Percent Difference Calculations
How do experimentally determined
values compare to actual values?
Covered in this lesson
• This lesson will demonstrate how to compare
a calculated density from an experiment to
the "actual" density reported in reference
sources.
• Comparing the actual or theoretical density to
a density determined by an experiment
(experimental density) involves a percent
formula.
Percent Error
• Percent error or percent difference
calculations use the following formula.
%error = theoretical value – experimental value X 100
theoretical value
Where the theoretical value is looked up in a resource book.
The answer is positive, an absolute value.
Example
• In a lab a piece of aluminum was found to
have an experimental density of 2.86 g/mL.
• According to the Mineralogy Database the
density of aluminum is 2.72 g/mL.
• Find the % error
• (http://webmineral.com)
Percent Error of Aluminum
• Experimental value 2.86 g/mL
• Theoretical value 2.72 g/mL
%error = theoretical – experimental
theoretical
X 100
• % error = 2.72 g/mL – 2.86 g/m = 5.15%
2.72 g/mL
Practice Problems
1. The experimental value of gold was found to
be 11.9 g/mL and the theoretical value is
19.3 g/mL. What is the percent error?
1. In an experiment the density of iron was
found to be 8.75 g/mL. If theoretical value is
8.71g/mL, what is the percent error?
(http://webmineral.com)
Answers Practice Problems
1. % error = 38.3% (is this really gold)
2. % error = 0.459%