Reef Chemistry Question of the Day #240 Conversion of CO2 to O2

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #240

This came up in another thread a few days ago and I thought I'd make a question of it.

If terrestrial house plants in your home converted ALL of the carbon dioxide (CO2) present in normal air (say, 400 ppm CO2) to oxygen (O2) by photosynthesis, what would be the final O2 concentration in the air?

Assume that the starting air is 20.95% O2, or 209,500 ppm.

A. 209,500 ppm
B. 209,791 ppm
C. 209,900 ppm
D. 210,322 ppm

Bonus question: Some homes have elevated indoor CO2 levels. If the indoor CO2 was twice as high as normal outside air before the plants do their thing (say, 800 ppm before and using it all up), is the answer to the above question higher, lower, or the same?


Good luck!



































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[Crimsonvice]

A, and the bonus answer would be the same.

 

[CNDReef]

Going with A because I know that plants also consume oxygen at night. Don’t know to what degree but I’ll stick to my answer

 

[Randy Holmes-Farley]

FYI, it is not because anything you guys said is right or wrong, but I altered the wording of the bonus question slightly:

"say, 800 ppm before and using it all up)"

to be sure folks knew I assumed the plants were still using up all the CO2.

 

[rkpetersen]

B, higher.

 

[MnFish1]

C and higher.
6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2

 

[sghera64]

So, you said "converted all the CO2 to O2". I presume that plants, or some other hypothetical means, is irrelevant. This is simply a mole fraction question right? What happens if the moles of CO2 are converted to moles of O2 1 for 1?

Unless there is something outside of the scope I laid out above, I have to agree MnFish1. It's a simple mass transfer and application of PV = nRT. (Where volume % is proportional to mole %.)

Ans: C and higher.

 

[JimWelsh]

B, and higher. 400 PPM CO2 becomes only 291 PPM O2 because of the carbon that went missing. The O2 molecule has less mass than the CO2 molecule.

 

[sghera64]

B, and higher. 400 PPM CO2 becomes only 291 PPM O2 because of the carbon that went missing. The O2 molecule has less mass than the CO2 molecule.

I agree with your math. I'm not sure the mass basis is correct. I guess the question in my mind is this: is ppm a mass basis, mole (or volume) basis?

From engineering school, we always use ~21% oxygen and ~79% nitrogen as volume (or mole) basis and not mass. Since Randy referred to them like this in his hint, I presumed all were on a mole or volume basis.

Thoughts? Anyone?

 

[rkpetersen]

ppm in aquarium water chemistry is usually mass-based, since ppm corresponds to mg/l. I am presuming the same applies in air here.

 

[MnFish1]

B, and higher. 400 PPM CO2 becomes only 291 PPM O2 because of the carbon that went missing. The O2 molecule has less mass than the CO2 molecule.

That seems plausible.

I agree with your math. I'm not sure the mass basis is correct. I guess the question in my mind is this: is ppm a mass basis, mole (or volume) basis?

From engineering school, we always use ~21% oxygen and ~79% nitrogen as volume (or mole) basis and not mass. Since Randy referred to them like this in his hint, I presumed all were on a mole or volume basis.

Thoughts? Anyone?

No. clue I think ppm is number not mass based. But that would be the question (lol)

 

[sghera64]

ppm in aquarium water chemistry is usually mass-based, since ppm corresponds to mg/l. I am presuming the same applies in air here.

You make a great point, but Emile Clapeyron didn't own an aquarium and preferred to play with gases and moles instead of liquids and grams. And I think he's partly to blame for starting the practice of using ppm (or percent) in terms of volume and moles when referring to gasses. I think it came with the rough assumption that many simple gases change volume and pressure 'similarly' on a mole basis regardless of their M.W. (PV=nRT).

That said, I do agree that with all my calculations with test kits, my ppm and ppb calculations are mass based.

 

[SDReefer]

I'm going to guess C for 400 ppm CO2 and 210,300 ppm for 800 ppm CO2.

6CO2 + 6H2O -> C6H12O6 + 6O2

Assuming a total of 1,000,000 particles -
400 CO2 molecules = 6.64 x 10^-22 mol
209,500 O2 molecules = 3.47 x 10^-19 mol

6.64 x 10^-22 mol CO2 = 6.64 x 10^-22 mol O2

(6.64 x 10^22 mol O2) + (3.47 x 10^-19 molO2) = 3.48 x 10^-19 mol O2

(3.48 x 10^-19 mol O2) x 6.022 x 10^23 = 209900 molecules = 209900 ppm

 

[MnFish1]

I

You make a great point, but Emile Clapeyron didn't own an aquarium and preferred to play with gases and moles instead of liquids and grams. And I think he's partly to blame for starting the practice of using ppm (or percent) in terms of volume and moles when referring to gasses. I think it came with the rough assumption that many simple gases change volume and pressure 'similarly' on a mole basis regardless of their M.W. (PV=nRT).

That said, I do agree that with all my calculations with test kits, my ppm and ppb calculations are mass based.

It will be interesting I never thought ppm was mass based in any sense. But it’s been a while

 

[theMeat]

I have know idea, but that's why it's good to have you around!

 

[JimWelsh]

I agree with your math. I'm not sure the mass basis is correct. I guess the question in my mind is this: is ppm a mass basis, mole (or volume) basis?

From engineering school, we always use ~21% oxygen and ~79% nitrogen as volume (or mole) basis and not mass. Since Randy referred to them like this in his hint, I presumed all were on a mole or volume basis.

Thoughts? Anyone?

Now that you've made me google this, I'm reasonably certain that you are correct.

 

[MnFish1]

Now that you've made me google this, I'm reasonably certain that you are correct.

Here is my google lol. This is why one of us (not me) has a PhD lolppm means parts-per-million. ... If ppm is expressed as THE VOLUME of particles to a unit volume of water, then ppm BY VOLUME is equal to µl/l. However, if ppm is expressed as THE MASS of particles in a unit volume of water, then ppm BY MASS is equal to mg/l.

 

[sghera64]

Here is my google lol. This is why one of us (not me) has a PhD lolppm means parts-per-million. ... If ppm is expressed as THE VOLUME of particles to a unit volume of water, then ppm BY VOLUME is equal to µl/l. However, if ppm is expressed as THE MASS of particles in a unit volume of water, then ppm BY MASS is equal to mg/l.

Very good. So, what if a liter of CO2 has the same number or moles as a liter or O2 (at the same pressure and temperature)?

[Sorry Randy if we are hijacking your QOTD thread here].

 

[JimWelsh]

Here is my google lol. This is why one of us (not me) has a PhD lolppm means parts-per-million. ... If ppm is expressed as THE VOLUME of particles to a unit volume of water, then ppm BY VOLUME is equal to µl/l. However, if ppm is expressed as THE MASS of particles in a unit volume of water, then ppm BY MASS is equal to mg/l.

But the question isn't about water. It is about air. That's different.

 

[MnFish1]

Life is like that