• # question_answer                 A 5% solution of cane sugar (mol. wt. = 342) is isotonic with 1% solution of a substance X. The molecular weight of X is:                                                                                                                                   A)                 34.2          B)                            171.2 C)                                            68.4        D)                                            136.8

Isotonic solutions are the solutions having same osmotic pressure.                 Applying Van't Hoff equation                 $\pi V=nST$                 $\pi =cST$                                         Osmotic pressure of 5% cane sugar solution                 $({{\pi }_{1}})\,C\times S\times T=\frac{50\,g/L}{342}\times 0.0821\times T$                 Osmotic pressure of 1% solution of substance $x\,({{\pi }_{2}})$                 $=\frac{10g/L}{M}\times 0.0821\times T$                 Both are isotonic                 So,          ${{\pi }_{1}}={{\pi }_{2}}$                 or            $\frac{50}{342}\times 0.0821\times T=\frac{10}{M}\times 0.0821\times T$                 $\therefore M\,(mol.\,wt.\,of\,X)=\frac{342}{5}=68.4$