Empirical Formula from combustion

[Natalie Do 3F]

I'm using this video as an example but there have been similar problems in UA meetings and other places https://www.youtube.com/watch?v=klkAzTWdYTw

Why is oxygen unknown but Carbon and Hydrogen we can find from the equation? Does it have to do with us not knowing which gas the oxygen is coming from since we aren't given coefficients?

[Stuti Pradhan 2J]

The reason the oxygen is unknown is because combustion reactions typically have a compound that reacts with an excess amount of oxygen to form carbon dioxide and water. Since all the carbon in the carbon dioxide and all the hydrogen in the water are from the original compound, you know that the mass of carbon and hydrogen that you find in the products is the exact mass in the compound. However, since the compound was reacted with excess oxygen, the oxygen in the carbon dioxide and water could be from the hydrocarbon or the excess oxygen. Since you do not know how much of the mass of the oxygen came from the hydrocarbon vs the excess oxygen that the compound was reacted with, you have to subtract the amount of carbon and hydrogen in the products from the original compound to find the mass of oxygen in the compound.

The stoichiometric coefficients are not related to figuring out the mass percent composition of the original compound.

Hope this helps!