A balloon contains 5 L of oxygen at a pressure of 90 kPa. How much oxygen will the balloon contain if the pressure is lowered to 67 kPa?

Answer is: volume will be 6,7 L.

Boyle's Law: the pressure volume law - volume of a given amount of gas held varies inversely with the applied pressure when the temperature and mass are constant.

p ₁V₁ = p₂V₂.

90 kPa · 5 L = 67 kPa · V₂.

V₂ = 90 kPa · 5 L / 67 kPa.

V₂ = 6,7 L, but same amount of oxygen.