October 7, 2015
Wednesday
Day 14
SWBAT:
Practice math skills to prepare for separation of matter lab.
Significant figures
Accuracy vs precision
Percent error
W-UPS1. What are significant figures and how are they important in science?
2. Differentiate between accuracy and precision.
3. What is percent error and how is it used in science? Define experimental value and accepted value.
ACCURACY and PRECISION
Impromptu lab-ish activity
Materials: 1 styrofoam plate, 3 paperclips
Procedure:
- toss the clips from arms-length – your goal is for the clips to land in the very center of the plate.
The center is about 8cm from the edge of the lower part of the plate
-do three trials each
-Sketch the three trials- indicating where (roughly) all of the clips landed/rested
-Measure how far from the center each clip landed
-Measure how far from each other they landed
-Label each trial- overall accurate, overall precise or both
Accuracy- when clips are close to target (center)
Precision- when clips are close to each other in a tight cluster
Both accuracy and precision- when clips are close to target and each other
How does this translate to measurement in chemistry?
PERCENT ERROR
|error|
_______________
Accepted value
x 100= ______________%
*Error = |experimental value (what you measured) – accepted value (actual value)|
*Don’t forget to include the absolute value symbols in the equation. We don’t really care if you measured over or
under the value, we DO care how far you were from the actual value.
*Don’t forget to label your final answer with the %
Practice problems-write the problems and answers in your notebook.
show your work and circle your final answer.
1. A 250.0 gram block is placed on a balance. The balance measures the mass of the block as 243.9 grams. What is
the percent error of the block?
2. A teacher calculates the mass of skittles as 62.1 g/bag. The true mass of skittles is 56.7 g/bag. Find the
teacher’s percent error.
3. There are 34 questions on a test. John answers 22 of them correctly. What is John’s percent error?
4. You bought a new car and estimated that your monthly payment would be $312. However, your actual
payment amount is $325. How much error was in your estimate?
5. A student buys a rope at the store. The label on the packaging says that the rope is 2.15 meters in length. The
student measures the rope as 1.85m. What is the student’s percent error?
Significant figures
#
Rule description
1
2
3
every non zero number
Zeros between non-zeros
Zeros to the left of non-zeros. If the zeros can be eliminated
when written in scientific notation, but the value remains
intact. This means that they are place holders.
Zeros at the right of non-zeros with no decimal point. They
are place holders. Why? The number could have been
rounded.
Zeros to the end at the right of the decimal indicate careful
measurement.
Whole objects- example: people - we don’t count them as
partial people regardless of their physical condition. They are
significant… they matter… 
4
5
6
Significant
or not?
YES
YES
NO
examples
NO
3000, 40, 90210
YES
17.00, 42390.800
YES
17 people -not 16.75 because one happens
to be missing a limb or two. It is still a
person, perhaps an unlucky person. In
order to represent a whole object, there
will never be anything indicting any more
or less than the number to the left of the
decimal 17.00000000000….(zeroes to
infinity)
Homework: Due Wednesday 10/14
-Read section 3.2 in your textbook and do practice problems
5-6 on page 58
7-8 on page 59
9-10 on page 60
11-12 on page 61
1, 2, 3, 4, 5, 6, 7, 8, 9
7011270006
0.0003404
13-16 on page 62