Accuracy and Precision

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?

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 4.184 J/gºC. Mark measures the specific heat of water as 4.250 J/gºC. What is Mark’s error? Error = exp.value – acc.value Error = 4.250 J/gºC – 4.184 J/gºC Error = 0.066 J/gºC

Percent Error │error │ %Error = x 100% acc.value Example: The accepted value for the molar mass of methane is 16.042 g/mol. Jenny measures the molar mass as 14.994 g/mol. What is Jenny’s percent error?

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

Accuracy and Precision

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 2.699 g/cm3.  What is the student's percent error?