Problem: If we approximate the atmosphere to be 79% nitrogen(N2) by volume and 2

Question

Problem: If we approximate the atmosphere to be 79% nitrogen(N2) by volume and 21% oxygen (O2), estimate the density of dry air(kg/m^3) at STP conditions (0 degrees Celcius, 1 atm). I haven't had a chemistry class in about 3 years, and amstumped on how to do a problem like this. Can anyone help? Problem: If we approximate the atmosphere to be 79% nitrogen(N2) by volume and 21% oxygen (O2), estimate the density of dry air(kg/m^3) at STP conditions (0 degrees Celcius, 1 atm). I haven't had a chemistry class in about 3 years, and amstumped on how to do a problem like this. Can anyone help?

Explanation / Answer

Given Pressure , P = 1 atm Temperature , T = 0 oC = 273 K We know that PV = nRT                        PV= ( m / M ) RT                           P= ( m / V ) * ( RT / M )                              = dRT / M From this d = PM / RT The average molecular mass of air (Approx. 79%nitrogen, N2; 21% Oxygen, O2) is 28.57g/molat standard temperature and pressure (STP).
So , d = PM / RT           = 1.274g / lit           = 1.274 *10^-3 Kg / 10^-3m^3            ( Since 1 Kg = 1000 g & 1 lit = 10^3m^3 )           =1.274 Kg / m^3

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