The volume of a given mass of a gas with some pieces of marble in a container at 750 mm Hg pressure is 100 mL. If the pressure is changed to 1000 mm Hg, the new volume is 80 mL. Find the volume occupied by the marble pieces, if the temperature remains constant.

Explanation

To solve the problem, we will use Boyle's Law, which states that for a given mass of gas at constant temperature, the product of pressure and volume is constant. This can be expressed mathematically as:

\[ P_1 V_1 = P_2 V_2 \]

### Step-by-Step Solution:

1. **Identify Given Values:**
- Initial pressure, \( P_1 = 750 \, \text{mm Hg} \)
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