• # question_answer 12 g of a non-volatile solute dissolved in 108 g of water produces the relative lowering of vapour pressure of 0.1. The molecular mass of the solute is A)  80                B)  60         C)  20                 D)  40

Correct Answer: C

Solution :

From Raoults law, relative lowering in vapour pressure $\Delta p=\frac{{{p}^{o}}-p}{{{p}^{o}}}=\frac{n}{N}=\frac{W}{m}\times \frac{M}{W}$ Here, $w=12g;W=108g,.m=?$ $M=18g,\Delta p=0.1$ $\Delta p=\frac{W}{m}\times \frac{M}{W}$ or $0.1=\frac{12}{m}\times \frac{18}{108}$ $m=\frac{12\times 18}{10.8}=20$

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