Answer : (2) Ethanoic acid has the lowest vapor pressure at .
Explanation :
Vapor pressure is the pressure which is exerted by vapor when the vapor form is in equilibrium with the solid or liquid form.
Boiling point is the temperature in which the vapor pressure of liquid is equal to the atmospheric pressure.
Vapor pressure is related with the boiling point :
vapor pressure is inversely proportional to the boiling point. As the boiling point increases, vapor pressure decreases and as the boiling point decreases, vapor pressure increases.
The standard boiling point of given liquids water, propanone, ethanoic acid and ethanol are 100, 56, 118 and respectively.
The image of graph of vapor pressure vs temperature is attached below.
Hence, Ethanoic acid is the substance which has the lowest vapor pressure.