Question

A solution is made by dissolving 45.5 g of sodium phosphate in 265 g of ethanol. The resulting solution has a volume of 225 mL. Using the above information, values found in Table 14.6, a vapor pressure of 5.87×10−2 atm at 20 oC, and the theoretical van’t Hoff factor, determine the following for the resulting solution:

A. M of Na3PO4
B. m of Na3PO4
C. Freezing point in oC
D. Boiling point in oC
E. Osmatic pressure at 20 oC
F. Vapor pressure at 20 oC

Answer 1

Mass of Na3PO4 = 45.5 g

Total volume of solution = 225 mL = 0.225 L

Mass of ethanol = 265 g

(A). Molar mass of Na3PO4 = 163.94 g/mol

Moles of Na3PO4 = 45.5 / 163.94

= 0.277

Molarity of Na3PO4 solution = Moles / Volume of solution

= 0.277 / 0.225

= 1.23 M

(B). Molality of solution = Moles / Mass of ethanol (kg)

= 0.277 / 0.265

= 1.04 m

(C). Freezing point of ethanol = - 114.1 oC

Freezing point constant, Kf = 1.99 oC kg/mol

Van't Hoff factor of Na3PO4 = 4

Depression in freezing point, Tf = i * Kf * m

= 4 * 1.99 * 1.04

= 8.27 oC

Freezing point of solution, Tf = To - Tf

= - 114.1 - 8.27

= - 122.37 oC

(D). Boiling point of ethanol = 78.4 oC

Boiling point constant, Kb = 1.22 oC kg/mol

Increase in boiling point of solution, Tb = i * Kb * m

= 4 * 1.22 * 1.04

= 5.07 oC

Boiling point of ethanol, Tb = To + Tb

= 78.4 + 5.07

= 83.47 oC

(E). Temperature, T = 20 oC = 293 K

Osmotic Pressure, = MRT

= 1.23 * 0.08206 * 293

= 29.57 atm