Question

5. 1.024 g of an unknown molecular solid are dissolved in enough water to produce 200.0 mL of solution. The osmotic pressure at 25C is 278.0 torr. What is the molar mass of the solid?

6a. Calculate the van't Hoff factor of a 0.085M potassium sulfate solution that has an osmotic pressure of 5.4atm at 25C

5. 1.024 g of an unknown molecular solid are dissolved in enough water to produce 200.0 ml. of solution. The osmotic pressure at 25 °C is 278.0 torr. What is the molar mass of the solid? (342.5 g/mol) 6. a. Calculate the van't Hoff factor of a 0.085 M potassium sulfate solution that has an osmotic pressure of 5.4 atm at 25°C. [2.6] b. Is this actual van't Hoff factor less than or greater than the ideal (expected) van't Hoff factor? Explain why the obsserved factor is different than the ideal factor.

Answer 1

Q5:

π = CRT

278.0/760 atm = (1.024g/MW)×(1/0.2000L) × 0.0821atm-L/K.mol × 298K

MW = 342.451 g/mol

= 342.5 g/mol (Answer)

Q6(a)

π = i.CRT

5.4atm = i * 0.085mol/L × 0.0821atm.L/K.mol × 298K

i = 2.597

i = 2.6 (Answer)

(b)

K₂SO₄ $\rightarrow$ 2 K⁺ + SO₄^2-

Theoretical van't Hoff factor = 3

The actual van't Hoff factor is less than the ideal van't Hoff factor . The observed van't Hoff factor is less than 3 because the solute molecules might not dissociated completely as shown in the reaction above .

(Percentage dissociation is less than 100% ) .