JEE Main 2023 — Chemistry Physical Chemistry

2023integermedium

$80$ mole percent of ${\mathrm{MgCl}}_{2}$ is dissociated in aqueous solution. The vapour pressure of $1.0$ molal aqueous solution of ${\mathrm{MgCl}}_{2}$ at ${38}^{o}C$ is _____ $\mathrm{mm}\mathrm{Hg}$ . (Nearest integer)

Given: Vapour pressure of water at ${38}^{o}C$ is $50\mathrm{mm}\mathrm{Hg}$

Official previous-year question

Held on 12 Apr 2023 · Verified 6 Jul 2026.

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Solution

One molal solution means one of solute present in one kg of solvent.

${\mathrm{MgCl}}_{2}\overset{}{\rightleftharpoons }{\mathrm{Mg}}^{2+}+2{\mathrm{Cl}}^{-} 100 1-0.80.81.6$

Hence, overall moles after dissociation $=2.6$

The relative lowering of vapour pressure

$\frac{{p}^{o}-{p}_{s}}{{p}^{o}}=\frac{\mathrm{moles}\mathrm{of}\mathrm{solute}}{\mathrm{moles}\mathrm{of}\mathrm{solute}+\mathrm{moles}\mathrm{of}\mathrm{solvent}}$

$\frac{p^{\circ}-{p}_{s}}{p^{\circ}}=\frac{2.6}{\frac{1000}{18}+2.6}$

For dilute solution

$\frac{p^{\circ}-{p}_{s}}{p^{\circ}}=\frac{2.6}{\frac{1000}{18}}$

$p=47.66\simeq 48\mathrm{mm}\mathrm{Hg}$

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JEE Main 2023 — Chemistry Physical Chemistry

2023integermedium

Given: Vapour pressure of water at ${38}^{o}C$ is $50\mathrm{mm}\mathrm{Hg}$

Official previous-year question

Held on 12 Apr 2023 · Verified 6 Jul 2026.

Did you get this right?

Solution

One molal solution means one of solute present in one kg of solvent.

${\mathrm{MgCl}}_{2}\overset{}{\rightleftharpoons }{\mathrm{Mg}}^{2+}+2{\mathrm{Cl}}^{-} 100 1-0.80.81.6$

Hence, overall moles after dissociation $=2.6$

The relative lowering of vapour pressure

$\frac{{p}^{o}-{p}_{s}}{{p}^{o}}=\frac{\mathrm{moles}\mathrm{of}\mathrm{solute}}{\mathrm{moles}\mathrm{of}\mathrm{solute}+\mathrm{moles}\mathrm{of}\mathrm{solvent}}$

$\frac{p^{\circ}-{p}_{s}}{p^{\circ}}=\frac{2.6}{\frac{1000}{18}+2.6}$

For dilute solution

$\frac{p^{\circ}-{p}_{s}}{p^{\circ}}=\frac{2.6}{\frac{1000}{18}}$

$p=47.66\simeq 48\mathrm{mm}\mathrm{Hg}$

Your note

JEE Main Physical Chemistry in other years

Keep practicing Physical Chemistry

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JEE Main 2023 — Chemistry Physical Chemistry

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Solution

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JEE Main Physical Chemistry in other years

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JEE Main 2023 — Chemistry Physical Chemistry

Did you get this right?

Solution

Your note

JEE Main Physical Chemistry in other years

More JEE Main Physical Chemistry questions

Other Chemistry topics for JEE Main

Organic Chemistry

Inorganic Chemistry

Keep practicing Physical Chemistry