The vapour pressures are 58.5 Torr, 70.0 Torr, and 70.1 Torr.

Raoult's Law states that the vapour pressure of a solvent#P_1# in a solution is equal to the vapour pressure of the pure solvent #P_1^°# times its mole fraction #x_1#.

#P_1 = x_1P_1^°#

Since the oxalic acid, H₂C₂O₄ (HOx), is non-volatile, its vapour pressure #P_2# = 0.

So, according to Henry's Law, the vapour pressure of the solution is

#P = P_1 + P_2 = P_1 + 0 = P_1#

It is often easier to solve these problems by using the mole fraction of the solute, #x_2#.

Since we have two components, #x_1 +x_2 = 1#.

Then #P_1 = (1 – x_2)P_1^° = P_1^° -x_2 P_1^°# and