Using Raoult’s Law, we can calculate for the vapor pressure using the formula:
P = x * P(pure)
Where x is the mole fraction of water, P(pure) = pressure of pure water, P = pressure when in solution
calculating for mole fraction of water:
x = (0.42kg / 18kg/kmol) / ((0.42kg / 18kg/kmol) + (0.075kg / 192.12kg/kmol))
x = 0.9835
Therefore:
P = 0.9835* P(71.93 mmHg)
P = 70.75 mmHg