Chemistry, Third edition

PERCENTAGE COMPOSITION BY MASS

Simplest formula

Just as the percentage composition by mass can be calculated from the formula of a

compound, the simplest formula of a compound can be found from the percentage

composition by mass of each element. The simplest formula is also known as the

empirical formula. Here, ‘empirical’ means ‘obtained from experimental results’.

To find the simplest formula:

1.Write down the symbols of the elements involved in the compound.

2.Write the percentage composition of each element under its symbol.

3.Divide each percentage by the molar mass of that element (‘the atomic mass in

grams’) to obtain a molar ratio.

4.Find the simplest, whole number, molar ratio to work out the formula of the

compound.

125

Calculating simplest

(or empirical)

formulae

(i) When methane is

analysed it is found to

contain 75% C and 25%

H. What is its empirical

formula?

(ii)15.3 g of aluminium

atoms combine with

13.6 g of oxygen atoms

to form an oxide. What

is the simplest formula

of the oxide?

(iii)0.4764 g of an oxide of

iron contained 0.3450 g

of iron. What is the

empirical formula of the

oxide?

(iv)A compound contains

sodium (32.3%), sulfur

(22.5%) and oxygen

only. What is its

empirical formula?

Exercise 8F

Example 8.5

Calculate the simplest, or empirical, formula for a compound

that contains 27.3% C and 72.7% O by mass.

Answer

Symbols of elements involved C O

Percentage composition 27.3 72.7

Divide by molar masses 27.3 72.7

12 16

Molar ratio 22.3 24.5

Simplest whole number ratio 22.3 24.5

(this can often be obtained by dividing 22.3 22.3

through by the smallest number)

This is approximately 21 22

Therefore, the simplest formula of the compound is CO 2.

Comment

Sometimes the composition of elements in the compound is given as mass of ele-

ments in a given mass of compound. In this case, the procedure is the same, but

the mass of each element is substituted for the percentage composition by mass.

Example 8.4

Calculate the percentage composition by mass of nitrogen in

ammonium nitrate, NH 4 NO 3.

Answer m(N)14 u

m(NH 4 NO 3 ) 14 (41) 14 (316)80 u

number of atoms of N  2

percentage composition by mass 

14  2  100

35%

80

Calculating

percentage

composition by

mass

Find the percentage

composition by mass of:

(i) sulfur in Na 2 SO 4

(ii)sodium in

Na 2 CO 3 · 10H 2 O

(iii)oxygen in Al(NO 3 ) 3

(iv)bromine in CaBr 2

(v)sulfate in (NH 4 ) 2 SO 4.

Exercise 8E