Percent Composition

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Calculating percent by mass for chemical compounds - Real Chemistry

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Key Questions

  • Percent composition in chemistry typically refers to the percent each element is of the compound's total mass.

    The basic equation = mass of element / mass of compound X 100%

    For instance, if you had a 80.0 g sample of a compound that was 20.0 g element X and 60.0 g element y then the percent composition of each element would be:

    Element X = 20.0 g X / 80.0 g total x 100% = .250 or 25.0 %
    Element Y = 60.0 g Y / 80.0 g total x 100 % = .750 or 75.0 %

    Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound. Video from: Noel Pauller

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  • Percent composition tells you which types of atoms (elements) are present in a molecule and their levels. Percent composition can also tell you about the different elements present in an ionic compound as well.

    Let's look at two examples

    The molar mass of #H_2O# is 18 g/mol
    The hydrogens make up 2g (since each mole of hydrogen is 1g)
    The oxygen makes up 16g.

    The percent composition of the compound is:
    H = (2g/18g) x 100 = 11.1%
    O = (16g/18g) x 100 = 88.9%

    Percent composition can be calculated the chemical formula of a compound, or it can be determined experimentally.

    Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound.
    Video from: Noel Pauller

    Hope this helps!

  • Here is an example for the compound C2HO4

    You will first need to find the molar mass of the compound.

    C = 2 x 12 = 24
    O = 1 x 16 = 16
    H = 4 x 1 = 4
    Total mass = 40 amu

    To find the percent composition you divide each part by the whole and multiply by 100 to convert to a %.

    C = (24/44) x 100 = 54.5%
    O = (16/44) x 100 = 36.4%
    H = (4/44) x 100 = 9.1%

    Percent composition can also be calculated using experimental data. The video below shows you how to do that type of calculation.

Questions

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