During a breath, the average human lung expands by about 0.50 l. part a if this expansion occurs against an external pressure of 1.0 atm, how much work (in j) is done during the expansion?

Work, in thermodynamics, is the amount of energy that is transferred from one system to another system without transfer of entropy. It is equal to the external pressure multiplied by the change in volume of the system. It is expressed as follows:

W = PdV

Integrating and assuming that P is not affected by changes in V or it is constant, then we will have:

W = P (V2 - V1)

Substituting the given values:
P = 1.0 atm = 101325 Pa
(V2 - V1) = 0.50 L = 
W = 101325 N/m^3 ( 0.50) (1/1000) m^3
W = 50.66 N-m or 50.66 J

So, in the expansion process about 50.66 J of work is being done.