Question

In each of four separate beakers (I, II, III, IV), X mL of 1M Fe$_2$O$_3$·xH$_2$O colloidal solution is present.
Equal volume and equal concentration of KCl, K$_4$[Fe(CN)$_6$], K$_3$PO$_4$ and K$_2$SO$_4$ was added into I, II, III and IV respectively.
The efficiency of precipitations in these beakers follows the order

• A. II > III > IV > I
• B. II > III > I > IV
• C. I > IV > III > II
• D. III > IV > I > II

Hint:

Remember Hardy–Schulze rule: The coagulating power of an ion is directly proportional to the valency of the ion when it is oppositely charged to the colloid.

Answer 1

The Correct Option is A

Step 1: Apply Hardy–Schulze rule. 
According to this rule, the coagulating power of an electrolyte increases with the valency of the oppositely charged ion. 
Step 2: Identify the charge on Fe$_2$O$_3$·xH$_2$O. 
This colloid is positively charged, so we need to focus on the anions of the added salts. 
Step 3: List anions and their valency: KCl → Cl$^-$ (valency = 1) K$_4$[Fe(CN)$_6$] → [Fe(CN)$_6$]$^{4-}$ (valency = 4) K$_3$PO$_4$ → PO$_4^{3-}$ (valency = 3) K$_2$SO$_4$ → SO$_4^{2-}$ (valency = 2) 
Step 4: Order of coagulation power: 
Higher the valency, greater the precipitation: 
[Fe(CN)$_6$]$^{4-}$ > PO$_4^{3-}$ > SO$_4^{2-}$ > Cl$^-$ So, order of beakers is: II > III > IV > I