Two solutions are isotonic when they have the same osmotic pressure. It is only possible when they have same molar concentration. A 25% solution of cane-sugar means 100g of solution contain 25g of cane-sugar. For dilute solution 100g is approximately equal to 100‌mL. Molarity of cane-sugar =
Number‌of‌mole‌of‌cane−sugar
Volume(in‌L)
=
25g
342g∕mol×100‌mL
×1000‌mL = 0.7309 ...(i) Now, 5% solution of substance A means 100g of the solution contain 5g of substance A. For dilute solution, 100g is approximately equal to 100‌mL Molarity of A=
5g
M′g∕mol×100‌mL
×1000‌mL =
50
M′
M ...(ii) Equating both equations (i) and (ii), we get
50
M′
=0.7309 M′=
50
0.7309
=68.408g‌mol‌m−1 Hence, molecular weight of A is 68.4gmol−1.