Correct Option
The correct option is 290 K.
Explanation
Ethanoic acid ($\text{CH}_3\text{COOH}$), commonly known as acetic acid, is a carboxylic acid. The physical properties of pure organic compounds, such as melting and boiling points, are distinct characteristics used for identification. Pure ethanoic acid is often referred to as glacial acetic acid due to its tendency to freeze at temperatures slightly below standard room temperature.
Analysis
The melting point of pure ethanoic acid is 290 K (approximately $17^\circ\text{C}$). At this temperature, the liquid solidifies into ice-like crystals, which is why it freezes during winters in cold climates.
The other options correspond to different physical properties of carbon compounds:
- 351 K ($78^\circ\text{C}$): This represents the boiling point of ethanol.
- 391 K ($118^\circ\text{C}$): This represents the boiling point of ethanoic acid.
- 111 K ($-162^\circ\text{C}$): This represents the boiling point of methane.
Key Takeaway: The melting point of pure ethanoic acid is 290 K. Its freezing nature in cold weather leads to the common name "glacial acetic acid."