A 5.25% solution of a substance is isotonic with a 1.5% solution of urea (molar mass `= 60 g mol^(-1)` ) in the same solvent. If the densities of both the solutions are assumed to be equal to `1.0 g cm^(-3)` , 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 same osmotic pressure.
`pi = C_1RT, pi_2 = C_2RT`
For isotonic solutions, `pi_1 = pi_2 :. C_1 = C_2`
or `(1.5//60)/V =(5.25//M)/V`
[Where, M=molecular weight of the substance]
or `(1.5)/60 = (5.25)/M or M=210`