In the combined gas law, we examine the relationship between pressure, volume, moles of gas, and temperature, saying P1V1/N1T1 equals P2V2/N2T2. This is essentially saying that pressure times volume divided by moles of gas and temperature in Kelvin is a constant, which we can call R. The ideal gas law uses this constant and rearranges the equation slightly, to P times V equals the ideal gas law constant, R, times moles of gas times temperature in Kelvin. The units of the ideal gas law constant dictate the units we need in the equation. We need to use liters for volume, atmosphere for pressure, moles for moles of gas, and Kelvin for temperature. When we do, the ideal gas law constant is 0.08206 liter atmospheres per mole Kelvin. Let's practice with the ideal gas law. Our problem reads, "Calculate the moles of gas in your tire at 30°C if the volume of the tire is 12 liters and the pressure of air in the tire is 35 PSI." We'll start with the ideal gas law equation. We can solve the equation for moles of gas. We have to be careful of units in this problem. Pressure must be converted to atmospheres by multiplying 35 PSI by 1 atmosphere per 14.7 PSI. Temperature must be converted to Kelvin by adding 273. If we look at our units, PSI cancels out, atmospheres cancel out, liters cancel out, and Kelvin cancels out; we're left with moles. Completing our calculation, we get 1.15 moles.