The acid which forms two series of salt is

Options

A. `H_(3)PO_(4)`
B. `H_(3)PO_(3)`
C. `H_(3)BO_(3)`
D. `H_(3)PO_(2)`

Answer

To determine the acid that forms two series of salts, we need to analyze the basicity of the given acids. Basicity refers to the number of protons (H⁺ ions) that an acid can donate. Let's evaluate the options step by step.

### Step-by-Step Solution:

1. **Understanding Basicity**:
- Basicity of an acid is defined as the number of replaceable hydrogen ions (H⁺) it can donate. An acid with a basicity of 2 can donate two protons, thus forming two series of salts.

2. **Analyzing the Options**:
- **Option 1: H₃PO₄ (Phosphoric Acid)**:
- Structure: It has three hydroxyl (OH) groups.
- Basicity: Since it can donate three protons, it is a tri-basic acid. Therefore, it can form three series of salts.
- Conclusion: This option does not satisfy the requirement as it forms three series of salts.

- **Option 2: H₃PO₃ (Phosphorous Acid)**:
- Structure: It has two hydroxyl (OH) groups and one hydrogen directly attached to phosphorus.
- Basicity: It can donate two protons (the two from the OH groups). Therefore, it is a di-basic acid and can form two series of salts.
- Conclusion: This option satisfies the requirement as it forms two series of salts.

- **Option 3: H₃BO₃ (Boric Acid)**:
- Structure: It has three hydroxyl (OH) groups.
- Basicity: It can donate only one proton, making it a mono-basic acid. Thus, it can form only one series of salts.
- Conclusion: This option does not satisfy the requirement.

- **Option 4: H₃PO₂ (Hypophosphorous Acid)**:
- Structure: It has one hydroxyl (OH) group and two hydrogens directly attached to phosphorus.
- Basicity: It can donate only one proton, making it a mono-basic acid. Therefore, it can form only one series of salts.
- Conclusion: This option does not satisfy the requirement.

3. **Final Conclusion**:
- The acid that forms two series of salts is **H₃PO₃ (Phosphorous Acid)**.

### Final Answer:
The acid which forms two series of salts is **H₃PO₃ (Phosphorous Acid)**.