The RQ is `"___________"` if respiratory substrate is oxalic acid.

Answer

To determine the respiratory quotient (RQ) when the respiratory substrate is oxalic acid, follow these steps:

1. **Understand the Definition of RQ**: RQ, or respiratory quotient, is defined as the ratio of the volume of carbon dioxide (CO2) produced to the volume of oxygen (O2) consumed during respiration. The formula is:
\[ RQ = \frac{\text{Volume of CO2 produced}}{\text{Volume of O2 consumed}} \]

2. **Identify the Respiratory Substrate**: In this case, the respiratory substrate is oxalic acid, which has the molecular formula C2H2O4.

3. **Determine the Products of Oxidation**: When oxalic acid is oxidized, it reacts with oxygen to produce carbon dioxide and water. The complete oxidation of oxalic acid yields:
- 4 molecules of CO2
- 2 molecules of H2O

4. **Calculate the Oxygen Consumed**: For the complete oxidation of oxalic acid, it is known that 1 molecule of oxalic acid will consume 2 molecules of oxygen. Therefore, for the oxidation of oxalic acid, the amount of oxygen consumed is:
- 2 molecules of O2

5. **Calculate the RQ**: Now, substitute the values into the RQ formula:
\[ RQ = \frac{4 \text{ (CO2 produced)}}{2 \text{ (O2 consumed)}} = 2 \]

6. **Conclusion**: Therefore, the RQ when the respiratory substrate is oxalic acid is 2.

**Final Answer**: The RQ is **2** if the respiratory substrate is oxalic acid.