JEE Main 2023 · 10 April · evening · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : 3.1500 g of hydrated oxalic acid dissolved in water to make 250.0 mL solution will result in 0.1 M oxalic acid solution.
Reason R : Molar mass of hydrated oxalic acid is 126 g mol⁻¹
In the light of the above statements, choose the correct answer from the options given below.
- (a) Both A and R are true but R is NOT the correct explanation of A
- (b) A is true but R is false
- (c) Both A and R are true and R is the correct explanation of A
- (d) A is false but R is true
Answer and solution
Answer (c) Both A and R are true and R is the correct explanation of A
Solution
Chemistry ma’am · 4 steps
Work out the molar mass of the dihydrate first. That one number is what makes the molarity come out to 0.1.
- Check Reason R. Hydrated oxalic acid is H₂C₂O₄.2H₂O. The acid part is 2(1) + 2(12) + 4(16) = 90, and the two waters add 2 × 18 = 36. So the molar mass is 126 g mol⁻¹, and R is true.
- Moles dissolved. n = 3.15/126 = 0.025 mol of oxalic acid. Each formula unit gives one oxalic acid molecule, so this is also the number of moles of acid in solution.
M = n/V = (0.025 mol)/(0.250 L) = 0.1 mol L⁻¹
- Check Assertion A. The solution is 0.1 M, so A is true. The value 126 is exactly what turns 3.15 g into 0.025 mol. This means R is the reason A works out.
- So. Both statements are true and R correctly explains A, which is option (c).
The idea to keep. For a hydrated salt, always include the water of crystallisation in the molar mass. Oxalic acid dihydrate is 126 g mol⁻¹, not 90. That is why 3.15 g in 250 mL makes a 0.1 M solution.
Answer from the NTA final answer key. Solution by PrepWiser.