Question

What is the molecular mass of sodium sulphate ?
A.154
B.119
C.165
D.142

Answer 1

Hint: To calculate the molecular mass of any substance we should know the atomic mass of each of the elements present in the molecule . In case of sodium sulphate the mass of sodium , sulphur and oxygen should be known .

Complete step by step answer:
Molecular mass of a substance refer to the relative mass of its molecule and is defined in a manner similar to the atomic mass as follows:
The molecular mass of a substance (element or compound ) is the number of times the molecule of the substance heavier than $1/12th$ the mass of an atom of carbon-12 isotope.
or
The molecule mass of a substance is the average relative mass of its molecules as compared with an atom of carbon-12 isotope taken as 12.
So , to calculate the molecular mass of sodium sulphate we will add the atomic masses of its constituent elements .
The chemical formula of sodium sulphate is $N{a_2}S{O_4}$ .
The atomic masses of its constituents are :
atomic mass of $Na = 23$ , atomic mass of $S = 32$ , atomic mass of $O = 32$
Therefore the molecular mass of sodium sulphate is (keeping in mind there is one atom of sulphur , two atoms of sodium and three atoms of oxygen ) .
$(23 \times 2) + 32 + (32 \times 4) = 142$

So, the correct answer is Option D .

Note:
Atomic masses can be expressed as either amu (atomic mass unit) or as gram atomic mass .
One atomic mass unit is equal to $\dfrac{1}{{12}}th$ of the mass of an atom of carbon-12 isotope .
The atomic mass of an element expressed in grams is called gram atomic mass .