Question

If $CuSO_{4}.5H_{2}O_{(s)} \rightleftharpoons CuSO_{4} .3H_{2}O_{(s)} + 2H_{2}O_{(l)} Kp = 1.086 \times 10^{-4} atm^{2}$ at $25°C.$ The efflorescent nature of $CuSO_{4} .5H_{2}O$ can be noticed when vapour pressure of $H_{2}O$ in atmosphere is

• A. $>7.29$ mm
• B. $<7.92$ mm
• C. $\ge 7.92$ mm
• D. None

Answer 1

Answer: (B)

An efflorescent salt is one that loses $H_{2}O$ to atmosphere. For the reaction
$CuSO_{4}.5H_{2}O (s) \rightleftharpoons CuSO_{4}.3H_{2}O(s) + 2H_{2}O_{(I)}$
$K_{P} = (P_{H_{2}O})^{2} = 1.086 \times 10^{-4}$
$P' _{H_{2}O} = 1.042 \times 10^{-2} $ atm $= 7.92$ mm
$\because If P' _{H_{2}O}$ at $25°C < 7.92$ mm only, then reaction will proceed in forward direction.