Posted by **Anonymous** on Monday, March 2, 2009 at 1:26am.

NH3+H2S-->NH4HS

<--

Kc=400 at 35 Celsius

what mass of NH4HS will be present at equilbrium?

My work

NH3 H2S NH4HS

initial 2 mol 2 mol 2 mol

change -x -x +x

equilibrium 2-x 2-x 2+x

Kc=[NH4HS]/[NH3][H2S]

400=(2+x)/[(2-x)(2-x)]

400=(2+x)/(4-4x+x^2)

1600-1600x+400x^2=2+x

1598-1599x+400x^2=0

x=0.500 mol

2-x=1.5

2+x=2.5

What is the mass of NH4HS at equilibrium?

2.50 mol NH4HS(52.0205g/mol)=130 grams of NH4HS

Now, the given Kc=400 but when I solved for Kc by hand by using numbers I got 1.11

Kc=[NH4HS]/[NH3][H2S]

=2.5/[(1.50(1.5)]

=1.111

So is answer for mass correct even if I keep getting a different Kc.

How do I solve for press of H2S at equilibrium.

Can i use the equation Kp=Kc(RT)^n (n is moles=1-2=-1)

Kp gives me the pressure, right?

- chemistry, problem. continued -
**Anonymous**, Monday, March 2, 2009 at 1:29am
NH3 H2S NH4HS

initial 2 mol 2 mol 2 mol

change -x -x +x

equilibrium 2-x 2-x 2+x

I will write this more clearly:

NH3

initial:2 mol

change: -x

equil: 2-x

H2S

initial: 2 mol

change:-x

equil:2-x

NH4HS

initial: 2 mol

change:+x

equil:2+x

- chemistry, plz check work -
**lalala**, Monday, March 2, 2009 at 10:06am
by any chance, are you taking chem 125 at calpoly? because my take-home quiz has the same exact question on it. anyways, the problem is that you need to use the molarity and not the moles. instead of having 2 mol for your initial concentrations, it needs to be 0.4M (2.00 mol/5.00 L) >>this is assuming that the volume of the container is 5.00 L. you should get the right Kc value if you replace 2 with 0.4. i hope this helps.

- chemistry, plz check work -
**DrBob222**, Monday, March 2, 2009 at 2:38pm
Another note here.

You made a math error two places. First the equation should be

400X^2 -1601X + 1598 = 0 (which won't make a large error at all).

And when you solve for X it should be 1.90 and not 0.5. All of that assumes, of course, that the volume is 1 L and from another post that appears not to be the case. In answer to your question, yes, the Kc should be 400 if you substitute the solution to the problem.

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