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

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Ans: (2) 40

Sol. Assuming dilute solution

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

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

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

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