Animated Solution for Chemistry - s and p-Block Elements: The ratio of number of water molecules in Mohr's salt and potash alum is ............ $\times 10^{-1}$. (Integer answer)

Have you ever lost marks in a JEE exam not because you didn't know the complex concepts, but because you forgot a simple chemical formula? This question is a classic example of why memorizing the standard formulas of common compounds is absolutely non-negotiable.

Today, we are going to dive into the world of double salts and look at two of the most famous ones: Mohr's salt and Potash alum. The question is straightforward—it asks for the ratio of their water molecules. But to get there, we need to pull their exact formulas from our memory banks. Let's break it down step-by-step.

First up is Mohr's salt. This is a very common reagent used in redox titrations in your chemistry lab. It is a double salt of ferrous sulphate and ammonium sulphate.

The complete chemical formula for Mohr's salt is ferrous ammonium sulphate hexahydrate, written as:

The key part of this formula for our question is the "hexahydrate" at the end. This tells us that there are exactly 6 molecules of water of crystallization per formula unit of Mohr's salt.

So, we have our first value: $n_1=6$.

Next, we have Potash alum, commonly known in Indian households as fitkari. It is widely used for water purification and as an antiseptic. Potash alum is a double salt of potassium sulphate and aluminium sulphate.

Here is where many students make a critical mistake. Sometimes, Potash alum is written in its empirical form as $\text{KAl}({\text{SO}}_4{)}_2\cdot 12{\text{H}}_2\text{O}$. However, the standard molecular formula that represents the complete double salt is:

For JEE questions asking for the number of water molecules in Potash alum, you must always use the standard molecular formula unless specified otherwise. This means Potash alum contains 24 molecules of water of crystallization.

So, we have our second value: $n_2=24$.

Now that we have both numbers, the rest is simple arithmetic. The question asks for the ratio of the number of water molecules in Mohr's salt to that in Potash alum.

Let's set up the ratio:

Simplifying this fraction, we get:

But wait! Before you rush to write down the answer, look at the format requested in the question. It asks for the answer in the form of `............ \times 10^{-1}`. This is a classic JEE trap to test your attention to detail.

We need to convert $0.25$ into scientific notation with a power of $-1$:

Therefore, the exact numerical value that fills in the blank is 2.5.

This question is a gentle reminder that inorganic chemistry often rewards direct knowledge. Make it a habit to maintain a list of common compounds—like Blue Vitriol (${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$), Green Vitriol (${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}$), Epsom salt (${\text{MgSO}}_4\cdot 7{\text{H}}_2\text{O}$), and Washing soda (${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$)—and memorize their water of crystallization. You never know when a simple ratio might fetch you an easy +4 marks!