Which of the following is most effective in coagulating positively charged hydrated ferric oxide sol?

Options

A. `NaNO_3`
B. `Na_2SO_4`
C. `(NH_4)_3PO_4`
D. `LiAlH_4`

Explanation

To solve the question regarding which substance is most effective in coagulating positively charged hydrated ferric oxide sol, we can follow these steps:

### Step 1: Understand the Concept of Coagulation
Coagulation is the process where colloidal particles aggregate to form a larger particle or floc. For coagulation to occur effectively, oppositely charged ions are needed to neutralize the charge of the colloidal particles.

### Step 2: Identify the Charge of the Colloidal Particles
In this case, we are dealing with a positively charged hydrated ferric oxide sol. This means that to coagulate this sol, we need negatively charged ions.

### Step 3: List the Given Options
The options provided are:
1. NO3⁻ (nitrate ion)
2. SO4²⁻ (sulfate ion)
3. PO4³⁻ (phosphate ion)
4. AlH4⁻ (aluminate ion)

### Step 4: Determine the Charge of Each Ion
- NO3⁻ has a charge of -1.
- SO4²⁻ has a charge of -2.
- PO4³⁻ has a charge of -3.
- AlH4⁻ has a charge of -1.

### Step 5: Compare the Negative Charges
According to the Hardy-Schulze rule, the effectiveness of coagulation increases with the charge of the coagulating ion. The more negative the charge, the better it is at coagulating the positively charged sol.

- NO3⁻: -1
- SO4²⁻: -2
- PO4³⁻: -3
- AlH4⁻: -1

### Step 6: Identify the Most Effective Coagulating Ion
Among the options, PO4³⁻ has the highest negative charge (-3). Therefore, it will be the most effective in coagulating the positively charged hydrated ferric oxide sol.

### Conclusion
The most effective ion for coagulating positively charged hydrated ferric oxide sol is **PO4³⁻** (phosphate ion), which corresponds to option 3.