Calculating Percent Error
What is it??
Percent Error is used to determine the
inaccuracy, in percentage, of a measured or
estimated value, compared to an accepted
value.
The Components of the Formula


Estimated Value (Measured Value)- The
value that has been derived from an
experiment, or an estimated value.
Actual Value (What you should have
gotten!)- This value is the exact value
excepted throughout the scientific
community, or the value which is determined
exact, at a later point in time.
The formula
The Formula for Percent Error is a follows:
 (EV – AV)
X 100 = Percent Error
AV
Remember:
EV= Estimated Value (measured value!)
AV= Actual Value
PS. We multiply by 100 to make to make the
decimal a percent.

What does it mean if the PE is
positive or negative ?


Negative if your measurement is too small
Positive if your measurement is too big
Practice Problems

Johnny calculates from an experiment that
he has 22.7 grams of carbon. However, he
should have gotten 21.8 grams of carbon,
which was the accepted value for this
experiment. What was his percent error?

Remember: (EV-AV) X 100 = PE
AV
Answer

(EV-AV) X 100 = PE
AV
(22.7-21.8) X 100 = PE
21.8
.900 X 100 = PE
21.8
.0413 X 100 = PE
4.13 % = PE
More Practice

The actual value for the absorption of carbon
is .135. Steven calculated that absorption
was .235. What is the percent error in
Steven’s experiment?
Answer

(EV-AV) X 100 = PE
AV
(.235 - .135) X 100 =PE
.135
.100 X 100 = PE
.135
.741 X 100 = PE
74.1 % = PE
Additional practice
1.
The boiling point of water is 100°C. During
an experiment, water came to a boil at
97°C according to the thermometer that
was being used. What is the percent error of
the thermometer?
answer
3.0%
Additional practice
2. An experiment was conducted to find the
mass of one mole of carbon atoms. The
results of the experiment showed that a mole
of carbon atoms had a mass of 15.78 g. The
accepted value of a mole of carbon atoms is
16.00 grams. What is the percent error in this
experiment?
answer
1.38%
Additional practice
3. An experiment performed to determine the
density of lead yields a value of 10.95 g/cm3.
The accepted value for the density of lead is
11.342 g/cm3. Find the percent error.
answer
3.46%
Additional practice
4. Find the percent error in a measurement of
the boiling point of bromine if the laboratory
figure is 40.6°C and the accepted value is
59.35°C.
answer
31.59 %