1. The problem statement, all variables and given/known data To what pressure must a piece of equipment be evacuated in order that there be only 10^8 molecules per ml at 25 degrees celcius 2. Relevant equations Ideal gas equation = PV = nRT Charles's Law = (V2/T2 = V1/T1) Avagadro's number (6.02 * 10^23) I know that at STP one mole of gas is 22.4L 3. The attempt at a solution I don't know where to begin but what I'm trying to do is find how many moles are in 10^8 molecules. Then use the Ideal Gas Equation to figure out the volume at STP and then using Charles's Law, figure out the pressure at 25 degrees or 298K.