Question

A 4% solution of sucrose is isotonic with 3% solution of an unknown organic substance.Calculate the molecular mass of unknown substance .

Answer 1

as both the sol. are isotonic therefore they have same concentration in moles
M(sucrose)=M(unknown)
4/342(1000/100)=4/x(1000/100) {342 is molecular mass of sucrose}
x=3×342/4
x=1026/4
x=256.4

Answer 2

Answer: 250 g

Explanation: Isotonic solutions are those solutions which have the same osmotic pressure.

$\pi =CRT$ if osmotic pressures are equal at the same temperature, concentrations must also be equal.

$\pi$ = osmotic pressure

C= concentration

R= solution constant

T= temperature

Thus: $C_{sucrose}=C_A$ where concentration is in molarity.

For sucrose solution: 4 g of sucrose is dissolved in 100 ml of solution.

For solute A: 3 g of A is dissolved in 100 ml of solution.

$Molarity=\frac{\text{Mass of solute}\times 1000}{\text{Molar mass of solute}\times \text{volume of solution in ml}}$

$C_{sucrose}=\frac{4\times 1000}{342}\times {100}=0.12$

$C_ A=\frac{3\times 1000}{M_A}\times 100$

As $C_{sucrose}=C_A$

$0.12=\frac{3\times 1000}{M_A}\times 100$

$M_A= 250g/mol$

The molecular mass of unknown substance is 250 grams.