1. Accuracy and Precision

2. Accuracy
Accuracy – how closely a measurement agrees with an accepted value.
For example:
The accepted density of zinc is 7.14 g/cm3
Student A measures the density as 5.19 g/cm3
Student B measures the density as 7.01 g/cm3
Student C measures the density as 8.85 g/cm3
Which student is most accurate?

3. Error Error = experimental value – accepted value
All measurements have some error.
Scientists attempt to reduce error by taking the same measurement many times.
Example: The accepted value for the specific heat of water is J/gºC. Mark measures the specific heat of water as J/gºC. What is Mark’s error?
Error = exp.value – acc.value
Error = J/gºC – J/gºC
Error = J/gºC

4. Percent Error │error │ %Error = x 100% acc.value
Example: The accepted value for the molar mass of methane is g/mol. Jenny measures the molar mass as g/mol. What is Jenny’s percent error?

5. Precision
Precision – describes the closeness of a set of measurements taking under the same conditions.
Good precision does not mean that measurements are accurate.

6. Accuracy and Precision

7. Example Problem
Working in the laboratory, a student finds the density of a piece of pure aluminum to be 2.85 g/cm3.  The accepted value for the density of aluminum is g/cm3.  What is the student's percent error?