Arrange the following in increasing order of ΔTb values?
A. 0.1 mol of sodium chloride dissolved in one kg of water
B. 0.1 mol of glucose dissolved in one kg of water
C. 0.1 mol of magnesium chloride dissolved in one kg of water
D. 0.1 mol of calcium phosphate dissolved in one kg of water
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → B, A, C, D
Elevation of boiling point is a colligative property: it depends on the number of solute particles, not on their nature. For an electrolyte the van’t Hoff factor i counts the particles formed from one formula unit:
\( \displaystyle \Delta T_b=i\,K_b\,m \)
All four solutions have the same molality (0.1 mol in 1 kg of water) and the same solvent, so ΔTb depends only on i. Assuming complete dissociation:
(B) Glucose: a non-electrolyte, i = 1
(A) NaCl → \( \ce{Na^{+}} \) + \( \ce{Cl^{-}} \): i = 2
(C) \( \ce{MgCl2} \) → \( \ce{Mg^{2+}} \) + \( \ce{2Cl^{-}} \): i = 3
(D) \( \ce{Ca3(PO4)2} \) → \( \ce{3Ca^{2+}} \) + \( \ce{2PO4^{3-}} \): i = 5
Shortcut: The non-electrolyte glucose (B) gives the smallest elevation and calcium phosphate (D), with five ions, the largest. Only Option (1) starts with B and ends with D.