Boyle's law, which states that at constant temperature, gas volume is inversely proportional to pressure, can solve this problem. The formula is:
\[
P_1 V_1 = P_2 V_2
\]
Where:
- \( P_1 = 1.0 \, \text{atm} \) is the initial pressure,
- \( V_1 = 5.0 \, \text{L} \) is the initial volume,
- \( P_2 = 2.0 \, \text{atm} \) is the final pressure,
- \( V_2 \) is the final volume to be determined.
To find \( V_2 \), rearrange the formula:
\[
V_2 = \frac{P_1 V_1}{P_2}
\]
Substituting the known values:
\[
V_2 = \frac{1.0 \times 5.0}{2.0} = 2.5 \, \text{L}
\]
Therefore, the gas volume at 2.0 atm pressure is \( 2.5 \, \text{L} \).