GATE 2022 MT – Question 53
The partial molar enthalpy of Au in an Ag–Au melt containing 25 mol% Au at 1400 K is −8300 J/mol. Assuming regular-solution behaviour, the activity of Au is ______ (three decimal places). Given $R=8.314$ J K$^{-1}$ mol$^{-1}$.
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Correct answer: 0.121 to 0.125
Explanation
For a regular solution, partial excess Gibbs energy equals partial mixing enthalpy, so $RT\ln\gamma_{Au}=-8300$ J/mol. Activity is $a_{Au}=x_{Au}\gamma_{Au}=0.25\exp[-8300/(8.314\times1400)]=0.12254$. Answer: **0.123**.