How do you find the empirical formula of a compound from its percent composition?


Quick Answer

Find the empirical formula of a compound by knowing the atomic weights of each individual element in the molecular compound. Take the atomic weights to convert each element to the number of moles, which is the relationship of each element to the number of atoms in the compound.

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Full Answer

  1. Find the overall mass

    Weigh the compound because the overall mass is needed to find the percentage of elements by weight. Masses convert to moles later in the process.

  2. Determine the chemical composition

    Find the components of the compound and break them down into each element. The percentage of each element determines the ratio of molar masses in the empirical formula.

  3. Convert everything to moles

    Use the atomic weights of each element to convert each element to moles. Moles describe the amount of atoms in each element.

  4. Follow this example

    Look at this easy example to follow for other chemistry problems. Assume a 100-gram substance contains 50.05 grams of sulfur and 49.95 grams of oxygen. Take 50.05 grams and divide by sulfur's atomic weight of 32.066 grams per mole to get 1.5608 moles. Next, do the same for oxygen to get 40.95 grams divided by 16.00 grams per mole to arrive at 3.1212 moles. The ratio of sulfur to oxygen is one to two, because 1.5608 times two is 3.1212. Therefore, the empirical formula is SO2 for sulfur dioxide.

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