A solution is prepared by dissolving 72 g of X (a molecular compound and a nonel
ID: 475988 • Letter: A
Question
A solution is prepared by dissolving 72 g of X (a molecular compound and a nonelectrolyte) in 200 g of water at 25 degree C. The solution freezes at -3.72 degree C What is the molar mass of X? K_f of water is 1.86 degree C/m. What is the boiling point of this solution? K_b of water is 0.512 degree C/m. What is the osmotic pressure of this solution assuming the volume of the solution does not change after the addition of X? The density of water is 1 g/mL. What is the vapor pressure of this solution? The vapor pressure of pure water is 23.8 torr at 25 degree C.Explanation / Answer
DTf = (Kf)(m)
DT represents the freezing point depression:
DT = Tf°- Tf
Tf° is the freezing point of the pure solvent.
Tf is the freezing point of the solution.
Kf is the molal freezing point depression constant.
Molality is calculated as follows:
m is the molality of the solution. m = moles of solute/ kg of solvent.
DT = Tf°- Tf = 25oC – (-3.72)oC = 28.72oC
DTf = (Kf)(m) = [(Kf)(g solute)] / [(MW)(kg solvent)]
28.72oC = [(1.86oC/m )( 72 g solute)] / [(MW)(0.2 kg solvent)]
MW = 23.31 g/mol
The molecular compound is magnesium.
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