Question

Calculate the final pressure on the addition of 2.0 g He(g) to a vessel of 10.0 L fixed volume already containing O2(g) at 25°C and 740 mm Hg

Answer 1

Use PV = nRT and solve for n = mols oxygen at the conditions listed.
Calculate mols from 2.0 g He, add to original mols O2 and recalculate final pressure from PV = nRT

Answer 2

2.19atm