Chemists count particles in a unit called the mole, symbol mol. One mole of any substance contains the same number of the stated particles as one mole of any other substance. The number is the Avogadro constant, 6.02 × 1023 per mole. The particles can be atoms, molecules, ions or electrons. So one mole of carbon (C) contains the same number of atoms as there are molecules in one mole of carbon dioxide (CO2).
The mass of one mole in grams is numerically equal to the relative formula mass, Mr (for an element, the relative atomic mass). Carbon has an Ar of 12, so one mole of carbon has a mass of 12 g. Carbon dioxide has an Mr of 44, so one mole of it has a mass of 44 g.
To change between mass and moles, use: amount in moles = mass in grams ÷ relative formula mass. Rearranged, mass = moles × relative formula mass. For example, 88 g of carbon dioxide is 88 ÷ 44 = 2 mol. Magnesium has an Ar of 24, so 0.5 mol of magnesium has a mass of 0.5 × 24 = 12 g.