Question

(a) Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen?
(b) If sulphur exists as S₈ Molecules, calculate the number of moles in 100 g of sulphur. (S = 32 u)

Answer 1

(a) Gram molecular mass is defined as the mass in grams of one mole of molecular substance or simply equal to the sum of all the atomic masses in the molecular formula of a compound in grams.

The atomic mass of oxygen atom(O) = 16

So, gram molecular mass of oxygen molecule(O₂) = (2 ⅹ 16) = 32 g

(b) The atomic mass of sulphur(S) = 32 u

Molecular mass of sulphur(S₈)= 8 ⅹ 32 = 256 u

Given mass of sulphur(S₈) = 100 g

$$\begin{gathered} \because \mathrm{Number}\mathrm{of}\mathrm{moles}=\frac{\mathrm{Given}\mathrm{mass}(\mathrm{in}\mathrm{grams})}{\mathrm{Molecular}\mathrm{mass}} \\ \therefore \mathrm{Number}\mathrm{of}\mathrm{moles}=\frac{100}{256}=0.3906 \end{gathered}$$