Page 142 - Chemistry ICSE Class X

128 ICSE Chemistry – 10

How to calculate the atomic mass of an element from the
mass numbers of its isotopes
The atomic mass of an element is the weighted arithmetic mean of the
atomic masses of its isotopes present in the sample of the element.
Let us consider a sample of an element X containing its two isotopes
X and X . Then
2
1
(Percentage of X × Mass number of X ) + (Percentage of X × Mass number of X )
Atomic mass of the element X = 1 1 2 2
100

This method is illustrated by taking the case of chlorine. The two
37
35
DĂƐƐ ŶƵŵďĞƌƐ ĂƌĞ ǁŚŽůĞ ŶƵŵďĞƌƐ͕ isotopes of chlorine, Cl and Cl occur in the ratio 3 : 1. Then,
17
17
ǁŚĞƌĞĂƐ ĂƚŽŵŝĐ ŵĂƐƐĞƐ ĂƌĞ ŐĞŶĞƌĂůůLJ
ĨƌĂĐƟŽŶĂů͘ (35 u × 3) + (37 u × 1) 105 u + 37u 142 u
Atomic mass of chlorine = = = = 35.5 u
3 + 1 4 4
79
EXAMPLE 5.14. If bromine occurs in the form of say two isotope Br (49.7%)
35
81
and Br (50.3%), then calculate the atomic mass of bromine atom.
35
79 u × 49.7 + 81 u × 50.3
Solution: Atomic mass of bromine = = 80.0 u
49.7 + 50.3
So, the atomic mass of bromine is 80.0 u.
EXAMPLE 5.15. The atomic mass of a sample of an element X is 16.2 u. What
16
18
are the percentages of isotope X and X in the sample?
8
8
Solution: Atomic mass of X = 16.2 u
16
Let the percentage of the isotope X be P . Then,
8
1
18
Percentage of the isotope X = (100 – P )
8
1
We can then write,
16
16
Atomic mass of (Percentage of X × 16 u) + (100 – Percentage of X) × 18 u
the sample = 8 100 8
(P × 16 u) + (100 – P ) × 18 u
16.2 u = 1 1
100
1620 u = 16 P u + 1800 u – 18 P u
1
1
This gives,
(18 P u – 16 P u) = 1800 u – 1620 u
1
1
2 P u = 180 u
1
180
P = = 90
1
2
16
Thus, Percentage of X = 90%
8
18
and Percentage of X = (100 – 90)% = 10%
8
Molecular Mass and Relative Molecular Mass
What is the molecular mass of a substance
Average mass of a molecule of any substance is known as its molecular
mass. Molecular mass is designated as M. Since molecular mass is actual
mass, hence it has the unit of mass, viz., gram, kilogram, atomic mass unit,
dŚĞ ŵŽůĞĐƵůĂƌ ŵĂƐƐ ŽĨ Ă ƐƵďƐƚĂŶĐĞ ŝƐ etc. The accepted standard unit of molecular mass is atomic mass unit
ĞƋƵĂů ƚŽ ƚŚĞ ƐƵŵ ŽĨ ĂƚŽŵŝĐ ŵĂƐƐĞƐ ŽĨ (u). 6JWU VJG OQNGEWNCT OCUU QH C UWDUVCPEG OC[ DG FGſPGF CU HQNNQYU
Ăůů ƚŚĞ ĂƚŽŵƐ ƉƌĞƐĞŶƚ ŝŶ Ă ŵŽůĞĐƵůĞ ŽĨ “The average mass of a molecule of a substance expressed in atomic
ƚŚĂƚ ƐƵďƐƚĂŶĐĞ͘
mass units is termed as its molecular mass.”