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Problem

Hydrogen-iodide

This problem is on the dissociation of hydrogen iodide gas into hydrogen gas and iodine gas.

$ 2HI \rightleftharpoons H_2 + I_2 $

If 1.0 mol of hydrogen iodide dissociates in a 1.0 L flask produces 0.125 moles of iodine gas, determine the equilibrium constant.

Solution

Construct an ICE table (initial, change, equilibrium).

$ 2HI \rightleftharpoons H_2 + I_2 $
HI H2 I2
I 1.0 0.0 0.0
C -2(0.125) +0.125 +0.125
E 0.250 0.125 0.125

The equilibrium constant is thus

$ K = \frac{\left[H_2 \right] \left[I_2 \right]}{{\left[HI \right]}^{2}} $
$ K = \frac{(0.125)(0.125)}{{(0.250)}^{2}} $
$ K = 0.0625 $
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