Students often assume that each
measurement that they make in the laboratory is true and accurate. Likewise, they
often assume that the values that they derive through experimentation are very accurate.
However, sources of error often prevent students from being as accurate as they
would like. Percent error calculations are used to determine how close to the true
values, or how accurate, their experimental values really are.
The value that the student comes up with
is usually called the observed value, or the experimental value.
A value that can be found in reference tables is usually called the true
value, or the accepted value. The percent error can be determined when the true
value is compared to the observed value according to the
equation below:
Let's look at an example of how the
formula would be used in a real-life situation.
Ex. 1 A student measures the mass and
volume of a piece of copper in the laboratory and uses his data to calculate the density o
the metal. According to his results, the copper has a density of 8.37 g/cm3.
Curious about the accuracy of his results, the student consults a reference table
and finds that the accepted value for the density of copper is 8.92 g/cm3.
What would be the student's percent error?
Solution - Step 1. Determine which values are
known.
The students result, or the observed value = 8.37 g/cm3.
The accepted, or true value = 8.92 g/cm3.
Step 2. Substitute these values in the percent error calculation, as shown below:
Step 3. Solve for the unknown, and round to correct significant digits.
Percent Error = -6.17%
Please note that the negative sign does not mean that the
error was less than zero, which would be impossible. It shows that the student's
calculated value was actually too low. Many teachers may ask you to
report the absolute value of your answer.
Now, be sure to check out the worksheets
and the online
quizzes!
