If $\begin{aligned}{\mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{s})} ightleftharpoons}{ } &…

If $\begin{aligned}{\mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{s})} ightleftharpoons}{ } & \mathrm{CuSO}_{4} .3 \mathrm{H}_{2} \mathrm{O}_{(s)}+2 \mathrm{H}_{2} \mathrm{O}_{(1)} \end{aligned}$
$\mathrm{K}_{\mathrm{p}}=1.086 \times 10^{-4} \mathrm{~atm}^{2}$ at $25^{\circ} \mathrm{C}$. The efflorescent nature of $\mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O}$ can be noticed when vapour pressure of $\mathrm{H}_{2} \mathrm{O}$ in atmosphere is
  1. $>7.29 \mathrm{~mm}$
  2. $ < 7.92 \mathrm{~mm}$
  3. $\geq 7.92 \mathrm{~mm}$
  4. None

Solution

An efflorescent salt is one that loses $\mathrm{H}_{2} \mathrm{O}$ to the atmosphere.
For the reaction
$\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{s})}+\mathrm{CuSO}_{4} \cdot 3 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{s})}+2 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{i})}$
$\mathrm{K}_{\mathrm{p}}=\left(\mathrm{p}_{\mathrm{H}_{2} \mathrm{O}}^{\prime}ight)^{2}=1.086 \times 10^{-4}$
$\mathrm{p}_{\mathrm{H}_{2} \mathrm{O}}^{\prime}=1.042 \times 10^{-2} \mathrm{~atm}=7.92 \mathrm{~mm}$
$\mathrm{Q}$ If $\mathrm{p}_{\mathrm{H}_{2} \mathrm{O}}^{\prime}$ at $25^{\circ} \mathrm{C} < 7.92 \mathrm{~mm}$ only then, reaction will proceed in forward direction, .

Asked in: JEE-TOPICTESTS-CHEMISTRY

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