Work done during an irreversible expansion against a constant external pressure is given by the formula: $W = -p_{ex}\Delta V = -p_{ex}(V_f - V_i)$

Given: External pressure, $p_{ex} = 2 \text{ bar}$ Initial volume, $V_i = 0.1 \text{ L}$ Final volume, $V_f = 0.25 \text{ L}$

Substituting the values into the equation: $W = -2 \text{ bar} \times (0.25 \text{ L} - 0.1 \text{ L})$ $W = -2 \text{ bar} \times 0.15 \text{ L} = -0.30 \text{ L bar}$

To convert the work done into Joules, we use the conversion factor $1 \text{ L bar} = 100 \text{ J}$: $W = -0.30 \times 100 \text{ J} = -30 \text{ J}$

Thus, the work done by the gas is $-30 \text{ J}$.