Question

Identify the correct order of solubility in aqueous medium of the following compounds:
a.\[CuS>ZnS>N{{a}_{2}}S\]
b.$ZnS>N{{a}_{2}}S>CuS$
c.$N{{a}_{2}}S>CuS>ZnS$
d.$N{{a}_{2}}S>ZnS>CuS$

Answer 1

Hint: Remember that ionic compounds are more soluble in an aqueous medium when compared to covalent ones, and that ionic character is directly proportional to the size of the cation.

Complete answer:
To start with, as is mentioned in the hint ionic compounds are much more soluble in an aqueous medium in comparison to their covalent counterparts, and this ionic character is directly proportional to the sizes of the compound’s cation.
Let us now analyse the sizes of their cation:
\[N{{a}^{+}}\] has an ionic radius of 97 pm.
\[Z{{n}^{2+}}\] has an ionic radius of 74 pm.
\[C{{u}^{2+}}\] has an ionic radius of 73 pm.

Using this information, we observe that in terms of size and, as a result of its proportionality to a compound’s ionic character, ionic character the following order is seen:
\[N{{a}^{+}}>\text{ }Z{{n}^{2+}}>\text{ }C{{u}^{2+}}\]
With this order in mind, we can conclude that the solubility order of the given compounds is as follows:
$N{{a}_{2}}S>ZnS>CuS$

Therefore, upon completion of our analysis, we can say that the answer to this question is option d $N{{a}_{2}}S>ZnS>CuS$.

Note:
To solve this question, it is not necessary to have an incredibly precise knowledge of the radii of different ions, however it is very important to know the trend running across the periodic table that determines the size of cations.
Another extremely important part of this question is the mention of an aqueous medium, and be very careful of bearing this in mind while answering this question.