A mixture of gases A and B is contained in a constant-volume vessel at a temperature of 400 K and a pressure of 1 bar. The ideal gas law can be used to describe the behavior of each gas in the mixture. The molar fractions of A and B are x_A and x_B, respectively, and the number of moles of A and B are n_A and n_B, respectively. If n_A = n_B = 1 mole and the partial pressure of A is 0.5 bar, determine the molar fraction of A.