Question

A 3% solution of x is isotonic with 6% solution of sucrose, calculate the molar mass of x

Answer 1

As both the solutions are isotonic they should have same concentration in mole/litre.
For sucrose solution concentration = 4 g/100 cm3
   = 40 g/litre
  = 40/342 moles/litre (Molar mass of sucrose (C12H22O11) = 342)
For unknown organic substance, let N be the molecular mass,
then the concentration = 3g/100 cm3
  = 30 g/ litre
  = 30/N moles/litre
Comparing both solutions,
 
30/N = 40/342
 N = (30 x 342)/40
 N = 256.5
 
The Molecular mass of organic substance is 256.5