A compound is found to have a molar mass of 190 g mol^-1. If 0.0673 g of the compound is dissolved in enough water to make 200 mL of solution at 25 °C, what is the osmotic pressure of the resulting solution? A) 1.21 × 10^-2 bar B) 6.12 × 10^-2 bar C) 4.39 × 10^-2 bar D) 3.97 × 10^-² bar E) 2.28 × 10^-2 bar

Question

Quizplus · Accepted Answer

First calculate the number of moles of the compound:
n = m/M = 0.0673 g / 190 g mol-1 = 3.54 × 10-4 mol
Then calculate the molarity of the solution:
M = n/V = 3.54 × 10-4 mol / 0.2 L = 1.77 mol L-1
Finally, use the formula for osmotic pressure:
π = MRT = (1.77 mol L-1)(0.0821 L bar mol-1 K-1)(298 K) = 4.39 × 10-2 bar = 4.39 × 10-2 × 100 kPa = 4.39 × 10-2 × 103 Pa = 4.39 × 10-5 MPa = 4.39 × 10-2 × 760 mmHg = 3.33 mmHg ≈ 4 mmHg ≈ 4 torr
Note: Choice D is incorrect because osmotic pressure should be reported in the same units as given in the options, i.e. bars.

A Compound Is Found to Have a Molar Mass of 190