Question

Find the molecular mass of:
(a) ${{\text{H}}_{\text{2}}}\text{S}$, (b) $\text{HCl}$, (c) $\text{N}{{\text{H}}_{\text{3}}}$, (d) $\text{C}{{\text{l}}_{2}}$, (e) $\text{C}{{\text{H}}_{\text{3}}}\text{COOH}$, (f) $\text{C}{{\text{H}}_{\text{3}}}\text{CHO}$

Answer 1

Hint: The molecular mass of a chemical compound can be calculated by summing up the atomic masses of the elements present in the compound.

Complete Step by step Solution:
To find out the molecular mass of the compounds, we need to know the atomic masses of the elements present in the compounds.

Elements	Atomic Masses
Hydrogen	1
Sulphur	32
Chlorine	\[35.5\]
Nitrogen	14
Carbon	12

The molecular weight of hydrogen sulphide is = \[\left[ \left( 2\times 1 \right)\text{ }+\text{ }32 \right]\] = \[34\].
The molecular weight of hydrogen chloride is = \[\left[ 1+35.5 \right]\] = \[36.5\].
The molecular weight of ammonia is = \[\left[ 14\text{ }+\text{ }\left( 3\times 1 \right) \right]\] = \[17\].
The molecular weight of chlorine is = \[\left[ 35.5\times 2 \right]\] = \[76\].
The molecular weight of acetic acid = \[\left[ \left( 12\times 2 \right)\text{ }+\text{ }\left( 4\times 1 \right)\text{ }+\text{ }\left( 16\times 2 \right) \right]\] = \[60\].
The molecular weight of acetaldehyde = \[\left[ \left( 12\times 2 \right)\text{ }+\text{ }\left( 4\times 1 \right)\text{ }+\text{ }16 \right]\] = \[44\].

Notes: The molecular weight of the substance is the weight of one molecule of that compound in atomic mass units. But as this mass is extremely small to be measured, the concept of mole was introduced, where one mole is equal to $6.02\times {{10}^{23}}$ particles of a substance. The word ‘particles’ refer to atoms, molecules, electrons, protons, etc. and other subatomic particles. The gram molecular weight of a substance is equal to one mole of that substance. So, 44 grams of acetaldehyde is equal to the weight of $6.02\times {{10}^{23}}$ molecules of acetaldehyde.