JEE Main 2021
LEVELJEE Main
Visualized Solution
The Sigma Insight: Stoichiometric and Volumetric Calculations
Analyzing the Setup
Imagine we have a beaker filled with exactly of water. Now, we are dropping of solid sodium oxide () into it.
Notice that the problem explicitly tells us to neglect any change in volume. This is a crucial assumption because adding a solid to a liquid usually changes the total volume slightly. By neglecting it, we can safely assume the final volume of the solution remains exactly .
The Master Equation
Now look at the chemical equation. Sodium oxide reacts vigorously with water to produce sodium hydroxide.
The balanced chemical equation is:
From this balanced equation, it is crystal clear that of sodium oxide yields exactly of sodium hydroxide. This stoichiometric ratio is the heart of our calculation.
Moles and Stoichiometry
Let's substitute the values. First, we need the moles of our reactant, sodium oxide. Its molar mass is calculated as:
So, the number of moles is the given mass divided by the molar mass:
This is where mistakes happen, do not forget the stoichiometry! Since one mole produces two moles, the moles of sodium hydroxide formed will be twice the moles of sodium oxide.
Final Calculation
Now, let's calculate the molarity. Molarity is defined as moles of solute divided by the volume of the solution in liters. Our volume is , which is .
Solving gives us approximately . But wait, the question asks for the answer in the format of something .
So, we write it as:
Rounding to the nearest integer, we get .
The Way Forward
Did you get the feel of it? Think about this: what if the question hadn't told us to neglect the change in volume?
In that case, we would absolutely need the density of the final solution to find the exact volume. Always watch out for such catches in physical chemistry problems!
Similar Questions
JEE Main 2021
LEVELJEE Main
Consider titration of NaOH solution versus oxalic acid solution. At the end point following burette readings were obtained. (i) (ii) (iii) (iv) (v) If the volume of oxalic acid taken was , then the molarity of the NaOH solution is ......... M. (Rounded off to the nearest integer)
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