Question

Oxygen in the Lungs: In the lungs, the respiratorymembraneseparates tiny sacs of air (pressure1.00x10⁵Pa) from theblood in the capillaries. These sacs are called alveoli. Theaverage radius of thealveoli is 0.125 mm, and the air insidecontains 14% oxygen. Assuming that the air behaves as an ideal gasat 310 K, find thenumber of oxygen molecules in one of thesesacs.

I don't even know where to start. Please help!