how would you begin this question?

Question

An empty 15.0 L cylinder, .500 moles of gaseous PCl5 are added and allowed to reach equilibrium. The concentration of PCl3 is found to be .0220M. (Temp at 375K)
PCl3(g) + Cl2(g) --> PCl5(g)

i need to fine how many moles of PCl5 remain at equilibrium

That is pretty easy. You started with .5moles PCl5, and it made .022*15 moles of PCl3. So looking at the balanced equation, it took .022*15 moles of PCl5 to do this.

So subtract the amount used from the initial amount.

so is it .17 moles of PCl5 remaining?

.5 - (.022 * 15) = 0.17

so, same question, we have to find the value of k..is my work right?
the equilibrium conc. of PCl5 = .01133 M
PCl3 = .022 M
Cl2 = .022 M
K = [PCl5]/[PCl3][Cl2}
K = [.01133]/[.022][.022]
K = 23.4

The set up is correct. I didn't check the calc work.

Answer 1

But yes, you can use the equilibrium concentrations and the balanced equation to calculate the equilibrium constant.