Let us first write down the relative atomic masses of all the elements involved. (Please not that these will differ from periodic table to periodic table. Familiarize yourself with the data sheet in your institution)
Al - 26.892
S - 32.066
O - 15.999
From the formula we can see that we
Two aluminium atoms = 26.892*2 = 53.784
Three sulphur atoms = 32.066*3 = 96.198
Twelve oxygen atoms = 191.988
Add them all up = 341.97
So the molar mass is 341.97 g/mol. The units have to assigned because this is the molar mass. Relative molecular mass has not units
