The ionisation cosntabnt of an acid, `K_(a)` is the meaure of strength of an acid. The `K_(a)` values of acetic acid, hypochlorous acid and formic acid are `1.74xx10^(-5)`, `3.0xx10^(-8)` and `1.8xx10^(-4)` respectively. Which of the following orders of ph of `0.1` mol `dm^(-3)` solutions of these acids is correct ? (A)Acetic acid > Hypochlorous acid > Formic acid (B)Hypochlorous acid > Acetic acid > Formic acid (C)Formic acid > Hypochlorous acid > Acetic acid (D)Formic acid > Acetic acid > Hypochlorous acid

Options

A. Acetic acid `gt` Hypochlorous acid `gt` Formic acid
B. Hypochlorous acid `gt` Acetic acid `gt` Formic acid
C. Formic acid `gt` Hypochlorous acid `gt` Acetic acid
D. Formic acid `gt` Acetic acid `gt` Hypochlorous acid

Explanation

To solve the problem, we need to determine the order of pH for 0.1 mol/dm³ solutions of acetic acid, hypochlorous acid, and formic acid based on their ionization constants (Ka values). The higher the Ka value, the stronger the acid, which means it will have a lower pH.

### Step-by-step Solution:

1. **Identify the Ka Values:**
- Acetic acid (CH₃COOH): \( K_a = 1.74 \times 10^{-5} \)
- Hypochlorous acid (HClO): \( K_a = 3.0 \times 10^{-8} \)
- Formic acid (HCOOH): \( K_a = 1.8 \times 10^{-4} \)

2. **Determine Acid Strength:**
- The strength of an acid is directly proportional to its Ka value. Thus, we can rank the acids based on their Ka values:
- Formic acid (HCOOH) has the highest Ka, making it the strongest acid.
- Acetic acid (CH₃COOH) is next.
- Hypochlorous acid (HClO) has the lowest Ka, making it the weakest acid.

3. **Rank the Acids by Strength:**
- From strongest to weakest:
1. Formic acid (HCOOH)
2. Acetic acid (CH₃COOH)
3. Hypochlorous acid (HClO)

4. **Relate Acid Strength to pH:**
- Since stronger acids have lower pH values, we can conclude:
- Formic acid will have the lowest pH.
- Acetic acid will have a higher pH than formic acid but lower than hypochlorous acid.
- Hypochlorous acid will have the highest pH.

5. **Establish the Order of pH:**
- Therefore, the order of pH from lowest to highest is:
1. Formic acid (lowest pH)
2. Acetic acid
3. Hypochlorous acid (highest pH)

6. **Select the Correct Option:**
- The correct order of pH is:
- Formic acid > Acetic acid > Hypochlorous acid
- This matches option (D).

### Final Answer:
The correct order of pH for 0.1 mol/dm³ solutions of these acids is:
**(D) Formic acid > Acetic acid > Hypochlorous acid**