The amount of a substance is measured in moles (mol). The mass of one mole of a substance, in grams, is equal to its relative atomic mass (Ar) for an element or its relative formula mass (Mr) for a compound. You find these values from the periodic table. For carbon dioxide, Mr = 12 + (16 × 2) = 44.
To find the number of moles, divide the mass in grams by the relative formula mass: moles = mass ÷ relative formula mass. To go the other way, multiply the number of moles by the relative formula mass: mass = moles × relative formula mass. The same method works with relative atomic mass for elements.
Worked examples. Iron has Ar = 56, so 5.6 g of iron is 5.6 ÷ 56 = 0.1 mol. Carbon dioxide has Mr = 44, so 22 g is 22 ÷ 44 = 0.5 mol. Sodium hydroxide has Mr = 40, so 0.25 mol has a mass of 0.25 × 40 = 10 g, and 0.5 mol of iron has a mass of 0.5 × 56 = 28 g.