Equimolal (0.1 m) aqueous solutions of acetic acid (CH3COOH), trichloroacetic acid (CCl3COOH) and trifluoroacetic acid (CF3COOH) are prepared. What is the correct order of the depression in freezing point of these solutions?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH3COOH < CCl3COOH < CF3COOH
Depression in freezing point is a colligative property: it depends on the number of solute particles. An acid that dissociates gives extra ions, so its van’t Hoff factor i is greater than 1:
\( \displaystyle \Delta T_f = i\,K_f\,m \)
All three solutions have the same molality, so the stronger the acid, the more it dissociates, the larger i and the larger ΔTf.
\( \ce{CH3COOH} \): the weakest of the three (pKa 4.76); least dissociation, smallest ΔTf.
\( \ce{CCl3COOH} \): three electron-withdrawing Cl atoms make it a much stronger acid; more dissociation.
\( \ce{CF3COOH} \): F withdraws electrons more strongly than Cl, so this is the strongest carboxylic acid (pKa 0.23); most dissociation, largest ΔTf.
The depressions are not equal (Option 4) because the acids are electrolytes that dissociate to different extents; equal values would hold only if none of them dissociated.
NCERT (2026-27 edition):
1.7 Abnormal Molar Masses – “…In case of association, value of i is less than unity while for dissociation it is greater than unity.” … “Depression of Freezing point, ΔTf = i Kf m”
8.9.1 Reactions Involving Cleavage of O–H Bond – “…The pKa of hydrochloric acid is –7.0, where as pKa of trifluoroacetic acid (the strongest carboxylic acid), benzoic acid and acetic acid are 0.23, 4.19 and 4.76, respectively.”
“…(based on pKa values): CF3COOH > CCl3COOH > CHCl2COOH > … > C6H5CH2COOH > CH3COOH > CH3CH2COOH”