1. The problem statement, all variables and given/known data Calculate the mass of a mole of dry air, which is mixture of N2(78 percent by volume, O2 (21 percent) and argon (1 percent). Use the ideal gas law to write the density of air in terms of pressure, temperature , and the average mass of the air molecules. Show , then, that the pressure obeys the differential equation dP/dz=-mgP/kT 2. Relevant equations PV=nkT 3. The attempt at a solution m(total)=.78(28 grams)+.21(16 grams)+.01(40 grams)= 29 grams rho=m/V , from Ideal gas law, V=nKT/P ==> rho=m*P/nkT I am having difficulties with show that dP/dz=-mgP/kT. I know other thread on the ideal gas law problem concerning the same problem I said that dP/dz=-rho*g. and I know that rho=mP/nkT, therefore I guess dP/dz=rho*g=mgP/nkT. The only problem is how would I get rid of n? Should I assume that they are talking about one mole of air and therefore n=1?