Question

A dilute aqueous solution of an organic compound soluble in water is formed by dissolving 2.35 g of the compound in water to form 0.250 L of solution. The resulting solution has an osmotic pressure of 0.605 atm at25∘CAssuming that the organic compound is a nonelectrolyte, what is its molar mass?

Answer 1

Answer:

• The molar mass is 380. g/mol (three significant figures)

Explanation:

1) Osmotic pressure is a colligative property, which means that it depends on the number of particles of solute present in the solvent.

2) The mathematical expression that relates osmotic pressure with the concentration of solute is:

• Π = i M R T

Where:

• Π is the osmotic pressure,
• i is the van't Hoff factor,
• M is the molarity concentration of the solute,
• R is the universal constant,
• T is the temperature in kelvins.

3) Here:

• i = 1, because the solute is a summed to be a nonelectrolyte,
• T = 25 + 273.15 K = 293.15,
• R = 0.08206 L atm mol⁻¹ K⁻¹,
• Π = 0.605 atm

4) Calculate M

• M = Π / ( i R T) =

             = 0.605 atm / (1 × 0.08206 L atm mol⁻¹ K⁻¹ × 298.15 K) =

             = 0.0247 atm (three significant figures)

5) Calculate number of moles, from molarity definition:

• M = n / V (liters)

            ⇒ n = M × V (in liters) = 0.0247 M × 0.250 liter =

                    = 0.00618 moles

6) Calculate molar mass:

• Molar mass = mass in grams / number of moles =

                             = 2.35 g / 0.00618 moles

                             = 380 g/mol ← answer