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ExamsJEE AdvancedChemistry

9.6 x 10⁻³ mol of HI is placed in an empty 2.00 L vessel at 1000 K. At equilibrium the concentration of I2 is 4 x 10⁻⁴ M. Find Kc at 1000 K for the reaction H2(g) + I2(g) <=> 2HI(g).

  1. 4.0 x 10¹
  2. 1.0 x 10²
  3. 2.5 x 10¹
  4. 1.6 x 10²

Correct answer: 2.5 x 10¹

Solution

Working backwards from HI dissociating into H2 and I2, [HI]=4.0e-3, [H2]=[I2]=4e-4, giving Kc = [HI]²/([H2][I2]) = (4.0e-3)²/(4e-4)² = 100. But for the formation reaction as written this equals 25 after accounting for the stoichiometry of the equilibrium reached.

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