Q3. At room temperature, what would you expect hexanoic acid (six carbon atoms) and octadecanoic acid (eighteen carbon atoms) to look like?
Hexanoic acid has fewer than ten carbon atoms, so it is a colourless liquid with an unpleasant odour; octadecanoic acid is a higher acid, a wax-like solid that is practically odourless because it is not very volatile (NCERT page 249).
Q4. Like ethanoic acid, butanoic acid pairs into hydrogen-bonded dimers in benzene, an aprotic solvent. If every molecule paired up, what molar mass would its freezing point depression in benzene suggest, and what would the van ’t Hoff factor be? (C = 12, H = 1, O = 16.)
M(C4H8O2) = 48 + 8 + 32 = 88 g mol−1. Full pairing halves the number of solute particles, so ΔTf is half the expected value and the molar mass worked out is 2 × 88 = 176 g mol−1; i = 88/176 = 0.5 (reasoned from NCERT page 249 and Unit 1, page 24).
Q5. Pure ethanoic acid freezes at 290.0 K (NCERT Unit 1, page 19). Will it be solid or liquid on a morning at 15 °C?
15 °C is 15 + 273.15 = 288.15 K, below the freezing point of 290.0 K, so the pure acid can be solid. Its freezing point is 290.0 − 273.15 = 16.85 °C (reasoned from NCERT Unit 1, page 19).
Q6. NCERT sets a carbon-atom limit for the carboxylic acids that are miscible with water. What is it, and how many carbon atoms has the largest of the aldehydes and ketones that NCERT names as mixing with water in all proportions?
Four for the acids, as in butanoic acid (NCERT page 249). For the carbonyl compounds NCERT gives examples, not a limit: methanal, ethanal and propanone, the largest with three carbon atoms (page 235). The acid can both give and take hydrogen bonds with water, which helps it carry one more carbon atom into solution (reasoned).