Calculate the equilibrium constant kc she obtained for this

Question

A student ran the following reaction in the laboratory at 638 K:

2HI(g) <-----> H₂(g) + I₂(g)  

When she introduced 0.313 moles of HI(g) into a 1.00 liter container, she found the equilibrium concentration of I₂(g) to be 3.10×10^-2 M.  

Calculate the equilibrium constant, K_c, she obtained for this reaction.  

K_c = ________