Total number of solutions from the following which has `pHlt7` at `25^(@)C`.
`(K_(a)(CH_(3)COOH)=1.8xx10^(-5),K_(b)(NH_(4)OH)=1.8xx10^(-5))`
(i) `10^(-8) MHCl`
(ii) `0.01 M` solution of `NH_(4)Cl`
(iii) `0.01 M` solution of `CH_(3)COONa`
(iv) `0.01M` solution is `B(OH)_(3)`
(v) `0.01M` solution of `CH_(3)COONH_(4)`

Explanation

To determine the total number of solutions that have a pH less than 7 at 25°C, we will analyze each solution provided in the question.

### Step-by-Step Solution:

1. **Analyzing HCl:**
- HCl is a strong acid that completely dissociates in solution.
- Therefore, any concentration of HCl will result in a pH less than 7.
- **Conclusion:** HCl contributes to a pH < 7.

2. **Analyzing NH4Cl:**
- NH4Cl is a salt formed from the weak base NH4OH and the strong acid HCl.
- In solution, NH4+ ions will hydrolyze to produce H+ ions, making the solution acidic.
- Therefore, the pH will be less than 7.
- **Conclusion:** NH4Cl contributes to a pH < 7.

3. **Analyzing CH3COONa:**
- CH3COONa is a salt formed from the weak acid acetic acid (CH3COOH) and the strong base NaOH.
- In solution, the acetate ion (CH3COO-) will hydrolyze to produce OH- ions, resulting in a basic solution.
- Therefore, the pH will be greater than 7.
- **Conclusion:** CH3COONa does not contribute to a pH < 7.

4. **Analyzing B(OH)3:**
- Boric acid (B(OH)3) acts as a weak acid in solution.
- It can accept OH- ions, leading to the production of H+ ions, thus making the solution acidic.
- Therefore, the pH will be less than 7.
- **Conclusion:** B(OH)3 contributes to a pH < 7.

5. **Analyzing CH3COONH4:**
- This salt is formed from acetic acid and ammonium hydroxide.
- The K_a of CH3COOH is equal to K_b of NH4+, which means that the solution will be neutral.
- Therefore, the pH will be equal to 7, not less than 7.
- **Conclusion:** CH3COONH4 does not contribute to a pH < 7.

### Summary of Contributions:
- HCl: pH < 7 (Yes)
- NH4Cl: pH < 7 (Yes)
- CH3COONa: pH > 7 (No)
- B(OH)3: pH < 7 (Yes)
- CH3COONH4: pH = 7 (No)

### Total Count of Solutions with pH < 7:
Thus, the total number of solutions with pH less than 7 is **3** (HCl, NH4Cl, and B(OH)3).

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