IGCSE›Cambridge IGCSE Chemistry›Mind mapsRelative Masses of Atoms and MoleculesCambridge IGCSE Chemistry: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsRelative atomic massAverage mass of isotopes of an elementCompared with 112\frac{1}{12}121 of a carbon-12 atomWeighted by natural abundanceNo units: it is a ratioRelative formula massMr = sum of the Ar values in the formulaIonic compounds: formula mass, still MrCount each atom by its subscriptRelative massAr and MrArMrExamplesMg has Ar 24: twice as heavy as carbon-12HX2O\ce{H2O}HX2O: (2×1)+16=18(2 \times 1) + 16 = 18(2×1)+16=18CaCOX3\ce{CaCO3}CaCOX3: 40+12+(3×16)=10040 + 12 + (3 \times 16) = 10040+12+(3×16)=100Link to molesMolar mass in g/mol equals Ar or Mrn=mMn = \frac{m}{M}n=Mm (mol = g ÷ g/mol)Rearrange the same equation for massExam tipsMultiply mass by the subscript in the formulaCheck the formula before adding masses