5/30/2013
Types of Observations and
Measurements
• We make QUALITATIVE observations of
reactions — changes in color and physical
state.
• We also make QUANTITATIVE
MEASUREMENTS,, which involve numbers.
MEASUREMENTS
• Use SI units — based on the metric system
• Use Dimensional Analysis to convert to proper
units
• Final answers have correct Significant Figures
and Label
Quick Review of Sig. Figs.
have infinite number of sig. figs.
• Atlantic-Pacific Rule
– All digits other than zero are always
significant
– Zeros between nonzero digits are significant
– Leading zeros after a decimal point are NOT
significant
– Ending zeros when no decimal point is
present are NOT significant
• Adding/Subtracting – least number of
decimal places to right of decimal point
• Multiplying/Dividing – least # sig. Figs. as in
problem
Practice Sig. Figs.
UNITS OF MEASUREMENT
How many sig. figs. are in these numbers?
Use SI units — based on the metric system
1. 0.00340
1. 3
Length
2. 23,010
Meter, m
2. 4
3. 5.600 x 107
Mass
3. 4
Kilogram, kg
How many sig. figs. should be in these answers?
4. 65.0 x 70.00 x 3450
4. 3 (don’t need to do problem
to determine this)
5. 1.2050
5. 3 (answer comes out 4.855
and 0.45 has only 2 to the right
so 4.86 is correct)
+ 3.200
Time
Seconds s
Seconds,
Temperature
Amount
Celsius degrees, ˚C
and/or kelvin, K
Mole
(6.02 x 1023 atoms or molecules)
+ (4.5 x 10-1) =
1
5/30/2013
Converting within the metric
system using units of length
Converting within the metric
system using units of length
• 1 kilometer (km) = ? meters (m)
• 1 kilometer (km) = 1 x 103 meters (m)
• 1 meter (m) = ? centimeters (cm)
• 1 meter (m) = 1 x 102 centimeters (cm)
• 1 centimeter (cm) = ? millimeter (mm)
• 1 centimeter (cm) = 10 millimeter (mm)
• 1 nanometer (nm) = ? Å
• 1 nanometer (nm) = 10 Å
M - - k h da base d c m - - μ - -n Å
Accuracy, Precision and Error
• Accuracy = how close your measurement is
to the “true” value
• Precision = How close your measurements
are to each other
• Deviation or Relative Error = distance your
answer is from the accepted or “true” valuealways positive
• Percent Error-formula applied is…
accepted value – experimental value x 100 =
accepted value * Absolute value = always positive
O—H distance =
9.4 x 10-11 m
9.4 x 10-9 cm
0.094 nm
Practice Error Problem
• During an experiment you found that the volume
of a gas was 236 mL and the teacher gave you
the accepted value as 0.225 dm3. What is the
percent error?
• First convert your answer into dm3
236 mL 1 cm3 1 dm3
= 0.236 dm3
1 mL 103 cm3
Now use the percent error formula to
calculate your percent error…
|0.225 – 0.236| x 100 = 4.9 % (positive)
0.225
2