Assume that 1-bromobutane and 1-chlorobutane form an ideal solution. At 273 K,  P^* _chloro = 3790. Pa and P^* _bromo = 1394 Pa. When only a trace of liquid is present at 273 K, y _chloro = 0.750. 

a. Calculate the total pressure above the solution.

b. Calculate the mole fraction of 1-chlorobutane in the solution.

c. What value would Z _chloro have in order for there to be 4.86 mol of liquid and 3.21mol of gas at a total pressure equal to that in part (a)? [Note: This composition is different from that of part (a).]