How many moles are in a solution with a concentration of 5 M and a volume of 0.25 L?

Calculate the partial pressure, in atmospheres, of o2 in the dry air outside an airliner cruising at an altitude of about 20000 ft (6096 m), where the atmoshperic pressure is 351 mm hg. how much must the outside air be compressed to produce a cabin pressure in which the partial pressure of o2 is 0.200 atm?