One form of Raoult's Law states that the vapor pressure of a solution of a non-volatile solute at certain temperature is equal to the vapor pressure of the pure solvent at the same temperature multiplied by the mole fraction of the solvent, this is:
p = X solvent * P pure solvent,
X solvent = number of moles of solvent / total number of moles.
Here the solute is 13.6 g of C12 H10 and the solvent is 26.4 g C6H6.
=>
moles of solvent = mass in grams / molar mass
molar mass of C6H6 = 6 * 12 g/mol + 6 * 1g/mol = 78 g/mol
moles of solvent = 26.4 g / 78 g/mol = 0.33846 mol
molar mass of C12H10 = 12 * 12g/mol + 10*1g/mol = 154 g/mol
moles of solute = 13.6 g / 154 g/mol = 0.08831 mol
