SolutionHard

Question

Two elements A and B form compounds having molecular formula AB2 and AB4. When dissolved in 20 g of C6H6, 1 g of AB2 lowers the freezing point by 2.55 K, whereas 1.0 g of AB4 lowers it by 1.7 K. The molar depression constant for benzene is 5.1 K-kg mol−1. The atomic masses of A and B are

Options

A.50, 25
B.50, 50
C.25, 50
D.75, 25

Solution

$\Delta T_{f} = K_{f}.m$

For AB2 solution: 2.55 = $5.1 \times \frac{1/(x + 2y)}{20/1000}(1)$

For AB4 solution: 1.7 = $5.1 \times \frac{1/(x + 4y)}{20/1000}(2)$

∴ Atomic mass of A = x = 50

Atomic mass of B = y = 25

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