This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Aldehydes and Ketones Physical Properties”.
1. Which of the following exists as a gas at 287K?
Explanation: Methanal exists as foul-smelling gas at room temperature. Ethanal is a volatile liquid which boils at 294K. Other aldehydes and ketones exist as either liquids or solids at room temperature.
2. If the boiling points of methoxyethane and propanol are 281 K and 370 K respectively, predict the boiling point of propanal.
a) 273 K
b) 281 K
c) 322 K
d) 370 K
Explanation: The boiling point of aldehydes are higher than those of non-polar hydrocarbons of comparable molecular masses. However, their boiling points are lower than comparable alcohols, due to the absence of intermolecular hydrogen bonding.
3. Which of the following is the least soluble in water?
Explanation: The lower aldehydes and ketones are miscible in water in all proportions due to the ability of the polar carbonyl group to form hydrogen bonds with water molecules. However, the solubility rapidly decreases as the length of the alkyl chain increases. Precisely, aldehydes and ketones with more than 4 carbon atoms are practically insoluble in water.
4. Propanal has a slightly higher boiling point than propanone.
Explanation: Propanone has two CH3 groups, one on either side of the CO group, whereas propanal has only one CH3CH2 group. This makes propanone more polar because of the higher electron releasing effect of two alkyl groups. The boiling point of propanal is 322K and that of propanone is 329K.
5. Two compounds A and B were being tested for their boiling points. It was observed that A started boiling after B, when both were subjected to same conditions. If the compound B is acetone, which of the following can be compound A?
Explanation: It can be inferred that B has a higher boiling point than A. Since hydrocarbons, ethers and aldehydes have lower boiling points than ketones of similar molecular masses, none of them can be compound A. Also, alcohols have a higher boiling point than similar ketones hence, the compound A is propanol.
6. Which of the following has the least pungent odour?
Explanation: As the size of the aldehyde molecule increases, the odour becomes less pungent and more fragrant. Methanal is the most pungent smelling aldehyde.
7. Which of the following is used as a flavouring agent?
Explanation: Methanal, ethanal and acetone are lower carbonyl compounds and are not much fragrant to be used as flavouring agents.
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