A gas sample in a balloon contains 4.0 moles of gas at a particular pressure, volume, and temperature. If your balloon springs a leak, and the volume decreases to 1/2 of the original amount before you can plug the leak, how many moles of gas have you lost. The temperature and pressure remain the same. A sample of o2 gas was collected over H2O in a 550ml container at 21C and at a barometric pressure of 743 torr. The vapor pressure of water is 19.0 torr at that temp. How many moles of O2 were collected?