An initial mixture of nitrogen gas and hydrogen gas is reacted in a rigid container at a certain temperature as follows: 3 H2 (g) + N2 (g) equilibrium reaction arrow 2 NH3 (g). At equilibrium, the concentrations are [H2] = 5.6 M, [N2] = 9.3 M, and [NH3] = 5.1 M. What were the concentrations of nitrogen gas and hydrogen gas that were reacted initially?

[H₂] = 13 M

[N₂] = 12 M

Explanation:

Let's consider the following reaction at equilibrium.

3 H₂ (g) + N₂ (g) ⇄ 2 NH₃ (g)

To find out the initial concentrations we will use an ICE chart. We recognize 3 stages: Initial, Change and Equilibrium and complete each row with the concentration or change in concentration. We will use letters for the unknown data.

3 H₂ (g) + N₂ (g) ⇄ 2 NH₃ (g)

I a b 0

C - 3x - x + 2x

E a - 3x b - x 2x

We know that,

[NH₃] = 2x = 5.1 M ⇒ x = 2.6 M

Then,

[H₂] = 5.6 M = a - 3x ⇒ a = 13 M

[N₂] = 9.3 M = b - x ⇒ b = 12 M