Question

Potash alum is represented by the formula:
(A) ${{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.24{{H}_{2}}O$
(B) $K.Al{{(S{{O}_{4}})}_{2}}.12{{H}_{2}}O$
(C) Both (A) and (B)
(D) None of the above.

Answer 1

Hint: Potash alum is the common name of potassium aluminium sulphate. It is used as a disinfectant and it also helps in stopping bleeding. To answer this question we will have to break the chemical formula of potash alum to get its chemical formula.

Complete answer:
Let’s look at the answer to the given question.
> The chemical name of potash alum is Potassium aluminium sulphate. This means that a molecule of potash alum contains aluminium and potassium metals along with sulphate ions.
> Now, we know that potash alum is a hydrated molecule and it has 12 molecules of water of crystallization. The number of crystallizations of water is very important and will help us in getting the right formula.
> In option (A) we can see that it contains potassium, aluminium and sulphate ions but the number of water of crystallization is 24. So, it is not the correct chemical formula of potash alum.
> In option (B) we can see that it contains potassium, aluminium and sulphate ions and the number of water of crystallization is also 12. So, it is the correct chemical formula of potash alum.

Hence, the answer of the given question is option (B).

Note: Students might get confused and mark both the formulas correct because in the second formula all the atoms are twice as that in the original chemical formula. But it should be remembered that the chemical representation is always unique and for one molecule only.