A solution contains 0.115 mol h2o and an unknown number of moles of nacl. the vapor pressure of the solution at 30°c is 25.7 torr. the vapor pressure of pure water at 30° c is 31.8 torr. calculate the number of moles of nacl

According to Raoult's low:

We will use this formula: Vp (Solution) = mole fraction of solvent * Vp (solvent)

∴ mole fraction of solvent = Vp (Solu) / Vp (Solv)

when we have Vp (solu) = 25.7 torr & Vp (solv) = 31.8 torr

So by substitution:

∴ mole fraction of solvent = 25.7 / 31.8 = 0.808

when we assume the moles of solute NaCl = X

and according to the mole fraction of solvent formula:

mole fraction of solvent = moles of solvent / (moles of solvent + moles of solute)

by substitute:

∴ 0.808 = 0.115 / (0.115 + X)

So X (the no. of moles of NaCl) = 0.027 m