Define an isolated system. Three moles of an ideal gas are expanded isothermally from 15 `dm^3` to 20 `dm^3` at constant external pressure of 1.2 bar, calculate the amount of work in Joules.

Answer

### Step-by-Step Solution:

1. **Define an Isolated System**:
An isolated system is defined as a physical system that does not allow the transfer of energy or mass to or from its surroundings. In other words, it is completely insulated from its environment.

2. **Identify Given Data**:
- Number of moles of gas (n) = 3 moles
- Initial volume (V1) = 15 dm³
- Final volume (V2) = 20 dm³
- External pressure (P_ext) = 1.2 bar
- The process is isothermal (temperature remains constant).

3. **Calculate the Change in Volume (ΔV)**:
\[ \Delta V = V2 - V1 = 20 \, \text{dm}^3 - 15 \, \text{dm}^3 = 5 \, \text{dm}^3 \]

4. **Calculate the Work Done (W)**:
The work done by the gas during expansion at constant external pressure is given by the formula:
\[ W = -P_{\text{ext}} \Delta V \]
Substituting the values:
\[ W = -1.2 \, \text{bar} \times 5 \, \text{dm}^3 \]
\[ W = -6 \, \text{dm}^3 \cdot \text{bar} \]

5. **Convert Work from dm³·bar to Joules**:
We know that:
\[ 1 \, \text{dm}^3 \cdot \text{bar} = 100 \, \text{J} \]
Therefore,
\[ W = -6 \, \text{dm}^3 \cdot \text{bar} \times 100 \, \text{J/dm}^3 \cdot \text{bar} = -600 \, \text{J} \]

6. **Final Result**:
The amount of work done during the isothermal expansion is:
\[ W = -600 \, \text{J} \]