Q. A 5% solution (by mass) of cane-sugar (M.W. 342) is isotonic with 0.877% solution of substance X. Find the mecular weight of X.
Isotonic solution have same concentration in mol L⁻¹.
For cane-sugar solution, we have
Concentration = 5g/100 cm³ (given)
= 50 gL⁻¹
= 40/342 mol L⁻¹
(Mol. mass of cane-sugar = 342 g mol⁻¹)
For substance X, if M is mol. mass, then, we have
Concentration = 0.877g/100 cm³ (given)
= 8.77g L⁻¹
= 8.77/M mol L⁻¹
∆ = 8.77/M = 40/342
Or. M = 342 × 8.77/40 = 74.98 g mol⁻¹