Chemistry Multiple Choice Questions on “Carboxylic Acids Physical Properties”.
1. Which of the following aliphatic carboxylic acids is a solid at room temperature?
a) Heptanoic acid
b) Octanoic acid
c) Nonanoic acid
d) Decanoic acid
Answer: d
Clarification: The members butanoic acid to nonanoic acid are colourless oily liquids at room temperature. The compounds with ten or more carbon atoms exists as waxy solids at room temperature.
2. Which among the following has the most unpleasant odour?
a) Caproic acid
b) Lauric acid
c) Myristic acid
d) Palmitic acid
Answer: a
Clarification: The first three members of carboxylic acids have pungent smell. The next six members have a faint unpleasant odour. The higher members (lauric acid, myristic acid, palmitic acid) are practically odourless due to their low volatility.
3. Benzoic acid is almost insoluble in which of the following solvents?
a) Cold water
b) Benzene
c) Ether
d) Alcohol
Answer: a
Clarification: The large hydrophobic benzene ring of benzoic acid prevents the carboxyl group from taking part in hydrogen bonding. However, benzoic acid is soluble in non-polar solvents like benzene, ether, alcohol, chloroform, etc.
4. The solubility of carboxylic acids _________ with the increase in size of alkyl groups.
a) increases
b) decreases
c) remains same
d) varies unpredictably
Answer: b
Clarification: This is because of the reduced polarity and hinderance provided by the large alkyl groups to the COOH group from involving in the hydrogen bonding with solvent molecules.
5. As a result of intermolecular hydrogen bonding, carboxylic acids exists as ______
a) acetals
b) aldols
c) hemiacetals
d) dimers
Answer: d
Clarification: The intermolecular hydrogen bonds between carboxylic acids are not broken even in vapour phase. In fact, most carboxylic acids exist as dimer in vapour phase or in aprotic solvents.
6. Which of the following is not a reason for carboxylic acids having higher boiling point than alcohols of comparable molecular masses?
a) Presence of electron withdrawing carbonyl group
b) Higher polarity of OH bond than in alcohols
c) Formation of two hydrogen bonds to form cyclic dimers
d) Presence of more alkyl groups in carboxylic acids
Answer: d
Clarification: The OH bond in carboxylic acids is more strongly polarised due to the presence of adjacent electron withdrawing CO group. This results in stronger hydrogen bonds. Also, the presence of alkyl groups should increase the polarity of OH bond as they are electron releasing in nature. Furthermore, carboxylic acids form two hydrogen bonds between molecules compared to the only one between alcohols.
7. What is the correct order of boiling points of the following?
a) HCOOH > CH3COOH > C2H5COOH
b) C2H5COOH > CH3COOH > HCOOH
c) HCOOH > C2H5COOH > CH3COOH
d) CH3COOH > HCOOH > C2H5COOH
Answer: b
Clarification: The boiling points increase with the increase in molecular mass, because as the size of alkyl group increases, the magnitude of van der Waal forces increases, thus making the bonds difficult to break.
8. If the boiling point of propanol is 370 K, predict the boiling point of acetic acid.
a) 322 K
b) 329 K
c) 370 K
d) 390 K
Answer: d
Clarification: Acetic acid and propanol both have same molecular mass. Acetic acid should have a higher boiling point than propanol because of its stronger hydrogen bonds and the fact that it forms cyclic dimers.
9. Boiling point of aromatic carboxylic acids are higher than that of comparable aliphatic carboxylic acids.
a) True
b) False
Answer: a
Clarification: This is because benzene ring has a planar structure and can pack more closely in the crystal lattice than aliphatic acids, which have a zig-zag structure. The melting of aromatic acids are also usually higher due to the same reason.