Arrange the following solutions in increasing order of their boiling point:
(A) 1% glucose
(B) 2% glucose
(C) 3% glucose
(D) 4% glucose
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B), (C), (D)
Boiling point is a colligative property: adding a non-volatile solute lowers the vapour pressure, so the solution boils at a higher temperature. The elevation of boiling point depends only on the number of solute particles, through the molality m:
\( \displaystyle \Delta T_b=i\,K_b\,m \)
Glucose is a non-electrolyte: it does not dissociate, so it gives 1 particle per molecule and i = 1. The solvent is the same in all four solutions, so Kb is also the same. Only the molality decides the order.
A x% (w/w) glucose solution has x g glucose (M = 180 g mol−1) in (100 − x) g water:
\( \displaystyle m=\frac{w_B/M_B}{w_A}\times1000 \)
\( \displaystyle (A)\ 1\%:\ m=\frac{1/180}{99}\times1000=0.056 \)
\( \displaystyle (B)\ 2\%:\ m=\frac{2/180}{98}\times1000=0.113 \)
\( \displaystyle (C)\ 3\%:\ m=\frac{3/180}{97}\times1000=0.172 \)
\( \displaystyle (D)\ 4\%:\ m=\frac{4/180}{96}\times1000=0.231 \)
The higher the molality, the larger ΔTb and the higher the boiling point.
Shortcut: For the same solute and solvent, more solute always means a higher boiling point. Only Option (4) starts with the 1% solution (A) and ends with the 4% solution (D).