Percentage mass is where you work out the percentage of a certain element:
for example, a question might ask ' what it the percentage mass of chlorine in hydrocholric acid?
You would then work out the Ar ( relative atomic mass) of each element in Hydrochloric acid.
Hydrogen (H)=1 CI=35.5 you then times these together ( onlt if theres more than one of the same substance e.g 03 three oxygen atoms) add them up.
35.5+1=36.5 Now, because you're trying to work out chlorine (CI) you need to do chlorine divided by the total mass of the compound, then x by 100 as you're finding a percentage.
this is how the equation would look like:
Percentage Mass= 35.5
-------- x 100 = answer, you would then write chlorine makes up ----- of hydrochloric acid.