1.66 x 10-24 g, i.e., 5.32 x 10-23 g.
The number of gram molecules of a substance present in a given mass of a substance can be
determined by the application of following formula:
Mass of a Substance in grams
No. of gram molecules =
Molecular mass of the substance in grams
Molar mass in grams
Mass of single molecule =
6.023 × 1023
= Molar mass in amu × 1.66×10−24 grams
Calculate the mass of 2.5-gram atoms of oxygen.

Solution: We know that,

Mass of the element in grams
No. of gram atoms =
Atomic mass of the element in grams

So, Mass of oxygen = 2.5 x 32 80.0g

Calculate the mass of 1.5-gram molecule of sulphuric acid.

Solution: Molecular mass of H

2S04 =2x 1+ 32+ 4 x 16= 98.0amu
ram-molecular mass of H2S04 =98.0g
Mass of 1.5-gram molecule of H2S04=98.0x 1.5=147.0g