How to Calculate Moles from Molecular Weight

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The mole is one of the more important concepts in chemistry. In technical terms a mole consists of 6.022 x 1023 molecules of a substance. In more practical terms a mole is the number of molecules necessary to have an amount of substance in grams equal to the substance’s molecular weight in atomic mass units, or amu. Therefore, if the molecular weight of a substance represents the number of grams required for 1 mole, then the number of moles represented by any given amount of substance will be equal to the grams of that substance divided by its molecular weight. Mathematically, this is represented by moles = grams ÷ molecular weight, or moles = g ÷ MW.

  1. Find the Molecular Formula

  2. Determine the molecular formula of the compound whose moles will be calculated. If this information is not already available, numerous reference books and online databases provide this information, including the National Institute of Standards and Technology website provide in the Resources section. As an example, suppose you wanted to determine the moles of aspirin in a 250 mg aspirin tablet. Typing “aspirin” into the NIST database reveals that the drug’s chemical name is 2-acetyloxy-benzoic acid and its molecular formula is C9H8O4. This indicates that one aspirin molecule contains nine carbon atoms, eight hydrogen atoms and four oxygen atoms.

  3. Calculate the Molecular Weight

  4. Calculate the compound’s molecular weight using the atomic weights provided in the periodic table of the elements. Multiply the number of each type of atom by its molecular weight and then sum the products. In the case of aspirin, the molecular weights of carbon, hydrogen and oxygen are 12.01, 1.01 and 16.00 amu, respectively. The molecular weight of aspirin is therefore 9(12.01) + 8(1.01) + 4(16.00) = 180.17 amu.

  5. Calculate the Moles

  6. Calculate the moles of substance by dividing the mass of substance in grams by the molecular weight in amu. In this case, the aspirin tablet contains 250 mg, or 0.250 g. Therefore, 0.250 g ÷ 180.17 amu = 0.00139 moles of aspirin.

    Things You'll Need

    • Periodic table of the elements
    • Calculator