Question

$\Delta_{r} G^{\circ}$ at $500 \,K$ for substance 'S' in liquid state and gascous state are $+100.7 \,kcal\, mol ^{-1}$ and $+103 \,kcal \,mol ^{-1}$, respectively. Vapour pressure of liquid ' S ' at $500\, K$ is approximately equal to $\left(R=2 \,cal K ^{-1} \,mol ^{-1}\right)$

• A. 0.1 atm
• B. 1 atm
• C. 10 atm
• D. 100 atm

Answer 1

Answer: (A)

0.1 atm

Answer 2

We have $S {\text{(liquid)}} \rightleftharpoons S {\text { (gas) }}$ $\Delta G_{\text {reaction }}^{\circ} =\Delta_{r} G^{\circ}(\text { vapor })-\Delta_{f} G^{\circ} \text { (liquid) }$ $=103-100.7$ $=2.3 \,kcal \,mol ^{-1}$ We know $\Delta G_{\text {Extion }}^{\circ}=-R T \ln K$ $2.3 \times 10^{3}=-2 \times 500 \times \ln K_{p}$ $ln \,K_{p}=-2.3$ $\Rightarrow K_{p}=0.1$ $\Rightarrow \text { vapour pressure }=0.1\, atm$