6.6: Mass Percent Composition of Compounds
- Page ID
- 48603
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- 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 to that of the composition of the peanut butter.
\[\% \: \text{by mass} = \dfrac{\text{mass of element}}{\text{mass of compound}} \times 100\% \nonumber \]
The sample problem below shows the calculation of the percent composition of a compound based on 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.
Solution
Steps for Problem Solving | When a \(20.00 \: \text{g}\) sample of the zinc-and-oxygen compound is decomposed, \(16.07 \: \text{g}\) of zinc remains. Determine the percent composition of the compound. |
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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. |
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?
- Answer
- 2.04% H, 32.65% S, and 65.3% O
Summary
- Processes are described for calculating the percent composition of a compound based on mass.