When RBC is kept in 0.5 M solution of sucrose, its volume does not alter, but if the same cell is placed in 0.5 M solution of sodium chloride the volume of cell

The sucrose molecule does not breaks into ions and is osmotically isotonic with the erythrocytes. In contrast, each salt molecule breaks up into two ions one of sodium and one chloride and is osmotically hypertonic as compared to the erythrocyte. Hence, when kept in 0.5M solution of salt the erythrocytes lose water due to exosmosis and their volume decreases.