[Malgrif]

I suspect the problem having problems but what do i know 

Consider the system

CO2(g) + H2(g)    CO(g) + H2O(g)

Initially, 0.25 mol of water vapour and 0.2 mol of carbon monoxide are placed in a 1.00 L reaction vessel. At equilibrium, spectroscopic evidence shows that 0.1 mol of carbon dioxide is present. Calculate K for the reaction.

So writing down the equilibrium law we get.
Kc = [CO][H2O]/[CO2][H2]

Now we need to find the unknowns. We know 3/4 of the reaction's concentrations but how do you find the forth? An ice table wont work since we don't have a Kc value and without a given H2 value we can't calculate Kc. Stoichometry sounds useless since the system is in equilibrium so what the heck. How do you solve this problem?

[enahs]

Stoichometry sounds useless since the system is in equilibrium so what the heck. How do you solve this problem?

I gotta disagree, it sounds like a winner here.

So, if you start with Carbon monoxide and water vapor, are you going to make just carbon dioxide, or will you make a mixture of carbon dioxide and hydrogen gas? What are the stochimetric proportions? Hint, the answer two both of these questions are in your chemical equation.

[Malgrif]

The system is in equilibrium and I believe stoichiometry is only usable in a complete chemical reaction. Since at equilibrium the moles of the reactants and products will change depending on temperature, pressure and the Kc value stoichiometry seems unusable(point out if i'm missing the obvious please ^^). Even if we know that 0.1moles of CO2 are present at equilibrium I don't see how you can use that to find the values of everything else.

[enahs]

You can not produce a chemical from nothing. It is produced from other chemicals. You can not also just produce "one" chemical if your system is a system that produces multiple chemicals.

If you start with pure CO and H₂O, and you make some CO₂, you must make some H₂. The stochimetry dictates how much is produced. Even at equilibrium, you still have to have a mass balance.

In this case, try and not think about the equilibrium as the concentrations of the individual species, but how skewed the reaction in the forward direction (as written).

[Malgrif]

Solving this problem by using stoichiometry would mean that one of the "products"'s concentration would be 0. Since stoichiometry is comparing mole ratios you would need to find the limiting reagent, which is the reagent that fully reacts leaving the [] at zero. That wouldn't make sense in a system that is in equilibrium. Essentially Kc=0 if solved this way.

The answer is Kc=1.7 in the back of the book. How they got this value I have no clue.

[enahs]

No.
If I have 1 mol of CO and 1 mol of H2O, and I make 0.5mol of CO2, I had to also make some H2!!! Stoichiometry tells me how many mols of H2 I had to make!

[Malgrif]

Maybe my stoichiometry skills are a bit rusty but... If what you're saying is correct the final concentration(which is the one we need to figure out Kc) of the limiting reagent(because we need that to solve for h2) would be 0. If we plug that into the equilibrium law it equals zero. What kind of stochiometric method am i suppose to use to solve this?

[sjb]

Could it be easier to think of things in two parts? One a completed reaction, and one a non-started one?

If you initially had 0.2 mol of CO, and you have formed 0.1 mol CO₂, by the balanced equation how much CO did you use up to make the carbon dioxide?

Similarly, by the balanced equation, how much water did you need to make this amount of carbon dioxide, and so how much is left?

If you used up this amount of water and carbon monoxide, how much H₂ did you make?

So what is your value for Kc

[Malgrif]

ok thanks all i got it. the real answer is 0.667 the text book is wrong. However I still don't know how you would solve this using stoichiometry>_>... this is the method i used.

                   CO2 + H2 --> CO + H2O
I                    0           0         0.2      0.25
C                  +x        +x         -x         -x
E                  0.1           x       0.2-x    0.25-x

x = 0.1 (from above)
[CO2] = 0.1
[H2] = 0.1
[CO] = 0.1
[H2O] = 0.15

0.6666

[enahs]

So, if you know what x is, how are you having a hard time understanding what the concentration of H2 is?!!

Are you sure about your typing into a calculator and such?

[Malgrif]

So, if you know what x is, how are you having a hard time understanding what the concentration of H2 is?!!

Are you sure about your typing into a calculator and such?

nope guess i'm wrong answer IS 1.5 ok ok ok.^____^ thanks for the help. now help me in my other thread please !!! 

[Borek]

I still don't know how you would solve this using stoichiometry

(...)

C                  +x        +x         -x         -x
E                  0.1           x       0.2-x    0.25-x

THIS IS stoichiometry. x moles of CO reacted to yield x moles of CO₂. x moles of CO reacted with x moles of H₂). Repeat, loud and clear: this is stoichiometry, this is stoichiometry, this is stoichiometry