# How to calculate Kp without knowing the volume? [closed]

I have the following chemical equation:

$$\ce{Sb2S3 + 3H2 <=> 2 Sb + 3H2S}$$

I have $$\pu{1000 grams}$$ of $$\ce{Sb2S3}$$ that reacts with $$\pu{10 grams}$$ of $$\ce{H2}$$ in a reactor at $$\pu{713 K}$$.

I would like to calculate $$K_p$$ (equilibrium constant using partial pressures).

I know that $$K_p$$ is equal to the quotient of the partial pressure of $$\ce{H2S}$$ cubed divided by partial pressure of $$\ce{H2}$$ cubed, and that I need to use the law of perfect gases.

$$K_p = \frac{P_{\ce{H2S}}^3}{P_{\ce{H2}}^3}$$

I have calculated the amount of moles of $$\ce{H2S}$$ to be $$\pu{2.264 mol}$$ and the amount for $$\ce{H2}$$ is $$\pu{2.02 mol}$$.

However there is just one problem I don't have the volume in this exercise, since it wasn't given. Thus, I am a bit stumped.

• Use the principle of partial pressures, Volume cancels out.. Jun 15 at 4:17
• It does not seem this problem can be solved with the information given, please add information so that one can arrive at the number of moles stated here. Or, incorporate clarifications which you stated in comments into your post. Jun 16 at 0:42

So you need $$K_p = \frac{p_{\ce{H2S}}^3}{p_{\ce{H2}}^3}$$ where $$p_{\ce{H2S}}$$ and $$p_{\ce{H2}}$$ are partial pressures of $$\ce{H2S}$$ and $$\ce{H2}$$ respectively.
Now notice that in the expression of $$K_p$$ , you have cube divided by cube, so the term of total pressure will get cancelled out.
So, you will end up having $$K_p = \frac{p_{\ce{H2S}}^3}{p_{\ce{H2}}^3}=\frac{(2.264 )^3}{(2.02 )^3}=1.407$$
• @StormCaster Initially I just put the values you had given without thinking much. Now when I think about it, how are you finding these values? Using the atomic mass of Sb as 122 g and sulphur as 32 g, we get the moles of $\ce{Sb_2S_3}$ as $\dfrac{1000}{122\times 2+ 32 \times 3}=2.941$ and moles of $\ce{H_2}$ as 10/2 =5. Now how will you find the equilibrium concentrations of these without knowing anything else? Are you sure you are not missing any other detail? May I know which book are you using? Jun 15 at 18:00