Chemical Equilibrium: Calculating K with Ideal Gas Law

[Anna_Kwong]

Homework Statement

It is a chem equilibrium question, involving the ICE table.

The following reaction:
S2(g) + 2H2 (g) ←→ 2SH2(g)
takes place at high temperatures. If the initial amount of H2 and SH2 is 1.75 g and 1.95 g, respectively, and at equilibrium the amount of S2(g) is found to be 0.00188 atm, what is the value of the equilibrium constant? Please give your answer to three significant figures. Given: T = 1670 K and Reaction Volume = 0.500 L. (Hint: you will need to use the ideal gas law).

Homework Equations

pv=nRT, ICE table

The Attempt at a Solution

What I did was find the the mols of H2 and SH2 since we are given n=m/M. So I have those 2 for "I" initial.
Then I found the equilibrium mols of S2, by plugging into the formula pv=nRt, solving for n=pv/Rt and I get 2.00E-5.

Essentially, I know K = [products]/[reactants]. But I only know the "I" of H2 and SH2, and I only know the "E" of S2.

Your help is greatly appreciated!

 

[Lchan1]

K = P(product)/ P(reactant) when everything is in gas state.

 

[Anna_Kwong]

how do I figure out the "C" in change? or is that even necessary..

 

[Charlie_Brown]

remember when dealing with gases, use partial pressure. when dealing with solutions, use concentration.

 

[Anna_Kwong]

ok so, kp= pSH2^2/ pH2^2 x pS2 , i know the value of pS2, which is 0.00188atm, how do I find the other 2 values needed to solve for the kp value? Thanks

 

[Anna_Kwong]

My kp value is 0.580, can anyone confirm this please and thanks

 

[Borek]

Use stoichiometry of the reaction.

Kc vs Kp - it depends on what the question asks. When dealing with gases it is more likely that the answer expected is Kp, but it is also perfectly possible to calculate Kc and it will be a valid answer as well. Whether it will be accepted by your teacher... that's another story.

Seems to me like your Kp value is much too low. Show details of your work.

 

[Anna_Kwong]

H2 :1.75g / 2.02g/mol = 0.866mol , use pv=nRT , p = nRT/v = (0.866mol)(0.082)(1670)/(0.5) = 237atm
SH2: 1.95g/34.06g/mol = 0.0572mol, use pv=nRT, p = nRT/v = (0.0572mol)(0.082)(1670)/(0.5)=15.7 atm
S2: 0.00188atm

kp= pSH2^2/pS2 x pH2^2 = (15.7)^2/((237)^2 x (0.00188)) = 2.33, eekk guess my calculations for the first run was incorrect, however, is the work done correctly?

 

[Borek]

Results looks much better now, although you have not accounted for the fact that your final mixture doesn't contain 1.75 g H₂ nor 1.95 g of H₂S - the first was produced together with S₂, the latter was decomposed. You should use reaction stoichiometry for that.

Not that it will change much, but you should check it to be sure.