Problem: Equal numbers of moles of H2 and I2 were mixed in a 3 liter flask and heated to 700 C. The initial concentration of each gas was 0.0088 M. They reacted according to the reaction H2(g) + I2(g) ⇄ 2 HI(g). 78.6% of the I2 was consumed in reaching equilibrium. What is Kc for the reaction?A. 0.25B. 530C. 4.2x10-5D. 4.0E. insufficient information is given to solve the problem

We are being asked to calculate the equilibrium constant, K_c for the given equilibrium reaction:

H_2(g) + I_2(g) ⇄ 2 HI_(g)

When dealing with equilibrium and K_c:

• K_c → equilibrium units are in molarity 
• K_c is an equilibrium expression:

$\boxed{{\mathbf{K}}_{\mathbf{c}}\mathbf{=}\frac{\mathbf{products}}{\mathbf{reactants}}}$

▪ only aqueous and gaseous species are included in the equilibrium expression
▪ the coefficient of each compound in the reaction equation will be the exponent of the concentrations in the equilibrium expression

Given:    

Initial Concentrations:

[H₂] = 0.0088 M
[I₂] = 0.0088 M
[HI] = 0 M