Question

A 5% solution of Cane sugar (Mol. Wt = 342) is isotonic with 1% solution of substance X.The molecular weight of X is

Answer 1

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Answer 2

Answer: 66.7 g/mol

Explanation: Isotonic solutions are those solutions which have the same osmotic pressure.  If osmotic pressures are equal at the same temperature, concentrations must also be equal.

$\pi =CRT$ for non electrolytes

$\pi$ = osmotic pressure

C= concentration

R= solution constant

T= temperature  

Thus: $C_{glucose}=C_{X}$ where concentration is in molarity.  

For sucrose solution: 5 g of sucrose $(C_{12}H_{22}O_{11})$ is dissolved in 100 ml of solution.

For solute X: 1.0 g of X is dissolved in 100 ml of solution.

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

$C_{sucrose}=\frac{5g\times 1000}{342}\times {100}=0.15M$

$C_ A=\frac{1.0\times 1000}{M_A}\times 100$

As $C_{urea}=C_X$

$0.15=\frac{1.0\times 1000}{M_A}\times 100$

$M_A= 66.7 g/mol$