JEE Main 2023 · 10 April · morning · Single correct · +4 / −1
Given below are two statements:
Statement I : Aqueous solution of K₂Cr₂O₇ is preferred as a primary standard in volumetric analysis over Na₂Cr₂O₇ aqueous solution.
Statement II : K₂Cr₂O₇ has a higher solubility in water than Na₂Cr₂O₇.
In the light of the above statements, choose the correct answer from the options given below:
- (a) Statement I is true but Statement II is false
- (b) Both Statement I and Statement II are false
- (c) Statement I is false but Statement II is true
- (d) Both Statement I and Statement II are true
Answer and solution
Answer (a) Statement I is true but Statement II is false
Solution
Chemistry ma’am · 4 steps
A primary standard must be weighed exactly, and sodium dichromate keeps pulling water out of the air.
- Primary standard needs. A primary standard is weighed out and dissolved to make a solution of exactly known strength. So it must be pure, stable, and not hygroscopic. If it absorbs water from the air, the weighed mass is partly water and the concentration is wrong.
Property K₂Cr₂O₇ Na₂Cr₂O₇ Takes up moisture from air No, stays dry Yes, hygroscopic Solubility in water Lower Much higher Can be weighed accurately Yes No - Statement I. K₂Cr₂O₇ stays dry and weighs true, while Na₂Cr₂O₇ absorbs moisture. So the potassium salt is the one used as the primary standard. Statement I is true.
- Statement II. It is the other way round: K₂Cr₂O₇ is less soluble than Na₂Cr₂O₇. That is why adding KCl to a sodium dichromate solution makes K₂Cr₂O₇ crystallise out. Statement II is false.
- So. Statement I is true and Statement II is false, which is option (a).
The idea to keep. For a primary standard, ask one question first: can I weigh it exactly? Hygroscopic salts like Na₂Cr₂O₇ and NaOH fail. And remember that K₂Cr₂O₇ is the less soluble dichromate.
Answer from the NTA final answer key. Solution by PrepWiser.