What is mass fraction?

Answer 1

It is a term used to denote the part, or fraction, of something that actually has mass, or the amount of a specific element within a compound.

For example, think of a jar full of marbles. There is some space in between the marbles that is empty, yes it contains air but for the purpose of this example consider it to be empty space. If the total volume of the jar the marbles occupy is 95%, then you could say that the mass fraction in the jar is 95% of the total volume.

You could also think of this in another way. Compounds are made up of various elements. Say you have common bleach, sodium hypochlorite, NaOCl, and you want to know what % of bleach is actually chlorine. The total weight of a molecule of NaOCl is 74 g/mol. Since Cl weighs 35g/mol the mass fraction of chlorine in NaOCl is 35/74 *100% or 47.29%.