Sig figs, Chemistry units

1. Units Of Measurement
Mrs. Mawhiney

2. Measurement of mass, length
and volume
• In the United States, we use a fairly awkward
system of measurement for most things - the
English system Scientists use the metric and SI
systems of units for the measurement of
physical quantities
• This system using standard units based on very
precisely known properties of matter and light
• Prefixes are used in from of the units to indicate
powers of ten

3. SI Units
Measurement Unit Symbol
Mass Kilogram kg
Length Meter M
Time Second s
Temperature Kelvin K
Quantity Mole mol
Energy Joule J
Pressure Pascal Pa

4. . Base Units
Mass - the quantity of matter that a sample
contains
• Note that weight is a measure of the attraction of
gravity for a sample and it varies depending on
the distance of the mass to a planet or moon
• Scientists often speak imprecisely of the “weight”
of an amount of substance. They really mean
mass.

5. Basic SI units/Derived units
Used to generate new Units
• Volume - space a given quantity of matter
occupies
• Volume - expressed in terms of length - m3
• m3
- an inconveniently large volume, so we
use liter (L; one cubic decimeter)
• We often use a mL (1 cubic centimeter) for
more manageable amounts of matter

6. Converting between units
• The standard method to convert between
two different units is the factor-label or
dimensional analysis method
• Dimensional analysis converts a
measurement in one unit to another by the
use of a conversion factor
• Conversion factors are developed from
relationships between units

7. Measurements and Units
Measurement - determines the quantity,
dimensions or extent of something
1.Consist of two parts
a. a numerical quantity (1.23)
b. a specific unit (meters)
Unit - a definite quantity adapted to as a
standard of measurement

8. Features of Measured
Quantities
When we measure a number, there are physical
constraints to the measurement
Instruments and scientists are not perfect, so the
measurement is not perfect (i. e., it has error)
The error in the measurement is related to the
accuracy and the precision of the measurement

9. Accuracy and Precision
Accuracy – how close the measurement is
to the “true” value (of course we have to
know what the “true” value is)
Precision – is a measure of how closely
individual measurements agree with one
another.

10. Example: Accuracy and
Precision

11. Equations for Precision and
Accuracy
1. Precision
2. Accuracy
Absolute Error
% AE = (True value-Avg Value) X 100
True Value

12. Significant Figures
•Any digit that is not zero is significant
1.234 kg 4 significant figures
•Zeros between nonzero digits are significant
6006 m 4 significant figures
•Zeros to the left of the first nonzero digit are not significant
0.08 L 1 significant figure
•One or more final zeros to the right of the decimal point are
significant
2.00 mg 3 significant figures
0.00420 g 3 significant figures
10.006000 8 sig figs

13. Counting Significant Figures
Atlantic / Pacific Method
a. Absent Decimal- Start on “atlantic” side
of number & cross out all zeroes until 1st
nonzero digit is reached, remaining digits
are significant
b. Present decimal- start on the “pacific” side
of the number & cross out all zeros until the
1st
nonzero digit Is reached, remaining digits
are significant

14. How many significant figures are in
each of the following measurements?
24 mL 2 significant figures
3001 g 4 significant figures
0.0320 m3
3 significant figures
6.400 x 104
molecules 4 significant figures
560 kg 2 significant figures

15. Significant Figures
Addition or Subtraction
The answer cannot be more accurate than any of the
original numbers.
89.332
1.1+
90.432 round off to 90.4
one significant figure after decimal point
3.70
-2.9133
0.7867
two significant figures after decimal point
round off to 0.79
370
-291.33
78.67
Number is rounded to “tens” place
round off to 80

16. Significant Figures
Multiplication or Division
The number of significant figures in the result is set by the original
number that has the smallest number of significant figures
4.51 x 3.6666 = 16.536366 = 16.5
3 sig figs round to
3 sig figs
6.8 ÷ 112.04 = 0.0606926
2 sig figs round to
2 sig figs
= 0.061

17. Significant Figures
Exact Numbers
Numbers from definitions or numbers of objects are considered
to have an infinite number of significant figures
The average of three measured lengths; 6.64, 6.68 and 6.70?
6.64 + 6.68 + 6.70
3
= 6.67333 = 6.67
Because 3 is an exact number
= 7

18. Scientific notation and
significant figures
1. When using scientific notation the base
must be written with the correct number of
significant digits
2. All zeroes are significant when using
scientific notation