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GATE 2019 MT – Question 15

Metallurgical Thermodynamics · Solutions and chemical potential · 1 mark · Multiple choice

Liquid steel is held in a graphite crucible long enough to establish equilibrium. What is carbon activity in the liquid, taking graphite as standard state?

  1. 0.0
  2. 0.5
  3. 1.0
  4. 2.0

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Correct answer: (C) 1.0

Explanation

Equilibrium with pure graphite equates the chemical potential of dissolved carbon to that of graphite. Since $\mu_C=\mu_C^\circ+RT\ln a_C$ and the standard is graphite, ln aC=0. Hence aC=1, C, even though carbon concentration is not 100%.

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