see I was reading that you cant include coefficents when calculating molar mass in a BCE. do you know if this is true?
Before finding the molar mass of anything, you need to first figure out what's being asked: atomic mass or molecular mass, because it could vary. O2, a molecule, has a total
molecular mass of: 2(16g/mol): 32 g/mol. The atom oxygen has an
atomic mass of 16g/mol. This is a very important distinction to make for several general chemistry problems.
What you seem to be confusing is the stoichiometric coefficient for a balanced reaction vs. the subscript ascribed to a given molecular element. For instance, consider this balanced reaction: 2 CH4 + 4 O2 --> 2 CO2 + 4 H2O. The molecular weight of O2 (which is composed of 2 oxygens) is the sum of their individual atomic weights, so 2(16g/mol): 32g/mol, as explained above. You would not multiple this by the stoichiometric coefficient.
Just be careful though, because if you're doing a limiting reagant question and you're asked to determine how much of a reactant is left, you'll have to do some interconverting between grams and moles, and in this instance, you'd need to use both: molecular weights AND the stoichiometric coefficients. It might help to brush up on this topic to avoid confusion. This is a very straightforward but important concept to know for most of general chemistry.
