Finding CS2 concentration at equilibrium for CH4 + 2H2S reaction

[cscott]

Reaction: $CH_4 + 2H_2S <-> CS_2 + 4H_2$
$k_{eq} = 100$ at a certain temperature

An unkown amount of reactants were intorduced into a 1.0L bulb and at equilibrium 0.10 mol of $CH_4$ and 0.30 of $H_2S$ were found. Calc $\[CS_2\]$ at equilibrium.

I don't seem to have enough information here. I can form the equation for $k_{eq}$ but I have two unknowns...

 

[Bystander]

Lousy problem statement: the idiot who wrote it mentally equates "reactants" with materials on "left-hand side of equation." Help?

 

[Borek]

Reactants or substrates? If reactants, there is no way to solve the question. If substrates, it is easy.

Two unknowns - you mean amounts of CS2 and H2?

Oops, Bystander was faster :)

 

[PPonte]

cscott I tried to resolve this. But before I show how I think this might be done, I want to make sure that I am right. Is the amount of $\[CS_2\]$ at equilibrium 0.323 mol ?

 

[cscott]

Lousy problem statement: the idiot who wrote it mentally equates "reactants" with materials on "left-hand side of equation." Help?

That idiot would be me . Sorry.

Reactants or substrates? If reactants, there is no way to solve the question. If substrates, it is easy.

I've never heard of "substrates" before. We just barely learned that the reactions go both ways , so I guess that's why I associated reactants with the LHS.

Two unknowns - you mean amounts of CS2 and H2?

Yes.

But before I show how I think this might be done, I want to make sure that I am right.

I didn't get the answers for this practice sheet yet. Maybe you can show your method anyway?

 

[Bystander]

That idiot would be me . Sorry. (snip)

Huh? You didn't write the problem, you're trying to solve it. All I was doing was translating what was meant by "reactants" as a hint for you.

 

[cscott]

Huh? You didn't write the problem, you're trying to solve it. All I was doing was translating what was meant by "reactants" as a hint for you.

The problem originally stated the two molecules on the LHS instead of "reactants". I was just being lazy.

I'm still not sure how to get the concentration of $H_2$ at equilibrium to in turn get it for $CS_2$

 

[PPonte]

I didn't get the answers for this practice sheet yet. Maybe you can show your method anyway?

Ok. Here it goes.

First, let's identify the initial amount of the various substances, reactants and products:
CH4 -> n mol
H2S -> m mol
CS2 -> 0 mol
H2 -> 0 mol

At equilibrium according to the stoichiometric coefficients of the substances their amount should be:
CH4 -> n - x mol
H2S -> m - 2x mol
CS2 -> x mol
H2 -> 4x mol

Nevertheless, the problem gives us the amount at equilibrium of CH4 and H2S which are respectively 0.10 mol and 0.30 mol.

Then, we must apply the chemical equilibrium formula using the equilibrium constant given by the problem:

100 = x(4x)^4) / (0.10 x 0.30^2)

Now, it's math.
x will be equal to 0.323 mol

Hope I could help.

 

[cscott]

Thanks PPonte.