I know P is the pressure, but is it the outside pressure or inside pressure? I know that in equilibrium, it doesn't matter, but then in this problem I got confused:

193 mL of O2 was collected over water on a day when the atmospheric pressure was 762 mmHg. The temperature of the water was 23.0 o C. How many grams of oxygen were collected?

How would you do that?

  • $\begingroup$ It is the pressure inside the volume of trapped gas, and also the pressure of the surrounding atmosphere (neglecting the tiny hydrostatic pressure difference between the trapped gas and the atmosphere). $\endgroup$ – Chet Miller Dec 4 '16 at 19:37