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A 20.0 L vessel initially holds 0.50 mol each of H2 and I2. They react and reach equilibrium for H2 + I2 <=> 2HI with K_eq = 49. What is the equilibrium concentration of HI?

1. 0.78 M
2. 0.039 M
3. 0.033 M
4. 0.021 M

Correct answer: 0.039 M

Solution

Solving the equilibrium expression gives x ~ 0.0194 M reacted, so [HI] = 2x ~ 0.039 M.

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