Problem: At a given temperature, the equilibrium constant for the reaction H2(g) + I2(g) ⇌ 2 HI(g) is 4.0. After equal moles of H2(g) and I2(g) are mixed, there are 0.20 mol of HI at equilibrium. How many moles of H2(g) are present at equilibrium?a. Need to know the volume of the reaction vesselb. 0.30 mol c. 0.10 mold. 0.20 mole. 0.23 mol

We're being asked to determine the moles of H₂ at equilibrium when 0.20 moles of HI is present at equilibrium at K = 4.0

When dealing with equilibrium and K_c:

• K_c → equilibrium units are in molarity/moles 
• 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

Set up an ICE chart to determine the # of moles of H₂ left after equilibrium.