Keq = [0.25 - x] / [0.25 - x][2x]

= [0.25 - ] / [0.25 - ][2()]

=

What numbers need to be substituted

If this refers to a problem I helped with earlier, you need to repost the entire problem. However, when I said to substitute into Keq, I meant to write the Keq expression and substitute the above number into it.

I2 + Cl2 ==> 2ICl (Is that the equation?)

initially:
(I2) = 0.5/2.0 = ??
(Cl2) = 0.5/2.0 = ??
(ICl)= 0

change:
(ICl) = +2x
(I2) = -x
(Cl2) = -x

equilibrium:
(ICl) = 0 + 2x = 2x
(I2) = (0.5/2.0) - x = 0.25 - x
(Cl2) = (0.5/2.0) - x = 0.25 - x

Set up Keq, substitute, and solve for x. Post your work if you get stuck. Check my work.

No.

Keq = (ICl)^2/(I2)(Cl2)
(2x)^2/(0.25-2)(0.25-x)
Solve for x.

To determine what numbers need to be substituted into the equation for Keq, we need more information about the context or specific problem you are working on. Typically, when using the equilibrium constant expression Keq, the values that need to be substituted are the concentrations (or pressures) of the reactants and products at equilibrium. These concentrations are represented by the variables in the equation.

Therefore, you would need to know the values of x, 0.25, and 2x, which represent the concentrations of the reactants and products at equilibrium. These values would be provided in the problem statement or you may need to calculate them based on given information, such as initial concentrations and reaction stoichiometry.

However, please note that the equations you have provided seem to have missing terms and may not be correct. If you can provide more information or clarify the equations, I would be happy to help further.