Question

A solution prepared by dissolving 0.822 g of it in 300 cm³ of water has an osmotic pressure of 149 mm Hg at 298 K. The molar mass of the solute is ____________.

Options

• 180 g mol⁻¹

• 256 g mol⁻¹

• 342 g mol⁻¹

• 422 g mol⁻¹

Answer 1

A solution prepared by dissolving 0.822 g of it in 300 cm³ of water has an osmotic pressure of 149 mm Hg at 298 K. The molar mass of the solute is 342 g mol⁻¹.

Explanation:

Osmotic pressure = π = 149 mm Hg

= `(149  "mm Hg")/(760 ("mm Hg"//"atm"))` = 0.196 atm

V = 300 cm³ = 0.3 dm³

R = 0.08205 dm³ atm K⁻¹ mol⁻¹

From formula,

M₂ = `("W"_2"RT")/(π"V")`

∴ M₂ = `(0.822  "g" xx 0.08205  "dm"^3  "atm K"^-1  "mol"^-1 xx 298  "K")/(0.196  "atm" xx 0.3  "dm"^3)`

= 342 g mol⁻¹