Question

If 8 g of a non-electrolyte solute is dissolved in 114 g of n-octane to reduce its vapour pressure to 80%, the molar mass (in g mol^–1) of the solute is [Given that molar mass of n-octane is 114 g mol^–1] 

(1) 20 

(2) 40 

(3) 60 

(4) 80

Answer 1

Ans: (2) 40

Sol. Assuming dilute solution

${\Delta P\over P_A^0}={n_B\over n_A}={w_B\over m_B}.{m_A\over w_A}$

${20\over100}={8\over m_B}.{114\over114}$

$m_B={8\times100\over20}=40\ \text{gmol}^{-1}$