# How to calculate atmospheric composition

Okay, so I'm a novice in chemistry and am trying to figure something out.

Say that you want to turn carbon dioxide into oxygen using photosynthesis, given an infinite supply of water. Say that the atmosphere's volume is 100km^3 and has a pressure of 5kPa. The current composition of the atmosphere is 90% Carbon Dioxide, 5% Argon, 5% Nitrogen. You want to get the Oxygen in the atmosphere to the point that it's at 20% of the atmospheric composition. How would you calculate the atmospheric composition after you do that?

A step by step guide with just formulas would be fine, you don't necessarily have to solve this exact problem; I just need to figure out how you go about solving it. I'm only asking because when I tried doing the math and putting it back into a percent via # mols or their mass, I thought that in the end it should still have 5% Argon and 5% Nitrogen in the atmospheric composition since the chemical equation is only using the Carbon Dioxide in the air to make Oxygen.

Keep in mind when explaining it to me that I've only just completed my first semester of college chemistry, so make sure you tell me what each variable is!

• As stated by @iad22agp, there is some important missing information to this very interesting, and potentially someday very relevant, idea. What is the proximity to a sun, other energy sources, is the planet geologically active, what are the dominant minerals, and very importantly does it have a magnetic field. Mars for example has bad-news answers to most of these questions. I'm not bringing up these things to shoot you down but to encourage you to give these complications some consideration, which will also help others to come up with more plausible scenarios to answer your question. – airhuff Jan 10 '17 at 9:03

A really interesting problem, but I don't think you have enough information to solve it. At first glance, it seems like you would get one oxygen molecule in the atmosphere for every $\ce{CO2}$ molecule consumed. If we assume we are only making carbohydrates, then the process would be $\ce{CO2 + H2O -> [CH2O] + O2}$. That would give a $70:20:5:5$ ratio of $\ce{CO2:O2:N2:Ar}$. However, plants will do more than just produce carbohydrates into their structures. They also produce fats, proteins, etc. Some of the $\ce{N2}$ would become incorporated. In addition, $\ce{CO2}$ is appreciably soluble in water (more so at lower temperatures). I think both factors would reduce the $\ce{CO2}$ and $\ce{N2}$ levels in the final mix.
• \ce{} command embraced with \$ is very convenient for LaTeX chemical formatting – jlandercy Jan 13 '17 at 16:59