Multiple choice Equilibrium (Chemistry) question? Explain please

Consider a reaction at STP and at equilibrium.

H2(g) + Cl2(g) <-> 2HCl(g)

Which change will result in an increase in the concentration of Cl2(g)?
a. Decreasing the pressure on the system
b. Decreasing the concentration of HCl
c. Increasing the concentration of H2(g)
d. Increasing the concentration of HCl

Can you explain the idea of how concentration affects equilibrium? I sort of get it & I'm thinking the answer is b. but I'm not positive as to why (I just used the process of the elimination).

Your thinking is on the right track but the actual answer is D Increasing the concentration of HCl.

Think of it like this. The question is asking what will happen directly after you add HCl. The system will no longer be at equilibrium because the concentration of products is too high. To adjust to the changes the reverse reaction will kick start, and the HCl will actually revert back to create more H2 and Cl2 (g) until equilibrium is reestablished. In this way, directly adding HCl here will lead to an increase of concentration of Cl2 (g)

yes the answer is b, this is an example of le chatilier's principle which states that

'If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established'

if you keep that in mind you can work out a number of similar problems