a mixture of gases contains 0.4 mole of N2, 0.5 moles of O2, and 0.7 moles of H2. What is the partial pressure of O2 if the total pressure is 1.3 atm?

Answer 1

There are several ways to do this. Probably the easiest is to use mole fractions.
total moles = 0.4 + 0.5 + 0.7 = 1.6 moles
mole fraction O2 = moles O2/total moles.
P_O2 = X_O2*P_total