We can combine our simple gas laws into one equation: P1 V1 over N1 T1 equals P2 V2 over N2 T2. Let's practice using the combined gas law. Our problem reads, "A tank of nitrogen originally stored at 25°C has a pressure of 2550 PSI and contains 212 moles of nitrogen. To save space in the lab, the tank is moved outside where the winter temperature is -12°C. After 72 moles of gas are removed from the tank for an experiment, what will the pressure be in the tank?" We can start with the combined gas law equation. Any variable that does not change in the problem can be removed. We're not told that the volume of the tank changes, so volume can be removed from the equation. Our resulting equation is P1 over N1 T1 equals P2 over N2 T2. We can solve this equation for P2 by multiplying both sides by N2 T2. When we plug in our numbers, we need to be careful with units and what the variables physically mean in the system. Our initial pressure, P1, is 2550 PSI. N2 is the moles of gas remaining in the tank after 72 moles of gas are removed, so N2 is 212 subtract 72 moles. We need units of Kelvin for temperature, so T2 is -12 plus 273 to convert from degree Celsius to Kelvin. N1 is our initial moles of gas, which is 212 moles. And we can convert T1 into Kelvin by adding 25 plus 273. Completing the calculation, we should get 1475 PSI.