Problem: A 16.0 -L tank is filled with helium gas at a pressure of 105 atm .How many balloons (each 2.00 L) can be inflated to a pressure of 1.00 atm, assuming that the temperature remains constant and that the tank cannot be emptied below 1.00 atm?

We're being asked to determine the number of balloons that can be inflated to a pressure of 1.00 atm

Recall that the ideal gas law is:

$\boxed{\mathbf{PV}\mathbf{=}\mathbf{nRT}}$

The pressure and volume of a gas are related to the number of moles, gas constant and temperature. The value nRT is constant.