Write the formulas of the two compounds

• Mohr's salt: $\mathrm{FeSO_4\cdot (NH_4)_2SO_4\cdot 6H_2O}$ So, number of water molecules in one formula unit $= 6$.

• Potash alum: $\mathrm{K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O}$ This is equivalent to: $\mathrm{KAl(SO_4)_2\cdot 12H_2O}$ Hence, number of water molecules in one formula unit of potash alum $= 24$ in the doubled form, or $12$ in the standard single formula unit form.

Interpret the question

In school/JEE chemistry, potash alum is usually taken as: $\mathrm{KAl(SO_4)_2\cdot 12H_2O}$ Therefore, compare water molecules as: $6 : 12 = 1 : 2$

So the ratio of number of water molecules in Mohr's salt and potash alum is: $\frac{6}{12} = \frac{1}{2}$

Use the given expression

The question says the ratio is: $\underline{\hspace{2cm}} \times 10^{-1}$

Let the required integer be $n$. Then: $n \times 10^{-1} = \frac{1}{2}$

So, $\frac{n}{10} = \frac{1}{2}$ $n = 5$

Final answer

$\boxed{5}$

Comparison with stored correct answer

Stored correct answer = $5$

This matches the derived answer.