Question

What will be the correct order of vapour pressure of water, acetone and ether at $30\,^\circ {\rm{C}}$. Given that among these compounds, water has maximum boiling point and ether has minimum boiling point.
A. Water < Ether < Acetone
B. Water < Acetone < Ether
C. Ether < Acetone < Water
D. Acetone < Ether < Water

Answer 1

Hint: As we know that, at a given temperature vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium in a closed container. The boiling point of a compound is inversely proportional to the vapour pressure of that compound. The highest boiling point containing compound has lower vapour pressure and lower boiling point containing compound has high vapour pressure.

Complete answer:
Main factors responsible for vapour pressure are:
i) H-bond is formed when hydrogen is bonded with more electronegative atoms like O, N and F. As we know that H-bonding produces a lower evaporation rate, so a lower vapour pressure result. Water produces H-bonding while acetone and ether does not produce H-bonding. In acetone and ether, there is no H-atom directly bonded with O.
ii) According to the question, water has the highest boiling point and ether has the lowest boiling point. The boiling point order is as follows:
Water > Acetone > Ether
At room temperature, with the lowest boiling point, a substance insists the highest vapour pressure and the largest boiling point substance insists the lowest vapour pressure. Hence, the correct order of vapour pressure is- Water < Acetone < Ether.

Thus, the correct answer is option (B) Water < Acetone < Ether.

Additional Information:
Lower boiling points of liquids results increase in vapour pressure. Vapour pressure increases by heating a liquid and allowing more molecules to enter the atmosphere.

Note: A student gets confused between boiling point and vapour pressure. The temperature at which the vapour pressure is equal to the atmospheric pressure is known as boiling point. On the other hand, force exerted by the vapour discharge by a liquid or solid substance in a closed vessel is called vapour pressure.