Question

Consider the following reaction: H2 (g) + I2 (g) ⇌ 2 HI (g)

Complete the following table. Assume that all concentrations are equilibrium concentrations in M .

Find [H2] at 340 ∘C .

Express your answer using two significant figures. (Please show me the step-by-step math needed to achieve this answer, that is what I'm having trouble with, thank you!)

T(∘C)	[H2]	[I2]	[HI]	[Kc]
25	0.0355	0.0388	0.922	−
340	−	4.50×10−2 M	0.391 M	90.6
445	4.85×10−2 M	4.72×10−2 M	−	50.2

Answer 1

Let's write the reaction again, and the general equation for calculating Kc:

H₂ + I₂ <-------> 2HI
Kc = [Products]/[Reactants]
Kc = [HI]² / [H₂][I₂]

Now, we elevate by 2 the concentration of HI because there is a 2 that balances the compound in the reaction. That's why we need to take account that contribution.

For 25 °C, it only miss the Kc so:
Kc = (0.922)² / (0.0388) * (0.0355)
Kc = 617.17

For 340 °C, we miss the [H2], so, from the above expression we solve for [H2]:
[H₂] = [HI]² / Kc[I₂]
[H₂] = (0.391)² / 90.6 * 0.045
[H₂] = 0.0375 M

Finally for 445 °C, [HI] is missing, so solving for this:
[HI] = (Kc[H₂][I₂])^1/2
[HI] = (50.2 * 0.0472 * 0.0485)^1/2
[HI] = 0.3389 M

Hope this helps