To save some time (skipping the concentration part of the equation), because 1 mole of khco3 will break down to 1 mole of CO2 = 22.4L. That 22.4L is reduced by a 1/5 when the dude takes 100ml out of the 500 ml. In other words the 1 mole of CO2 in the 500 mL is reduced to a certain # of moles in the 100mL. Going from 500 ---> 100 is 1/5. So reducing the 1 mole or 22.4L (at stp) by a 5th is: 22.4/5 is around 5L.