The species which does not have a conjugate base is

Options

A. `H_(3)PO_(4)`
B. `H_(2)PO_(3)^(-)`
C. `H_(2)PO_(2)^(-)`
D. `HPO_(4)^(2-)`

Answer

To determine which species does not have a conjugate base, we need to understand the concept of conjugate acids and bases. A conjugate base is formed when an acid donates a proton (H⁺). Therefore, for a species to have a conjugate base, it must contain at least one acidic hydrogen that can be released as a proton.

Let's analyze the given species one by one:

1. **H₃PO₄ (Phosphoric Acid)**:
- Structure: H₃PO₄ has three acidic hydrogens.
- Upon dissociation, it can lose one proton to form H₂PO₄⁻, which is its conjugate base.
- Conclusion: H₃PO₄ has a conjugate base.

2. **H₂PO₃⁻ (Dihydrogen Phosphite Ion)**:
- Structure: H₂PO₃⁻ has two acidic hydrogens.
- It can lose one proton to form HPO₃²⁻, which is its conjugate base.
- Conclusion: H₂PO₃⁻ has a conjugate base.

3. **H₂PO₂⁻ (Dihydrogen Phosphinate Ion)**:
- Structure: H₂PO₂⁻ has two hydrogens, but we need to check if they are acidic.
- Upon analysis, it turns out that this species does not have any acidic hydrogen that can be released as a proton.
- Conclusion: H₂PO₂⁻ does not have a conjugate base.

4. **HPO₄²⁻ (Hydrogen Phosphate Ion)**:
- Structure: HPO₄²⁻ has one acidic hydrogen.
- It can lose this proton to form PO₄³⁻, which is its conjugate base.
- Conclusion: HPO₄²⁻ has a conjugate base.

From the analysis, we can conclude that the species that does not have a conjugate base is **H₂PO₂⁻**.

### Summary of the Solution:
- H₃PO₄ has a conjugate base.
- H₂PO₃⁻ has a conjugate base.
- H₂PO₂⁻ does not have a conjugate base.
- HPO₄²⁻ has a conjugate base.
- Therefore, the answer is H₂PO₂⁻.