Any species capable of accepting a lone pair of electrons acts as a lewis acid, where as a species capable of donating a proton acts as a Bronstead acid.
Which of the following can not act as a Bronstead acid is aqueous medium?

Options

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

Answer

To determine which of the given species cannot act as a Brønsted acid in an aqueous medium, we need to analyze each option based on their ability to donate protons (H⁺ ions).

### Step-by-Step Solution:

1. **Understanding Brønsted Acids**:
- A Brønsted acid is defined as a species that can donate a proton (H⁺) to another species.

2. **Analyzing Each Option**:
- **Option 1: S2O7²⁻ (Disulfate ion)**:
- This species can donate protons, thus it can act as a Brønsted acid.

- **Option 2: H2PO2⁻ (Hypophosphite ion)**:
- This species does not readily donate protons; it acts more like a base. Therefore, it cannot act as a Brønsted acid.

- **Option 3: H2PO3⁻ (Phosphorous acid)**:
- This species can donate protons, so it can act as a Brønsted acid. It is also amphoteric, meaning it can act as both an acid and a base.

- **Option 4: H2PO4⁻ (Dihydrogen phosphate ion)**:
- This species is a weak acid and can donate protons, thus it acts as a Brønsted acid.

3. **Conclusion**:
- Among the given options, **H2PO2⁻** is the only species that cannot act as a Brønsted acid in an aqueous medium.

### Final Answer:
- The species that cannot act as a Brønsted acid in aqueous medium is **H2PO2⁻**.