Knowing that work is distance times force, and that pressure is force per area, consider a piston cylinder moving a distance h against an opposing force due to the external pressure of the air above it.
a) Show that the work done by the piston is -Pext(delta)V. You may want to use a diagram.
b) Exhaling during breathing is doing work since air is moved against atmospheric pressure. Calculate the work done in J when 0.500L of air is exhaled into an empty balloon at an atmospheric pressure of 1.00 atm.