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 (temperature and pressure remain the same)?

Explanation:

When volume of gas is V , pressure is P and temperature is T , if n be number of moles of gas

PV / RT = n , R is universal gas constant .

When volume becomes .5 V , pressure P and temperature T

number of moles = Px .5 V / RT = .5 PV / RT = .5 n

Number of moles of gas becomes half .

so 4 moles becomes  2 moles

No of moles lost = 4 - 2 = 2 moles.