What is the correct order of pKa?

Options

A. `CH_(4) gt NH_(3) gt H_(2)O gt HF `
B. `NH_(3) gt CH_(4) gt H_(2)O gt HF`
C. `H_(2)O gt CH_(4) gt NH_(3) gt HF`
D. `HF gt CH_(4) gt NH_(3) gt H_(2)O `

Explanation

To determine the correct order of pKa for the compounds CH4, NH3, H2O, and HF, we need to analyze their acidity. The pKa value is inversely related to acidity; the lower the pKa, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Central Atoms**:
- CH4 (Methane) - Central atom: Carbon
- NH3 (Ammonia) - Central atom: Nitrogen
- H2O (Water) - Central atom: Oxygen
- HF (Hydrofluoric acid) - Central atom: Fluorine

2. **Understand Electronegativity Trends**:
- Electronegativity increases from left to right across a period in the periodic table.
- The more electronegative an atom, the more it can stabilize the negative charge of the conjugate base, making the acid stronger.

3. **Rank the Compounds Based on Electronegativity**:
- Fluorine (F) is the most electronegative, followed by Oxygen (O), then Nitrogen (N), and finally Carbon (C).
- Therefore, the order of acidity based on electronegativity is: HF > H2O > NH3 > CH4.

4. **Determine the Corresponding pKa Values**:
- Since stronger acids have lower pKa values, we can infer the following order:
- HF has the lowest pKa (strongest acid).
- H2O has a higher pKa than HF.
- NH3 has a higher pKa than H2O.
- CH4 has the highest pKa (weakest acid).

5. **Final Order of pKa Values**:
- The order of pKa from lowest to highest is:
- HF < H2O < NH3 < CH4

### Conclusion:
The correct order of pKa values is:
- pKa(HF) < pKa(H2O) < pKa(NH3) < pKa(CH4)