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Percent Element in a Compound

The following example shows how the percentage of an element in a compound can be calculated.

 

EXAMPLE - Calculating the Percentage of an Element in a Compound:   

What is the percent carbon in glucose, C6H12O6?

Solution:

First, we need to decide what units we want in our answer. Chemists assume a percentage is a mass percentage unless otherwise stated. We could use any mass units, but it is easiest to use grams. Remember that percentage is a ratio of the number of units of the part to 100 units of the whole. Thus we can calculate the percentage carbon in glucose by dividing the mass in grams of carbon by the mass in grams of glucose and multiplying by 100.

Image of the beginning of the calculation identifying that what you want to calculate is the grams of carbon over the grams of glucose, C6H12O6, times 100

Because we want a ratio of units of carbon over units of glucose when we are done, we need to begin with a value on the right side of our ratio that has a comparable arrangement (units of carbon over units of glucose). There are no values given in the question, but the chemical formula provides us with a ratio of moles of element to moles of compound. The formula, C6H12O6, tells us that there are six moles of carbon for each mole of glucose.

Image showing the beginning of the unit analysis setup with 6 moles of carbon over 1 mole of C6H12O6

To convert this information to a ratio of grams of carbon to grams of glucose, we use the molar masses of carbon and glucose.

Image that shows the rest of the unit analysis setup where moles af carbon are converted into grams with the molar mass of carbon and the moles of C6H12O6 are converted into grams using its molar mass. When the result is multiplied by 100, the answer is 40.002% carbon in glucose.

                                          =  40.002% C in C6H12O6

The following general formula provides a shortcut for determining the percentage of an element in a compound.

  Image that shows the shortcut for calculating the percent of an element in a compound. The number of atoms in the compound formula is multiplied by the atomic mass of the element, and this is divided by the formula mass of the compound. The result is multiplied by 100.

Applying it to the example above gives:

Image of the setup for calculating the percent carbon in glucose with the shortcut, 6 times 12.011 divided by 180.158 times 100   

=  40.002% C in C6H12O6

 

Sample Study Sheet: Calculating the Percentage of an Element in a Compound

Tip-off - You want to calculate the percentage of an element in a compound.

General Steps - This type of problem can be solved with unit analysis or with a shortcut.

Unit analysis

Image of the general setup for doing these calculations the long way

Shortcut

  Image that shows the shortcut for calculating the percent of an element in a compound. The number of atoms in the compound formula is multiplied by the atomic mass of the element, and this is divided by the formula mass of the compound. The result is multiplied by 100.