Explain why 'orthophosphoric acid, `H_3 PO_4`, is tribasic but phosphorus acid, `H_3 PO_3`, is dibasic'.

Explanation

To explain why orthophosphoric acid (H₃PO₄) is tribasic while phosphorus acid (H₃PO₃) is dibasic, we can follow these steps:

### Step 1: Understand the Structures of the Acids
- **Orthophosphoric Acid (H₃PO₄)**: The structure consists of one phosphorus atom (P) at the center, bonded to four oxygen atoms. Among these, one oxygen is doubly bonded to phosphorus, and the other three oxygens are part of hydroxyl (–OH) groups. Thus, the structure can be represented as:
```
O
||
HO-P-OH
...