Chem – Empirical and Molecular Formula from Mass and Percent Part 5



When you compared the oxygen to the sulfur in a mole ratio, you came out with a nice, neat number like 3. This doesn’t usually happen, but because I wanted you to focus on the process and not the exact numbers, I made a simpler problem for you. However, if you are comparing mole ratios and you come out with a number like 2.98 or 3.05, then you can safely assume it rounds off to 3. If you happen to come out with a number like 5.45 or 5.52, then you can safely assume it rounds off to 5.5. This is a bit trickier to deal with, but I just want to give you an idea of the rounding. We will try a problem like this next. WARNING: some teachers will even expect you to round to a number like 1.33 or 1.66. I am very against teaching this to students because it can cause unnecessary confusion. It is very hard to determine if you are rounding correctly in a problem like that. Since empirical and molecular formulas are not a huge part of chemistry, I don’t try to emphasize them too much. This is not something that will come up in most chapters so if you are struggling with it get down the basics and then move on.


PRACTICE PROBLEMS:  Solve the empirical and molecular problems below. Use this periodic table if needed.


What is the molecular formula of a compound that has a molar mass of 478.8 g/mol and is composed of 70% Iron and 30% Oxygen?

Answer: Fe6O9









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