Chemists often need to estimate the number of atoms or molecules present in a sample. Although mass is a convenient measure for weighing out a specific quantity, the number of atoms or molecules is more important for reaction stoichiometry (the relationships between products and reactants in a reaction) and for determining concentration. Unfortunately, the number of particles present in even a small sample is astronomically large, and it's inconvenient to calculate equations involving quintillions or septillions of particles when solving a problem. Consequently, chemists use a unit called the mole instead.
Measure the weight of the sample you're working with. Make sure you measure its weight in grams. If your scale doesn't measure in grams, convert from ounces to grams by multiplying the weight by 28.35 grams per ounce.
Determine the molar mass of the substance in your sample using the periodic table and the molecular formula. The periodic table lists the atomic mass of each element in atomic mass units (amu); the molar mass of an element is the same as its atomic mass, only the units are in grams, not amu. To find molar mass, multiply the number of atoms of each element in the formula by the molar mass of that element, then add all these numbers together. Glucose, for example, has the molecular formula C6H12O6, so the molar mass of glucose is 6 x molar mass carbon + 12 x molar mass hydrogen + 6 x molar mass oxygen.
Divide the weight in grams of your substance by the molar mass from step 2 to find out how many moles you have.
Multiply the number of moles by 6.022 x 10 to the 23rd power to find the number of molecules or particles in your sample. Remember that a mole is just 6.022 x 10 to the 23rd power atoms or molecules.