Chemistry - Masses of atoms and moles

Atoms are much too small to weigh and so we use 'relative' atomic masses.

The relative atomic mass of an element (Ar), is an average value that depends on the isotopes the element contains. However, when rounded to a whole number it is often the same as the mass number of the main isotope of the element.

The relative formula (Mr) of a substance is found by adding up the relative atomic masses of the atoms in its formulae. For example:
Worked example                               Solution
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 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. For example:
Worked example                                     Solution

What is the mass of one mole of            Ar of Na = 23, Ar of O = 16, Ar of H = 1
NaOH?                                                   so 23g +16g + g +1g = 40g