Atoms are too small to weigh so we use 'relative' atomic masses. These are shown in the periodic table.
The relative formula mass (Mr) of a substance is found by adding the relative atomic masses of the atoms in its formula. E.g.
Calculate the Mr of CaCl2 - Ar of Ca = 40, Ar of Cl= 35.5 so 40+(35.5 x 2) = 111
The relative formula mass of a substance in grams is called 'one mole' of that substance. Using moles of substances allows us to calculate and weigh out in grams and masses of substances with the same number of particles. One mole of sodium atoms contains the same number of atoms as one mole of chlorine atoms. E.g.
What is the mass of one mole of NaOH? - Ar of Na = 23, Ar of O = 16, Ar oH=1
so 23g + 16g + g + 1g = 40g
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