There was a question which was asked in JEE Main 2020 held on 8th January. The question was:

As per Hardy–Schulze formulation, the flocculation values of the following for ferric hydroxide sol are in the order :

(1) $\ce{K3[Fe(CN)6] < K2CrO4 < KBr = KNO3 = AlCl3 }$

(2) $\ce{K3[Fe(CN)6] < K2CrO4 < AlCl3 < KBr < KNO3}$

(3) $\ce{K3[Fe(CN)6] > AlCl3 > K2CrO4 > KBr > KNO3}$

(4) $\ce{AlCl3 > K3[Fe(CN)6] > K2CrO4 > KBr = KNO3}$

Answer given by the offocial body was (1).

Yes, Hardy-Schulze rule does say that for a positive sol as the charge of the anion in the salt increases the flocculating value decreases(if the charges of anions in 2 salts are different then does not depend on the number of negative ions per molecule of salt and the rule still holds good). But I have never read anywhere that if the charge on the anions is the same then the flocculating values are equal.

Shouldn't the flocculating value of AlCl3 be less than flocculating value of KBr and KNO3? because it will give 3 anions per molecule whereas the other two ( KBr and KNO3) give only 1.


1 Answer 1


The Joint Entrance Examination board should update their outdated set of questions relying on rote memorization. Why do they assume the student has to memorize the charge of the ferric hydroxide particles and in what medium? Charge on the colloids is dependent on the medium and pH! They should also mention that the molar concentration of the anions is the same. With this information, the question becomes complete.

If the zeta potential of ferric hydroxide is experimentally found to be positive, then the anion should affect them. So, notice the charge on the anion in (1): (-3), (-2), and then monovalents.


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