Homework Help: chemistry, plz check work

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.

Answer this Question

Related Questions

AP CHEM - I would really appreciate some help on this problem. =) At 218 C, Kc...
CHEM HELP! - NH4HS<-->NH3+H2S What is the minimum mass of NH4HS ...
Chemistry - NH4HS(s) NH3(g) + H2S(g) ∆H_ = +93 kilojoules The ...
chem - Consider the decomposition of ammonium hydrogen sulfide: NH4Hs(s) &lt...
chemistry - Ammonium bisulfide, NH2HS, forms ammonia NH3, and hydrogen sulfide, ...
Chemistry - NH4HS(s )<=>NH3(g)+ H2S(g) ∆H = +93 ...
Chemistry - For the following reaction, the equilibrium constant KC = 97.0 at ...
Chemistry - For the following reaction, the equilibrium constant KC = 97.0 at ...
Chemistry - For the following reaction, the equilibrium constant Kc = 97.0 at ...
chemistry - At 23°C, Kp = 0.075 for the equilibrium: NH4HS (s) ↔ ...

For Further Reading

First Name:
School Subject:
Answer: