Chemistry
posted by Wolfgang .
Imagine that you have a 5.00 L gas tank and a 4.00 L gas tank. You need to fill one tank with oxygen and the other with acetylene to use in conjunction with your welding torch. If you fill the larger tank with oxygen to a pressure of 105 atm, to what pressure should you fill the acetylene tank to ensure that you run out of each gas at the same time? Assume ideal behavior for all gases

2C2H2 + 5O2 ==> 4CO2 + 2H2O
The long way is to use PV = nRT and solve for n= number of mols O2 in the 5L tank. Convert that to mols C2H2 used and resubmit to PV = nRT and solve for P.(Note that you don't have a T listed but you can make up one and use it for both sets of data.)
The shorter way is to see that
PV= nRT can be modified to
PVk = n
(V1/V2) x (2 mol C2H2/5 mols O2) = ? 
0.4444