Q22 / 125 · BITSAT 2009 · Chemistry

The degree of dissociation of $\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}$ in dilute aq. solution containing $7.0 \mathrm{~g}$ of salt per $100 \mathrm{~g}$ of water at $100^{\circ} \mathrm{C}$ is $70 \%$. If vapour pressure of water at $100^{\circ} \mathrm{C}$ is $760 \mathrm{~mm} \mathrm{Hg}$. The vapour pressure of solution is

  • A
    735
  • B
    $\quad 730$
  • C
    760
  • Correct answer
    D
    $\quad 746$

Solution

$\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2} \rightleftharpoons \mathrm{Ca}^{2+}+2 \mathrm{NO}_{3}^{-}$ $1-0.71 \quad 0.7 \quad 2 \times 0.7$ $\quad\left(\because \alpha=\frac{70}{100}=0.7\right)$ $\quad \mathrm{n}_{1}=\frac{100}{18}=5.55$ $\frac{\mathrm{p}^{0}-\mathrm{p}_{\mathrm{s}}}{\mathrm{p}^{0}}=\frac{\mathrm{n}_{2} \times \mathrm{i}}{\mathrm{n}_{1}+\mathrm{n}_{2}}, \frac{760-\mathrm{p}_{\mathrm{s}}}{760}$ $=\frac{0.042 \times 2.4}{5.55+0.042}$ $\therefore \mathrm{p}_{\mathrm{s}}-746 \mathrm{~mm} \mathrm{Hg} .$

Hence, the correct answer is option: 'D'.

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