Consider the same expansion, but this time against a constant external pressure of 1 atm.

Explanation

To solve the problem of isothermal expansion of an ideal gas against a constant external pressure of 1 atm, we can follow these steps:

### Step 1: Understand the System
We have an ideal gas that is expanding isothermally from an initial volume \( V_1 = 2 \) liters to a final volume \( V_2 = 10 \) liters against a constant external pressure \( P_{ext} = 1 \) atm.

### Step 2: Apply the First Law of Thermodynamics
The first law of thermodynamics states:
\[
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