At 15°C, the value of Kw is 4.5 × 10⁻¹⁵. What is the equilibrium concentration of OH⁻ at this temperature?
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By Raoult's law:
-(1)
Where represents the vapor pressure measured for the solution,
denotes the mole fraction of the solvent, and
specifies the vapor pressure of the pure solvent.
The vapor pressure for benzene is recorded as (as given).
Adding a non-volatile solute decreases the vapor pressure by 10.0% (as given).
The vapor pressure of the solution component is thus given by .
Substituting the known values into formula (1):
The number of moles of benzene is .
Calculating to find moles of benzene yields .
Since,
Thus,
Therefore, the moles of non-volatile solute required is .