4.6: Percent Composition
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- Jan 5, 2021
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Is there anything healthy in this jar?
Packaged foods that you eat typically have nutritional information provided on the label. The label on a jar of peanut butter (shown above) reveals that one serving size is considered to be 32g. The label also gives the masses of various types of compounds that are present in each serving. One serving contains 7g of protein, 15g of fat, and 3g 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 what was just indicated for the peanut butter.
%by mass=mass of elementmass of compound×100%
Percent Composition from Mass Data
The sample problem below shows the calculation of the percent composition of a compound based on mass data.
Example 4.6.1: Percent Composition from Mass
A certain newly synthesized compound is known to contain the elements zinc and oxygen. When a 20.00g sample of the compound is decomposed, 16.07g of zinc remains. Determine the percent composition of the compound.
Solution
Step 1: List the known quantities and plan the problem.
Known
- Mass of compound =20.00g
- Mass of Zn=16.07g
Unknown
- Percent Zn=?%
- Percent O=?%
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.
Step 2: Calculate.
Mass of oxygen=20.00g−16.07g=3.93gO
%Zn=16.07gZn20.00g×100%=80.35%Zn
%O=3.93gO20.00g×100%=19.65%O
Step 3: 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.
Percent Composition from a Chemical Formula
The percent composition of a compound can also be determined from the formula of the compound. The subscripts in the formula are first used to calculate the mass of each element in one mole of the compound. That is divided by the molar mass of the compound and multiplied by 100%.
%by mass=mass of element in1molmolar mass of compound×100%
The percent composition of a given compound is always the same as long as the compound is pure.
Example 4.6.2: Percent Composition from Chemical Formula
Dichlorineheptoxide (Cl2O7) is a highly reactive compound used in some organic synthesis reactions. Calculate the percent composition of dichlorineheptoxide.
Solution
Step 1: List the known quantities and plan the problem.
Known
- Mass of Cl in 1molCl2O7=70.90g
- Mass of O in 1molCl2O7=112.00g
- Molar mass of Cl2O7=182.90g/mol
Unknown
- Percent Cl=?%
- Percent O=?%
Calculate the percent by mass of each element by dividing the mass of that element in 1 mole of the compound by the molar mass of the compound and multiplying by 100%.
Step 2: Calculate.
%Cl=70.90gCl182.90g×100%=38.76%Cl
%O=112.00gO182.90g×100%=61.24%O
Step 3: Think about your result.
The percentages add up to 100%.
Percent composition can also be used to determine the mass of a certain element that is contained in any mass of a compound. In the previous sample problem, it was found that the percent composition of dichlorine heptoxide is 38.76%Cl and 61.24%O. Suppose that you need to know the masses of chlorine and oxygen present in a 12.50g sample of dichlorine heptoxide. You can set up a conversion factor based on the percent by mass of each element:
12.50gCl2O7×38.76gCl100gCl2O7=4.845gCl
12.50gCl2O7×61.24gO100gCl2O7=7.655gO
The sum of the two masses is 12.50g, the mass of the sample size.
Science Friday: Stained Glass Conservation
Stained glass from the Middle Ages is often hundreds of years old. Unfortunately, many of these relics are in need of cleaning and maintenance. In this video by Science Friday, conservator Mary Higgins discusses the methods used to protect the stained glass.
Summary
- Processes are described for calculating the percent composition of a material based on mass or on chemical composition.
Review
- What is the formula for calculating percent composition?
- What information do you need to calculate percent composition by mass?
- What do subscripts in a chemical formula tell you?