Question

3.795 g of sulphur is dissolved in 100 g of CS₂. This solution boils at 319.81 K. What is molecular formula of sulphur in solution? The boiling point of CS₂ is 319.45 K. 

(Given that Kb for CS₂ = 2.42 K kg mol^–1 and atomic mass of S = 32.)

Answer 1

Given: 

Mass of sulphur = W₂ = 3.795 g = 3.795 x 10^-3 kg 

Mass of solvent = W₁ = 100 g = 100 x 10^-3 kg 

Boiling point of solution = T_b = 319.81 K 

Boiling point of pure solvent = \(T^°_b\) = 319.45 K 

Molal elevation constant = K_b = 2.42 K kg mol^-1 

Atomic mass of sulphur = 32 

To find: Molecular formula of sulphur in solution 

Formula: Molar mass of solute = M₂ = \(\frac{K_b\times W_2}{ΔT_b\times W_1}\)

Calculation: For solution of sulphur in CS₂ (solvent),

∴ Molecular formula of sulphur in CS₂ = S₈