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Chemistry LibreTexts

6.6: Mass Percent Composition of Compounds

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  • Skills to Develop

    • Determine percent composition of each element in a compound based on mass.

    Packaged foods that you eat typically have nutritional information provided on the label. The label on a jar of peanut butter reveals that one serving size is considered to be \(32 \: \text{g}\). The label also gives the masses of various types of compounds that are present in each serving. One serving contains \(7 \: \text{g}\) of protein, \(15 \: \text{g}\) of fat, and \(3 \: \text{g}\) of sugar. By calculating the fraction of protein, fat, or sugar in one serving size of peanut butter and converting to percent values, we can determine the composition of peanut butter on a percent by mass basis.

    Percent Composition

    Chemists often need to know what elements are present in a compound and in what percentage. The percent composition is the percent by mass of each element in a compound. It is calculated in a similar way that we just indicated for the peanut butter.

    \[\% \: \text{by mass} = \dfrac{\text{mass of element}}{\text{mass of compound}} \times 100\%\]

    The sample problem below shows the calculation of the percent composition of a compound based on mass data.

    Example \(\PageIndex{1}\): Percent Composition from Mass Data

    A certain newly synthesized compound is known to contain the elements zinc and oxygen. When a \(20.00 \: \text{g}\) sample of the compound is decomposed, \(16.07 \: \text{g}\) of zinc remains. Determine the percent composition of the compound.


    Steps for Problem Solving  
    Identify the "given"information and what the problem is asking you to "find."

    Given : Mass of compound = 20.00 g

     Mass of Zn = 16.07 g

    Find: % Composition (% Zn and %O)

    List other known quantities

    Subtract to find the mass of oxygen in the compound. Divide each element's mass by the mass of the compound to find the percent by mass.

    Mass of oxygen = 20.00 g - 16.07 g = 3.93 g O

    Cancel units and calculate.

    \[\% \: \ce{Zn} = \dfrac{16.07 \: \text{g} \: \ce{Zn}}{20.00 \: \text{g}} \times 100\% = 80.35\% \: \ce{Zn} \nonumber\]

    \[\% \: \ce{O} = \dfrac{3.93 \: \text{g} \: \ce{O}}{20.00 \: \text{g}} \times 100\% = 19.65\% \: \ce{O} \nonumber\]

    Calculate the percent by mass of each element by dividing the mass of that element by the mass of the compound and multiplying by \(100\%\).

    Think about your result. The calculations make sense because the sum of the two percentages adds up to \(100\%\). By mass, the compound is mostly zinc.

    Exercise \(\PageIndex{1}\)

    Sulfuric acid, H2SO4 is a very useful chemical in industrial processes. If 196.0 g of sulfuric acid contained 64.0g oxygen and 4.0 g of hydrogen, what is the percent composition of the compound?

    2.04% H, 32.65% S, and 65.3% O


    • Processes are described for calculating the percent composition of a compound based on mass.