The partial pressure of water above a solution of water and a nonvolatile solute at 25oC is 18.6 mm Hg. What is the mole fraction of the solute?
According to Raoult's Law,
Psolution = XsolventPsolvent
18.6mmHg = X(H2O)*P(H2O at 25o)
Look up the water vapor pressure at 25, substitute above, and solve for X(H2O).
18.6mmHg = X(H2O) * 23.76mmHg
X(H2O) = .7838
What is next?
To find the mole fraction of the solute, you need to know the partial pressure of the solute and the total pressure.
In this case, the total pressure is the partial pressure of water, which is 18.6 mm Hg.
To calculate the mole fraction, you can use the following formula:
Mole Fraction = moles of solute / (moles of solute + moles of solvent),
where moles of solute represent the number of moles of the solute and moles of solvent represent the number of moles of water in this case.
Since the solute is nonvolatile, it does not contribute to the vapor pressure, and only water's partial pressure is given. Therefore, the mole fraction of the solute will be zero (0).
So, the mole fraction of the solute in this solution is 0.