At elevated temperatures, hydrogen iodide may decompose to form hydrogen gas and iodine gas, as follows: 2HI(g) ⇌ H2 (g) + I2 (g)

In a particular experiment, the concentrations at equilibrium were measured to be [HI] = 0.85 mol/L, [I2] = 0.60 mol/L, and [H2] = 0.27 mol/L. What is Kc for the above reaction?

Explanation:

Since Kc is

You can insert the Hydrogen and Iodine gas on top, and Hydrogen Iodide in the denominator.

Note: you can only include gases and aqueous species in an equilibrium expression, and all the species in this reaction are gaseous so you're good.

Inserting their molarity at equilibrium into their places, and you can solve. Don't forget to make the coefficient of HI turn into a power.

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explanation: