Question

In magnesium sulphide, the ratio by mass of Mg and S is 3 :4. What is the ratio of the number of Mg and S atoms ?

Answer 1

• The law of multiple proportions states that if two elements are combined to form more than one compound between them, then the ratios of the masses of the second element which combines with a fixed mass of the first element will always be ratios of small whole numbers.
• As, Formula of Magnesium sulphide =$\mathrm{MgS}$

Given ration by mass of $\mathrm{Mg}$ and $S$=3:1

Number of atoms in 3 g of $\mathrm{Mg}$=$\frac{N_A}{24}\times 3=\frac{N_A}{8}$

Number of atoms in 3 g of $S$ =$\frac{N_A}{32}\times 4=\frac{N_A}{8}$

• Therefore the ratio of number of $\mathrm{Mg}$ and $S$=1:1