Question

For the coagulation of positively charged hydrated ferric-oxide sol, the flocculating power of the ions is in the order:

• A. \( \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^- > [\text{Fe(CN)}_6]^{4-} \)
• B. \( \text{Cl}^- > \text{SO}_4^{2-} > \text{PO}_4^{3-} > [\text{Fe(CN)}_6]^{4-} \)
• C. \( \text{SO}_4^{2-} = \text{Cl}^- = \text{PO}_4^{3-} = [\text{Fe(CN)}_6]^{4-} \)
• D. \( [\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^- \)

Hint:

Flocculating power increases with ion charge per the Schulze-Hardy rule.

Answer 1

The Correct Option is D

The question asks about the flocculating power of different ions for the coagulation of positively charged hydrated ferric-oxide sol. To answer this, we need to understand the concept of flocculation and the dependence on the charge of the coagulant ions.

Coagulation is the process where charged particles in a colloidal solution come together to form larger particles, leading to sedimentation. The efficiency of an ion to cause coagulation is determined by its valency or charge, which is explained by Schulze-Hardy rule:

Schulze-Hardy Rule: The greater the charge (valency) of the ion, the greater is its ability to cause coagulation of a sol containing particles of opposite charge. 

In this case, we have the following ions with their respective charges:

• \(\text{PO}_4^{3-}\): Tri-negative ion (valency = 3)
• \(\text{SO}_4^{2-}\): Di-negative ion (valency = 2)
• \(\text{Cl}^{-}\): Mono-negative ion (valency = 1)
• \([\text{Fe(CN)}_6]^{4-}\): Tetra-negative ion (valency = 4)

According to the Schulze-Hardy rule, the flocculating power of ions is proportional to the fourth power of its valency. Therefore, ions with higher valency will have greater flocculating power.

Given the ions and their charges, the order of flocculating power is:

• \([\text{Fe(CN)}_6]^{4-}\) (highest flocculating power due to highest negative charge)
• \(\text{PO}_4^{3-}\) (next highest due to tri-negative charge)
• \(\text{SO}_4^{2-}\) (less flocculating power due to di-negative charge)
• \(\text{Cl}^-\) (least flocculating power due to mono-negative charge)

Thus, the correct order of flocculating power for the given ions is:

\(\([\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^-\)

Hence, the correct answer is:

\( [\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^- \)