An ideal gas is allowed to expand from 1 L to 10 L against a constant external pressure of 1 bar. The work done in kJ…

🧠 Work against constant external pressure: $w = -P_{\text{ext}} \cdot \Delta V$ The gas expands against a fixed opposing pressure, so use the irreversible work formula — not $nRT\ln(V_f/V_i)$.

🗺️ Calculation $w = -P_{\text{ext}} \times (V_f - V_i) = -1\text{ bar} \times (10 - 1)\text{ L} = -9\text{ L.bar}$

Convert using $1\text{ L.bar} = 0.1\text{ kJ}$: $w = -9 \times 0.1 = -0.9\text{ kJ}$

Negative sign → work is done by the system (expansion).

⚠️ Trap: Don't use the reversible work formula $nRT\ln(V_f/V_i)$ — that applies only to reversible isothermal expansion. This is expansion against a constant external pressure.

$\boxed{\text{Answer: (c)}}$