Question

Among (A) water (B) ethanol and (C) mercury, the correct order of the vapor pressure at room temperature is:
(a) $A > B > C$
(b) $A > B > C$
(c) $B > A > C$
(d) $B > C > A$

Answer 1

Hint: Vapor pressure of the substance is caused due to the evaporation and depends on the strength of the intermolecular forces between the substances. Vapor pressure and the intermolecular forces are related to each other as: $vapor\text{ }pressure=\dfrac{1}{\text{intermolecular forces}}$ . Now you can easily answer the statement on this basis.

Complete Solution :
First of let’s discuss the water, ethanol and mercury. Water is a polar molecule and is made up of two atoms i.e. hydrogen and the oxygen atoms and acts as a solvent for many organic compounds.
 Ethanol is a member of the alcohol family and consists of two carbon atoms along with the -OH group and its common name is ethyl alcohol and has a high boiling point and is readily soluble in water. The chemical formula of ethanol is as $C{{H}_{3}}C{{H}_{2}}OH$.
Mercury is a metal which is liquid in nature and has an atomic number as 80 mass number as 200 and belongs to the zinc family and is a d-block element.

- Now coming to the vapor pressure. By vapor pressure we mean the pressure which is due to the evaporation of liquids.
We can determine the vapor pressure of any substance through the intermolecular forces. The substances which have high intermolecular forces will have low vapor pressure because intermolecular forces and vapor pressure are inversely proportional to one another. On the contrary the substances which have low intermolecular forces will have high vapor pressure.
So, from the above given compounds the decreasing order of intermolecular forces is as;
 $C > A > B$
So, thus the increasing order of the vapor pressure of the above given compounds is as;
 $B > A > C$
So, the correct answer is “Option A”.


Note: Vapor pressure of a substance depends not only on the intermolecular forces but also on the temperature and the surface area of the substance and the vapor pressures of the substances changes at different temperatures and with the surface area.