Question

Gaseous `HI` is synthesized from `H_2 (g)` and `I_2 (g)` at a temperature where `K_c = 1.00 xx 10^2`. Suppose that `HI` at `5.000 xx 10^(-1) atm`, `H_2` at `1.000 xx 10^(-2) atm`, and `I_2` at `5.000 xx 10^(-3) atm` are mixed in a `5`-`L` flask?

Calculate the equilibrium pressures of all species.

I'm kind of confused because I have a different answer than the book, but my answer yields the correct `K`.

Answer 1

Consider,

`H_2(g) + I_2(g) rightleftharpoons 2HI(g)` where `K_c = ([HI]^2)/([H_2][I_2]) = 100`

Under the given conditions, `Q approx 5000`, which generally means the equilibrium will shift left.

Hence, using the `"ICE"` table,

`K_c = ((0.5 - 2x)^2)/((0.01+x)(0.005+x)) = 100`

`= x approx 0.0355"atm"`

Therefore,

`[H_2] = 0.0455"atm"`, `[I_2] = 0.0405"atm"`, and `[HI] = 0.429"atm"`

Note: the flask size is relatively insignificant here. Some mean professor probably put it there to confuse students: it's generally factored in if we can derive how many moles of gas there are.