Amount of Substance

• The actual mass of any atom is too small to find by weighing, so the atoms are compared, and relative masses are used. 
• Scientist now use the isotope carbon-12 as a baseline upon which all other elements are compared to. Mass spectrometry has allowed us to measure the relative masses of atoms, and 1/12th the atomic mass of carbon-12 is given a value of exactly 1. 
• Relative atomic mass = average mass of one atom of an element ÷ 1/12th mass of one atom of carbon-12. 
• Molecules can be handled the same way as atoms, by comparing the mass of one molecule to a 1/12th of carbon-12. 
• Relative molecular mass = average mass of one molecule ÷ 1/12th mass of one atom of carbon-12. 
• You can find the relative molecular mass by adding up the relative atomic masses of all atoms present in the molecule. 
• Relative formula mass is used for ionic compounds because they don’t exist as molecules, though this has the same symbol as Mr (relative molecular mass).