Dry air was passed successively through a solution of 5 g of a solute in 80 g of water and then thro — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

By Ostwald-Walker method, $\frac{P^\circ - P}{P} = \frac{\text{loss in pure solvent}}{\text{loss in solution}} = \frac{0.04}{2.5}$. Thus $\frac{n}{N} = \frac{0.04}{2.5} \implies \frac{5/M}{80/18} = \frac{0.04}{2.5}$. $\frac{90}{80M} = 0.016 \implies M = \frac{90}{80 \times 0.016} = 70.31$ g/mol. Therefore, correct answer is A.