Ratio of components of a gas vs. temp/pressure/etc

This gives you the percentage of oxygen in air by volume (and number of molecules).

https://www.engineeringtoolbox.com/air-composition-d_212.html

Air can be considered an ideal gas, and a mole or Avagadro's number of molecules (R = 6.023 x 10²³ occupy 22.4 liters of volume at STP (273 deg K, 14.7 psi). (Note that the oxygen molecule consists of 2 atoms.)

https://planetcalc.com/7916/

For other pressures and temperatures, you can use the ideal gas law to calculate the new volume.

P₁V₁/T₁ = P₂V₂/T₂

https://www.schoolphysics.co.uk/age16-19/Thermal%20physics/Gas%20laws/text/Ideal_gas_equation/index.html

Don't forget that altitude will also change oxygen levels.

The ideal gas law (PV=nRT) will identify how many molecules you will have. However, this will not identify the partial pressure (Dalton's law) distribution of any components, particularly oxygen. Any chemical reaction in the volume will add the complexity of gas stoichiometry to the situation. (Plants in an enclosed terrarium will increase the oxygen level during daylight and decrease it at night.) Additionally, if any of the gasses can undergo a phase change then things get even more complicated. (Let it snow, let it snow.)

Do you want to calculate all the oxygen atoms including those bonded to hydrogen that make up the water in the air, or do you just want diatomic oxygen?

A little more background ....

The problem is the performance of a model airplane engine, in a class which is right at the edge of making maximum power. The class is an FAI class for Control Line speed models, known as F2A. Engines are 2.5cc, tuned pipe, single-blade prop, weird models with only a long inboard wing. Engines turn 40k when performing properly, airspeed is on the order of 290-300kph.

Making the engine perform on any given day is strongly affected by the ambient atmospheric conditions. We routinely note and record barometric pressure, temperature, dew point, %RH, density altitude. And then what the model did under those conditions. What this question was intended for was to try and measure the effect of displacing molecules of air gasses when water vapor is introduced. IOW, what happens to the available oxygen, critical for best performance, as the atmospheric conditions change.

"More humidity makes your engine run richer, reduces power (less oxygen)

More heat makes your engine run richer, reduces power (thinner air, less oxygen to burn)

More altitude makes your engine run richer. (thinner air, less oxygen)"

So I would think a little more nitromethane to fuel mix added on hot and/or humid days...

..."The atmospheric concentration of O2 has remained constant for several hundred years at 20.95%. The percentage is the same at sea level or on Mount Everest. However, the absolute O2 concentration does not remain constant. An O2 tank is required to climb Mount Everest, even though the relative O2 concentration is still 20.95% (Fig. 1A). The absolute O2 concentration determines the rate of most biological and chemical processes, but the relative O2 concentration is typically reported. This is analogous to discussing relative humidity when absolute humidity is what determines evaporation rates."...

https://www.ictinternational.com/casestudies/understanding-oxygen-in-air/

https://www.rcuniverse.com/forum/gas-engines-142/11136269-humidity-effect-performance.html

Walbro carbs..

https://www.spektrumrc.com/Products/Default.aspx?ProdId=EVOE62GXI

https://power4flight.com/

That baby is honkin'

Looks like a one-winged one-horn flyin' purple people eater....

https://www.motortrend.com/how-to/hrdp-1304-what-is-nitromethane-anyway/

https://www.rccaraction.com/fast-facts-nitro-fuel/

It occurs to me that the quality of the fuel also plays an important role, it has a shelf life, and degrades in direct sunlight and exposure to the air...so it must be fresh, the fuel container must be kept sealed, and must be stored in a dark cool place for best results...

https://grassrootsmotorsports.com/articles/fuel-tips-what-specific-gravity/

This is a very common issue in drag racing and talked about widely on drag racing forums. Try searching the drag racing forums I think you will find a wealth of information. This site give up to date air density altitude corrected for water vapor for race tracks all around the world.

Absolute humidity is what you should be interested in,not relative humidity.

https://www.zehnderamerica.com/absolute-vs-relative-humidity-whats-the-difference/

For a model aircraft, the characteristics of air at "normal temperature and pressure [NTP]", a Chemist's term, are applicable: "1 mole of gas occupies 22.4 litres at NTP" is a good level of approximation. The effective molarity of air, which is a mixture and not a pure substance, can be extracted by interpolation of its composition; nitrogen, oxygen, argon, water vapour, carbon dioxide and a few also-rans can be used to give an equivalent molar mass to a good level of approximation.

The "international standard atmosphere" gives the properties of air with altitude. Charts can be found by any internet search engine.

The ideal gas law gives a good level of approximation of the properties of air as its temperature and pressure changes.

The rest is a load of arithmetic.

Enjoy!