Question

Two elements A and B form compounds having formula AB₂ and AB₄. When dissolved in 20 g of benzene (C₆H₆), 1 g of AB₂ lowers the freezing point by 2.3 K whereas 1.0 g of AB₄ lowers it by 1.3 K. Tin molar depression – constant for benzene is 5.1 K kg mol^-1. Calculate atomic masses of A and B.

Answer 1

Suppose atomic masses of A and B are ‘a’ and ‘b’ respectively. Then 

Molar mass of AB₂ = a + 2b = 110.87 g mol^-1 

Molar mass of AB₄ = a + 4b =196.15 g mol^-1 

Equation (ii) - Equation (i) gives 2b = 85.28 or b = 42.64 

substituting in equation (i) we get 

a + 2 × 42.64 = 110.87 or a = 25.59 

Thus atomic mass A = 25.59 u 

Atomic mass of B = 4.64 u.