Question

(a) (1 pts) Rank these compounds in order of increasing vapor pressure above the condensed phase (either liquid or solid). Assume they are all at the same temperature. HO CCl FeS Cl2 lowest vp. highest v.p. (b) (1 pts) Explain why you put them in this order

Answer 1

b) The substance with the highest boiling point will have the lowest vapour pressure. This is because, higher the boiling point, more is the energy required to release from the intermolecular forces and lesser the tendency to form vapours. Hence, the lesser vapour pressure.

From the above mentioned molecules, Cl₂ is a gas, CCl₄ and H₂O are liquids and FeS is a solid at the room temperature. This implies that FeS has the highest boiling point and hence the lowest vapour pressure and Cl₂ has the lowest boiling point and hence the highest vapour pressure among the 4. Also, H₂O is expected to have higher intermolecular forces and therefore a higher boiled point than CCl₄ due to the presence of extensive hydrogen bonding. So H₂O will have lower vapour pressure when compared to CCl_4. Therefore, the order of vapour pressures is as follows.

a) Lowest V.P.    FeS < H₂O < CCl₂ < Cl₂ Highest V.P.