Suppose the elements X and Y combine to form two compounds XY₂ and X₃Y₂. When 0.1 mole of XY₂ weighs 10 g and 0.05 mole of X₃Y₂ weighs 9 g, the atomic weights of X and Y are

Let, Atomic mass of 'X'=A ;  Atomic mass of 'Y'=B

0.1 mole of XY₂ weighs 10 g and 0.05 mole of X₃Y₂ weighs 9 g

Mass of 1mole of XY₂=$\frac{1}{0.1}\times 10=100\mathrm{g}$

Mass of 1mole of X₃Y₂ = $\frac{1}{0.05}\times 9=180$

Molecular mass of XY₂ = A+2B=100 ---(1)

Molecular mass of X₃Y₂ = 3A+2B=180 ---(2)

(2)-(1)$\Rightarrow$A=40 and B=30.