Transcript Some Review from CHM 1010 by Dr. Gage

Some Review…
Did I learn that in
Gen Chem I?
CHM 1030
PGCC
Barbara A. Gage
Data and Results
• Data = any measured values recorded in
the course of an activity
temperature, mass from balance, volume
in a graduated cyclinder
• Results = any value that is the result of
manipulating the data (subtracting,
converting, finding the log of…)
volume dispensed from a buret, mass of
solid that was massed in a container
CHM 1030
PGCC
Barbara A. Gage
How many decimal places do I
need?
• Don’t ask this question!
• You need to determine this from the
data you are using and any conversions
or constants
• The answer will depend on the number
of significant figures…remember those?
CHM 1030
PGCC
Barbara A. Gage
Significant Figures
They don’t go away…
CHM 1030
PGCC
Barbara A. Gage
Significant Figures
• When making a measurement you must
record all digits you are sure of and one
that is a reasonable estimate
(regardless of where the decimal place
falls)
• The object below is 4.86 cm. You can
be certain of the 4 and 8. The 6 is an
estimate and can vary +/- 1 between
measurements.
CHM 1010
PGCC
Barbara A. Gage
Significant Figures
• The graduated cylinder at the
right contains 38.34 mL of liquid.
It is assumed that you can visually
divide the space between lines
into ten parts.
CHM 1030
PGCC
Barbara A. Gage
Significant Figures –Addition and
Subtraction
•When you are adding and subtracting
numbers you only count the columns
where you are sure of all the values in
that column.
CHM 1030
PGCC
Barbara A. Gage
Significant Figures –
Multiplication and Division
• When multiplying or dividing two or more
values, the answer should contain the number of
digits in the value with the least number of
significant figures.
0.003570 4 sig figs
X
23.4 3 sig figs
0.083538 5 sig figs (calculator answer)
You can only trust the answer to 3 sig figs so the
correct answer is 8.35 x 10-2.
CHM 1030
PGCC
Barbara A. Gage
Exact Numbers
• Some numbers are “exact” such as the
50 in 50 drops (if you count exactly 50)
or 60 s = 1 min
• Exact numbers do not affect the
number of significant figures in an
answer
CHM 1030
PGCC
Barbara A. Gage
Volume Measuring Devices
Why do the numbers run
different ways?
CHM 1030
PGCC
Barbara A. Gage
Measurement Devices
• To Contain (TC)
• Designed to hold a given amount
• Beakers, Erlenmeyer flasks, graduated
cylinders
CHM 1030
PGCC
Barbara A. Gage
Reading a graduated cylinder
Locate the bottom of the meniscus and determine the value
CHM 1030
PGCC
Barbara A. Gage
Measurement Devices
• To Deliver (TD)
• Designed to dispense a given amount
• Burets, pipets
CHM 1030
PGCC
Barbara A. Gage
Reading a buret
Locate the bottom of the meniscus and determine the value
CHM 1030
PGCC
Barbara A. Gage
Balances
• Top loaders in PGCC labs generally measure
to the nearest 0.01g. These should be
used when you do not require data to more
than 0.01g.
• Analytical balances measure to the nearest
0.0001g and are only used for small masses
that require more accuracy.
• 0.0001 g = 0.1 mg !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
• Using the wrong balance is a SERIOUS
error. Be sure to read your activity
carefully!
CHM 1030
PGCC
Barbara A. Gage
Do fingerprints make a
difference?
• Because the analytical balances are so
sensitive, your fingerprints, hand lotion,
hand sanitizer or breakfast residue may
affect the reading.
• Measured objects should be placed on
and taken off the balance with a lab
wipe.
• Did your fingerprints make a
difference?
CHM 1030
PGCC
Barbara A. Gage
Some Miscellaneous Things…
• The analytical balance has some degree
of variability in the last digit. If you
did not transfer any material from your
fingers, there is a chance that the mass
after touching will be 0.0001 g less than
before.
• % error can be positive or negative.
CHM 1030
PGCC
Barbara A. Gage