Question

Which of the following has the highest solubility of water?
A. $LiOH$
B. $KOH$
C. $CsOH$
D. $RbOH$

Answer 1

The lower alcohols are generally highly soluble in water. But, the solubility of the alcohols decreases with the increase in molecular mass of the alcohol. The solubility of alcohols in water is mainly because of the formation of hydrogen bonds between alcohol and water molecules.

Complete step by step answer:
The extent of solubility of an alcohol in water is mainly dependent upon the ability of the alcohol molecules to form hydrogen bonds with water molecules. When the molecular mass of an alcohol increases, the hydrocarbon part, i.e., the alkyl group of the alcohol grows larger and this resists the formation of hydrogen bonds with water molecules and hence the solubility of the alcohol in water goes on decreasing.
This is due to the fact that with the increase in branching, there is decrease in the surface area of the non-polar hydrocarbon part of the alcohol and hence, there is an increase in the solubility of the alcohol. Based on these facts, secondary butyl alcohol will have higher solubility in water than tertiary butyl alcohol. $CsOH$ has the highest solubility of water because of low lattice energy due to large size.

So, the correct answer is Option C.

Note: Alcohols can also form hydrogen bonds with water like alcohols and so, it is expected that they are soluble in water. But because of the presence of the larger hydrocarbon part, i.e., the benzene ring in phenols, their solubility is much lower than that of alcohols. Due to the same reason, phenols are also soluble in many organic solvents like ether.