Equilibrium problems

Homework Statement

(1) For the given system at equilibrium, NH4HS(s) <-----> NH3(g) + H2S(g), the K value is 1.6x10^-4 at 24oC, when pressures are in atmospheres.
What is the minimum mass of solid NH4HS(s) that can be introduced into a 5.0 L evacuated container so that equilibrium will result?

(2) Carbon Tetrachloride (CCl4) was once commonly used as a fire extinguisher. However, at high temperatures, CCl4 reacts with oxygen to form two deadly gases, phosgene and Cl2, according to the following chemical equilibrium.

2CCl4(g) +O2(g) <----> 2COCl2(g) + Cl2(g)

at 500oC, 0.562 mol of O2 and an unknown amount of CCL4 was introduced into a 1.00 L reaction vessel. Once the system had reached equilibrium, there was 0.263 moles of Cl2 present. Calculate the initial amount of CCL4 that was admitted into the vessel. K for this reaction at 500oC is 2.00 mol/l

The Attempt at a Solution

1) My textbook says as long as some solid is present the amount is a constant and it will not effect the concentration of ions in solution so I believe the answer would be any amount greater then zero. It is a six mark question though so I believe that is not the correct answer.

2) not to sure on this one

K=2.00Mole/L=([COCl2]^2 [Cl2]^2)/([CCl4]^2[O2])

sqrt{2.00[0.562]/[0.263]^2}= [COCl2]/[CCl4]

so this would give me the ratio of the concentrations of COCl2 over CCl4 but they should be equal to one because of the stoichiometry shouldn't they?

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1) I took another shot at this one

K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]

[NH3]/[H2S]= 0.012649 moles/L

mass NH4HS= 0.012649moles NH3/H2S /L x 5.0L x 1mole NH4HS/ 1mole NH3/H2S x 51.11249 g/ mole NH4HS = 3.2g

Borek
Mentor
1) My textbook says as long as some solid is present the amount is a constant
Amount? Or activity?

Borek
Mentor
K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]
What is X?

Seems to me that you are doing tricks that have nothing to do with correct algebra :grumpy:

What is X?

Seems to me that you are doing tricks that have nothing to do with correct algebra :grumpy:
I used x because I thought that because of the stoichiometry NH3=H2S so it just makes the calculation simpler.

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it says the concentration is a constant. " So, as long as at least some solid is present, how much solid is present is not relevant to the concentration of ions in solution."

Borek
Mentor
I used x because I thought that because of the stoichiometry NH3=H2S so it just makes the calculation simpler.
You have still not answered my question, I suppose you assume I will guess that x=NH3=H2S. What if I am too dumb?

K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]
This is wrong. You have taken square root only of some parts of the equation. Besides, you have already named [NH3] and [H2S] x, why do you mix now x with [NH3] and [H2S]?

"So, as long as at least some solid is present, how much solid is present is not relevant to the concentration of ions in solution."
It doesn't mean amount is constant, it means amount doesn't matter. That's not the same.

You have still not answered my question, I suppose you assume I will guess that x=NH3=H2S. What if I am too dumb?
Yes I was assuming that. I should of been more clear

This is wrong. You have taken square root only of some parts of the equation. Besides, you have already named [NH3] and [H2S] x, why do you mix now x with [NH3] and [H2S]?
the square root of K = [NH3] = [H2S] = x . I have used NH3/H2S/x to indicate NH3 or H2S or x again I apologize I should have been more clear.

It doesn't mean amount is constant, it means amount doesn't matter. That's not the same.
I misunderstood apparently I need to be a bit more careful in my reading and writing

Borek
Mentor
Now, that it is all cleared, we can try to solve the question

Hint: for the equilibrium to exist you need a solid. If the amount of solid is too small, it will all decompose.There will be no solid left, there will be no equilibrium between solid and gases.

So there must be an appreciable amount of solid.

Now that I have really started to think about this one I realize that this has nothing to do with concentration of ions in solution. this is about a solid decomposing into gas. So the equilibrium constant K is not concentration but pressure right? " the K value is 1.6x10^-4 at 24oC, when pressures are in atmospheres." That statement indicates that If the unit of pressure were to change the K value would also change. And since it is a equilibrium constant which equals [NH3][H2S] the K value must be the product of the the partial pressures of [NH3][H2S]. am I on the right track?

Borek
Mentor
Seems OK so far.