A sample containing 7.20g O2 gas has a volume of 33.0L Pressure and temperature remain constant. What is the volume after 4.00g He is added to the O2 gas already in the container?

Answer 1

7.2 gO2/32 g/mol = 0.225 moles O2.
I would use PV = nRT and solve for P, then add 4/4 = 1.0 mole He to make a total of 1.225 mol combined gases and plug into PV = nRT again and solve for V.
You also can use a ratio/proportion.
(33/0.225) = (V/1.225) and solve for V.