What is the total pressure after 1.00 mol of H2 (g), 1 mol of O2(g) and 3 mol of CO2 (g), are injected into a rigid 22.4L container at 273 K?

Volume, V = 22.4 L
No. of moles of gases, n = (1.00 + 1.00 + 3.00) mol = 5.00 mol
Gas constant, R = 0.08206 L atm / (mol K)
Temperature, T = 273 K

Gas law : PV = nRT
Pressure, P = nRT/V = 5.00 × 0.08206 × 273 / 22.4 atm = 5.00 atm

You can think of it this way.....

All are in agreement that there are 5.00 moles of gas molecules. We all know that 1 mol of any ideal gas at STP has a volume of 22.4L. If there are 5.00 moles, still with a volume of 22.4L and standard temperature, then the pressure must be five times greater, so instead of a pressure of 1.00 atm (as in STP), the pressure will be 5.00 atm.