air volumes

Kathleen Archer Kathleen.Archer at
Mon Feb 26 17:08:50 EST 2001

Dear Folks,
I posed a challenge question to my plant phys students, and now I have to
solve it myself in order to know who has the correct answer.  I appeal to
those of you who are comfortable thinking about gases to tell me whether I
have thought about this in the right way.

The question:  How much air must a leaf process in order to get enough CO2
to make a gram of G-3-P?

1) 1 gram of G-3-P = 0.00588 moles of G-3-P.  Therefore, 3X times that
should be the moles of CO2 needed = 0.0176 moles

2) Air is 0.035% CO2, or 0.035 moles CO2 in 100 moles of air

3) If there are 0.035 moles CO2/100 moles air, then there must be 0.0175
moles CO2 in 50.29 moles air.

4) Our problem now becomes what volume is occupied by 50.29 moles of air?
Asuming room temperature (25C) and atmospheric pressure, then the perfect
gas law says
	volume = nRT/P, or moles X gas constant X temp (in Kelvin)/ 1 atmosphere.
	V = (50.29 moles)(0.0821*)(298)/1 atmosphere = 1,258.9 liters
*R in units of liter atmospheres per degree Kelvin mole

Does that seem reasonable?

Kathleen Archer


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