The average lung capacity of an adult male human is 6 liters. The composition of air gives the percentage of oxygen as 20%. Of the 6 liters that are inhaled the volume of oxygen is 6*0.2 = 1.2 liters.

The pressure is given as 90 kPa and the temperature is 22 C or 295.15 K. Use the ideal gas law P*V = n*R*T where P is pressure, V is the volume, T is the temperature, n is the amount of gas in moles and R is a constant. This gives:

1.2*90 = n*295.15*8.314

=> `n ~~ 0.44` mole

The molar mass of oxygen is 32. This gives the approximate mass of oxygen in grams as 1.4 g.

**Further Reading**