I was recently asked the following question
When breathing, approximately 5 percent of each exhaled breath is carbon dioxide. Neglecting any difference in water-vapour content, estimate the typical difference in mass between an inhaled breath and an exhaled breath. Assume that one’s lung capacity is about half a litre and that 20% of the air that is breathed in is oxygen. Note that in the process of breathing, approximately ¼ of the O inhaled is replaced by CO2. Air is 80% nitrogen and 20% oxygen.
given the molar mass of oxygen is 32 grams per mole, CO2 is 44 grams per mole and nitrogen is 28 grams per mole
I know you can use min = density of air * volume of 2 lungs.
And then work out 5% of this mass ( A = 0.05min)
Then use NCO2 = the 5% mass / molar mass of oxygen
This gives us the number of carbon dioxide moles and we can use
dm = change in mass = NCO2*(44-32)
using this method I get about 23 mg
However, before using this method, I used another which is provided below (my original method):
Why is it that these two methods disagree? In my original method, which statement causes the difference between the two methods answers?
(Which answer is more correct and why)
(Surely my original method follows from logic e.g. The first statement, where the total number of moles = the sum of moles of oxygen, nitrogen and CO2, and that 1 part in 5 of that sum is oxygen (when breathing in) etc.)