Consider the following gas-phase equilibrium;

H2(g) + I2(g) <--> 2HI(g)

At a certain temperature, the equilibrium constant Kc is 4.0. Starting with equimolar quantities of H2 and I2 and no HI, when equilibrium was established, .2 moles of HI was present. How much H2 was used to start the reaction?

Initial concentration of H2 and I2 = "x" and final concentration of HI =.2

[.2]^2 / x^2= 4.0 gives x = .1 mole. The correct answer is .2 moles. The mistake I'm making is that I need to put something along with "x" in the denominator, "x" minus something. What? thanks