Understanding the Oxygen-Air Density Formula

So, I've been diving into the formulas as of late with the intentions of understanding them on a comprehensive level as opposed to just memorizing things i have no intellectual connection to. if this is in a previous post under EFI fundamentals im sorry, i looked briefly but there are so many pages even after dialing down in the search bar. so...

Air Density formula: D= (2.7xP)/T and is solved for lb/ft3

whereas 2.7 is a constant.. I have been trying to figure out just where the constant comes from. Ive gathered so far that it has something to do with unit conversions but I cannot replicate the 2.7 when plugging in with the "ideal gas density formula" maybe this is where im wrong, perhaps this is the wrong equation to refference to but i cant find anything else really.. it makes sense, densities of different gasses are calculated the same way using Temp (in Kelvins), Pressure (in Bar), Molecular weight (unique to each element/compound) and a gas constant of .08206atm.Liter/Mol/K and im gathering it is used to simulate "ideal" conditions in gass' behavior as there are apparently some inter-molecular forces that also effect a gass' density. there is another gas constant but it is in a different SI conversion of units in order to help calculate something to do with energy opposed to volume. the molecular weight of oxygen is 31.998g/mol.

Ideal gas density formula: D= (Bar Pressure x molecular weight)/(ideal gas constant x Kelvin Temp.) and is solved for g/L (grams per Liter)

When i plug in the Standard Conditions given in the respective module here at HP Academy, I get .08562lb/ft3 after the conversion from g/L. So in order to understand the 2.7 constant given in the HP Academy class, I replaced that 2.7 with "X" and solved for X given the density I calculated using the Ideal Gas Density formula:

.08562= X(P)/T .08562= X(14.7)/518.67 and I got 3.020988. which could probably be rounded down to 3.. What I am wondering is if this is wrong and if I am misunderstanding something here, and if there is anybody that could help me to better understanding the instructors 2.7 constant, or if it even matters much, because The resulting A/F ratios will be different if the air density is calculated at about .086 than if calculated at about .077, noticeably I would imagine since there is about 13% increase in the outcome

Are you calculating the molecular weight of air?

Air is only about 20% Oxygen.

Nitrogen Makes up about 70% and the other 10% is various gasses.