[Gaz2.2]

Hi,

If anybody could help me with these problems it would be great. On a sample paper we were given theres 2 questions called Equilibrium Constants? I haven't any idea how to do them. Help much much appreciated as i only have a days study left!!!

Thanks a lot!
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In an experiement to study the formation of HI
H2 + I2 ----> 2HI
          <----

H2 and I2 were placed in a sealed container at a certain temperature. At equilibrium, [H2] =0.650 M, [I2] =0.106 M, [HI] =0.187 M. Calculate the Kc for this reaction.
_______________________________________________________________________

Hydrogen fluoride, HF, can be made from the reaction

H2 + F2   ---->    2HF
             <----

In one experiement, 0.05 mol of H2 and 0.025 mol of F2 are introduced into a 0.25 L container. At equilibrium there is 0.005 M of F2 present. Calculate the Kc for this reaction

[DevaDevil]

Read this site and you can calculate them without any problems

[english]

First you must remember that molar amounts (mol) are useless.

Convert all amounts to molar concentrations (mol/L).

[SBinzel]

Remeber that K_c (aka K_eq) is equal to [products] / [reactants] where [y] is concentration of subtance y.

So, for the first question it's simply [HI]/ {[H2]*[I2]}

For the second one, the problem (I think) is impossible unless you have more information, or you mislabeled something in your rendering of the problem.

Basically, you have to do an Initial, Change, Equilibrum (ICE) setup. Start by finding the concentrations of each of the reagents and products by dividing mols/vol (L) to get M. Recall that the ratio of reactant to reactant = 1:1, so when [F2] decreases by some amount to reach 0.005, [H2] will decrease by that same amount

       H2 +            F2 <--->        2HF
I      .05/.25= .2   .025/.25= .1     0
C     -0.095          -0.095             +X
E     1.05                  0.005         X

Now you have to have a flash back to algebra. Set it up like you did for the last one, [prodcuts]/[reactants] except for [products] use a a variable. And you'd set it equal to Kc, which you don't have. And you can't find Kc because you don't know the final value of [2HF], for which you need Kc. What is the exact wording and units of the problem?

[Gaz2.2]

Hi, thanks for the reply. Em that is the exact wording, i thought exactly what you are thinking regarding the question needing more information. But it is striaght from text.

[Yggdrasil]

Using SBinzel's method you have enough information to solve the problem:

       H2 +            F2 <--->        2HF
I      .05/.25= .2   .025/.25= .1     0
C     -0.095          -0.095             +X
E     1.05                  0.005         X

From the coefficients in your balanced chemical reaction, you know that for every mole of F₂ used up, you will create 2 moles of HF.  Therefore, you can calculate the concentration of HF at equilibrium and use that value to calculate K_c.