A 5.25% (w/v) solution of a substance is isotonic with a 1.5%(w/v) solution of urea `("molar mass =60 g mol"^(-1))` in the same solvent. Molar mass of the substance will be

Options

A. `"210.0 g mol"^(-1)`
B. `"90.0 g mol"^(-1)`
C. `"115.0 g mol"^(-1)`
D. `"105.0 g mol"^(-1)`

Answer

A

Explanation

Isotonic solutions have the same osmotic pressure.
`pi_(1)=C_(1)RT=pi_2=C_2RT`
so `C_(1)=C_(2)`. Let M be the molar mass of the substance. Now as the densitites are same `(5.25)/M=(1.5)/(60) rArr M=5.25 xx (60)/(1.5)=210`