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

Explanation

As the solutions are isotonic, the molarities of the two solutions are equal.
`M_("sucrose") = M_("unknown")`
`4/342 xx 1000/100 = 3/x xx 1000/100`
` x = (3 xx 342)/(4) = (3 xx 171)/(2) = 513/2 = 256.5 g "mol"^(-1)`