Which of the following is the strongest acid:

Options

A. `HCOOH `
B. `CH_3COOH`
C. `CH_3 CH_2 COOH`
D. `(CH_3)_2 CHCOOH`

Answer

To determine which of the given compounds is the strongest acid, we need to analyze the stability of their conjugate bases. The stronger the conjugate base, the weaker the acid, and vice versa. The stability of the conjugate base is influenced by the substituents attached to the acidic hydrogen.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- A: HCOOH (formic acid)
- B: CH3COOH (acetic acid)
- C: CH3CH2COOH (propanoic acid)
- D: (CH3)2CHOH (tert-butanol)

2. **Write the Conjugate Bases**:
- For A: HCOOH → HCOO⁻ (formate ion)
- For B: CH3COOH → CH3COO⁻ (acetate ion)
- For C: CH3CH2COOH → CH3CH2COO⁻ (propionate ion)
- For D: (CH3)2CHOH → (CH3)2CHO⁻ (tert-butoxide ion)

3. **Analyze the Stability of the Conjugate Bases**:
The stability of the conjugate base is crucial in determining the strength of the acid. The more stable the conjugate base, the stronger the acid.
- **Formate Ion (HCOO⁻)**: The negative charge is delocalized over the two oxygen atoms, which stabilizes the ion.
- **Acetate Ion (CH3COO⁻)**: The negative charge is also delocalized over the two oxygen atoms, but the presence of the methyl group (CH3) exerts a +I effect, which slightly destabilizes the ion.
- **Propionate Ion (CH3CH2COO⁻)**: Similar to acetate, but the ethyl group (CH3CH2) has a stronger +I effect than the methyl group, further destabilizing the ion.
- **tert-Butoxide Ion ((CH3)2CHO⁻)**: The negative charge is destabilized by the presence of three methyl groups, which exert a strong +I effect.

4. **Compare the Acidity**:
- Formate ion is the most stable due to effective resonance stabilization.
- Acetate ion is less stable than formate due to the +I effect of the methyl group.
- Propionate ion is even less stable due to the stronger +I effect from the ethyl group.
- Tert-butoxide ion is the least stable due to the destabilizing effect of three methyl groups.

5. **Conclusion**:
Based on the stability of the conjugate bases, we conclude that **Option A (HCOOH)** is the strongest acid among the given options.

### Final Answer:
**The strongest acid is Option A (HCOOH).**