Question

Calculate the molar mass of the following substances.
A.Ethylene, ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}$
B.Sulphur molecule, ${{\text{S}}_{\text{8}}}$
C.Phosphorus molecule, ${{\text{P}}_{4}}$(Atomic mass of phosphorus = 31)
D.Hydrochloric acid, $\text{HCl}$
E.Nitric acid, $\text{HN}{{\text{O}}_{\text{3}}}$

Answer 1

The molecular mass or the molar mass of a compound can be calculated by summing up the atomic masses of the component elements.

Complete answer:
A.Ethylene – In the ethylene molecule, there are two carbon atoms and two hydrogen atoms. The atomic weight of carbon is 12 while that for hydrogen is 1. Hence the molecular weight of ethylene is $\left[ \left( 12\times 2 \right)+\left( 1\times 2 \right) \right]=24+2=26$
B.Sulphur – The atomic weight of sulphur is 32. Hence the molecular weight is $\left[ 32\times 8 \right]=256$. Here the atomicity of the sulphur molecule is eight, hence the atomic weight was multiplied by eight.
C.Phosphorous – The atomic weight of sulphur is 31. Hence the molecular weight is $\left[ 31\times 4 \right]=124$. Here the atomicity of the phosphorus molecule is four, hence the atomic weight was multiplied by four.
D.Hydrochloric acid – The atomic weight of hydrogen is 1 and that of chlorine is $35.5$. Hence the molecular weight of hydrochloric acid is $35.5+1=36.5$
E.Nitric Acid – The atomic weight of nitrogen is 14, hydrogen is 1, and that of oxygen is 16. Therefore the molecular weight of nitric acid is $\left[ 1+14+\left( 16\times 3 \right) \right]=63$

Note:
The molecular weight of a substance is equal to one mole of molecules of that compound. The atomic weight of an element is equal to one mole of atoms of that element. One mole of a substance is defined as $6.023\times {{10}^{23}}$ particle, where particles can be atoms, molecules, and different subatomic particles.