a student mixed O2, N2, and He gas together in a container. the total pressure of the gas mixture was found to be 1.86 atm. before mixing, the student recorded N2 at 10.8 psi and He at 578 mmHg, but forgot to record O2. Determine the pressure of O2 before mixing (1 atm=14.7 psi = 760mmHg)