The maximum pressure of $CO_2$ in the container is determined by the equilibrium constant $K_p$ of the given reaction. For the reaction: $SrCO_3(s) \rightleftharpoons SrO(s) + CO_2(g)$ $K_p = p_{CO_2} = 1.6\text{ atm}$. Thus, the maximum pressure of $CO_2$ that can be attained is $1.6\text{ atm}$. Applying Boyle's law ($p_1V_1 = p_2V_2$) to find the volume at which this maximum pressure is reached: $0.4\text{ atm} \times 20\text{ L} = 1.6\text{ atm} \times V_2$ $V_2 = \frac{0.4 \times 20}{1.6} = 5\text{ L}$.