Chemical Equilibrium of Hydrogen Iodide

Problem

This problem is on the dissociation of hydroiodic acid (more commonly known as hydrogen iodide) into hydrogen gas and iodine gas.

$2HI\rightleftharpoons H_{2}+I_{2}$

If 1.0 mol of hydroiodic acid 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

$K={\frac {\left[H_{2}\right]\left[I_{2}\right]}{{\left[HI\right]}^{2}}}$

$K={\frac {(0.125)(0.125)}{{(0.250)}^{2}}}$

$K=0.0625$