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.

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.

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

                                              40.002% C in C6H12O6

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

 

Applying it to the example above gives:

  

                           =  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

Shortcut