Question

H2+I2->2HI
If 3 moles of H2, I2, and HI are in a 3L flask, what will be the equilibrium amount of each. Kc=5

The answer is H2= I2= .71M, HI= 1.584M(given by teacher)

Just need to know how to do the work.

Answer 1

Do you mean the problem to sound the way it does. Are these the EQUILIBRIUM concentrations? I don't think so. Otherwise, K = 1
(H2) = 3 mols/3 L = ??
(I2) = same
(HI) = same
Write the equilibrium constant expression, plug in the numbers, and solve

Answer 2

I think you meant to say that 3 mols HI, H2, and I2 were placed in a 3 L container and allowed to come to equilibrium. Calculate the concentrations of each.
Write Kc = (HI)^2/(H2)(I2)
Concentrations at the beginning are 1 M for each.
At equilibrium,
(HI) = 1+2x
(H2) = 1-x
(I2) = 1-x
Plug into Kc expression and solve the quadratic for x, then evaluate 1-x, 1-x, and 1+2x.
Post your work if you get stuck.