JEE Main 2023 · 1 February · 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) : An aqueous solution of KOH when used for volumetric analysis, its concentration should be checked before the use.
Reason (R) : On aging, KOH solution absorbs atmospheric CO₂.
In the light of the above statements, choose the correct answer from the options given below :
- (a) (A) is correct but (R) is not correct
- (b) (A) is not correct but (R) is correct
- (c) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (d) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Answer and solution
Answer (d) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Solution
Chemistry ma’am · 4 steps
KOH quietly reacts with the CO₂ in air, so the strength on the label drifts. That is why we check it.
- What ageing does. KOH is a strong base. Every time the bottle is opened, it absorbs CO₂ from the air and turns some of its hydroxide into carbonate.
2KOH + CO₂ → K₂CO₃ + H₂O
- Effect on concentration. Two OH⁻ ions are used up for every CO₂ absorbed. So the actual KOH concentration falls below the value it was made up to, and a titration using the old value gives wrong results.
- Link A and R. This is why a KOH solution is standardised, for example against oxalic acid or potassium hydrogen phthalate, just before volumetric use. (R) is true, and it is exactly the reason (A) is needed.
- So. Both statements are correct and (R) correctly explains (A), which is option (d).
The idea to keep. Alkali hydroxides like NaOH and KOH absorb CO₂ and moisture from air, so they are never primary standards. Always standardise their solutions before titrating.
Answer from the NTA final answer key. Solution by PrepWiser.