Answer:The volume of the gas at 0∘C and800 mmHg is 19.38 liters._______________________________The combined gas law is [tex]\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}[/tex].Since $T_1 = T_2$, the temperature is constant. The combined gas law simplifies to Boyle's Law:$P_1 V_1 = P_2 V_2$Solve for $V_2$:[tex] \tt \scriptsize{(760\text{ mmHg})(20.4\text{ L}) = (800\text{ mmHg})(V_2)}[/tex][tex]V_2 = \frac{(760\text{ mmHg})(20.4\text{ L})}{800\text{ mmHg}}[/tex][tex]V_2 = \frac{760 \times 20.4}{800}\text{ L}[/tex][tex]V_2 = \frac{15504}{800}\text{ L}[/tex][tex]V_2 = 19.38\text{ L}[/tex]