Question:

Liquid 'M' and liquid 'N' form an ideal solution. The vapour pressures of pure liquids 'M' and 'N' are 450 and 700 mmHg, respectively, at the same temperature. Then correct statement is:
(\(x_M\) = Mole fraction of 'M' in solution ;
\(x_N\) = Mole fraction of 'N' in solution ;
\(y_M\) = Mole fraction of 'M' in vapour phase ;
\(y_N\) = Mole fraction of 'N' in vapour phase)

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Apply Raoult's law and Dalton's law: \(y_M/y_N=\frac{x_MP_M^\circ}{x_NP_N^\circ}\).
Updated On: Nov 5, 2026
  • \(\frac{x_M}{x_N}\lt\frac{y_N}{y_N}\)
  • \((x_M-y_M)\lt(x_N-y_N)\)
  • \(\frac{x_M}{x_N}=\frac{y_N}{y_N}\)
  • \(\frac{x_M}{x_N}\gt\frac{y_M}{y_N}\)
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The Correct Option is D

Solution and Explanation

Partial pressures: \(p_M=x_MP_M^\circ=450x_M\) and \(p_N=x_NP_N^\circ=700x_N\). In the vapour, \(\frac{y_M}{y_N}=\frac{p_M}{p_N}=\frac{x_M}{x_N}\times\frac{450}{700}=\frac{9}{14}\frac{x_M}{x_N}\).
Hence \(\frac{x_M}{x_N}\gt\frac{y_M}{y_N}\), which is option (4).
Option (2): since \(x_M+x_N=y_M+y_N=1\), \(x_M-y_M=-(x_N-y_N)\); N is more volatile so \(y_N\gt x_N\), making \(x_N-y_N\lt0\lt x_M-y_M\); statement (2) is false. Options (1) and (3) compare with \(y_N/y_N=1\) (printed typo) and are not generally true.
Answer: (4) \(\frac{x_M}{x_N}\gt\frac{y_M}{y_N}\)
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