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WAEC Chemistry 2022 Paper

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Question : 46

Total: 50

If the molar mass of

\mathrm{X}(\mathrm{HCO}_3)_2

\mathrm{mol}^{-1}

, determine the relative atomic mass of X .

[\mathrm{H}=1.0,\ \mathrm{C}=12.0,\ \mathrm{O}=16.0]

40
48
61
101

Solution:

Concept:

The molar mass of a compound is the sum of the molar masses of all atoms in its formula.

Explanation:

The formula is

\mathrm{X}(\mathrm{HCO}_3)_2

.

Mass of one

\mathrm{HCO}_3

group

= 1.0 + 12.0 + (3 \times 16.0) = 61.0

g mol

^{-1}

.

Since there are two such groups:

2 \times 61.0 = 122.0

g mol

^{-1}

.

Total molar mass:

X + 122.0 = 162

g mol

^{-1}

.

Hence,

X = 162 - 122 = 40

.

The relative atomic mass of

X

is therefore 40.

Answer:

A. 40

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