Transcript File - Ms. Snyder Chemistry

In science, we very
specific definitions for
accuracy and precision
in regards to how we
measure quantities.
Being
means
that the equipment
reports a measurement
that is close to the
actual,
.
When we talk about the
of equipment, we
mean the
of the values it produces. If we
take 10 measurements of the same sample, do we get
the same value every time?
Precision vs. Accuracy
One of the most common analogies for remembering
the difference between these words is that of a
marksman and a target. For each target, decide if the
marksman was very accurate, very precise or both.
allow instruments
to be more
accurate and more
precise!
The degree of accuracy and precision that an instrument
can reach is related to the size of the
. The
increment is the
(gram, decigram, milligram, etc.)
It is important to understand that the size of the
container is irrelevant;
is related to precision. The smaller the
increment, the more precise the equipment.
Which of the following instruments will measure
mass with the greatest precision?
Electronic
Scale
Maximum
Mass
Smallest
Increment
A
1000 g
10 g
B
500 g
1g
C
600 g
0.5 g
D
500 g
smallest
0.1 g increment
smallest increment
When you look at
different instruments,
you can easily see
which is more
accurate and precise.
But we need to be
able to describe the
level of precision in
our lab reports.
We do this by defining
the significance of the
numbers we use.
The first scale reads 12.7 grams. You may have learned that 12.7
and 12.700 are the same, but you can see that saying this object
had a mass of 12.700 would be inaccurate.
In science our numbers almost always represent measurements,
so we cannot just add zeroes. This would imply we used a more
precise instrument than we did.
There are rules for determining how
significant a number is. You will not
have to memorize them, but it is
important that you understand each
of these rules and why we use them.
Rule 1a: All non-zero digits are significant
12.34 has 4 significant digits.
(All of the digits are significant.)
Rule 1b: Zeroes between non-zero digits are
significant.
101.25 has 5 significant digits.
(All of the digits are significant.)
Rule 2: Leading zeroes are not significant
(Leading zeroes are in front of the numbers.)
0.025 has 2 significant digits.
Rule 2: Leading zeroes are not significant
Some instruments, like the
micropipette shown, only
measure very small amounts of a
substance. They can only
measure between 0.001 mL and
0.100 mL. Because of this, we
don’t consider the leading zeroes
significant. Leading zeroes are
just placeholders.
0.007 mL has 1 sig fig.
Photo by Patrick McAleer
Rule 3: Zeroes to the right of non-zero numbers
are only significant if the decimal point is shown.
120 has 2 significant digits.
120.0 has 4 significant digits.
This idea takes a
little more
explanation. To
start, lets look at
this beaker.
It appears to hold 20 mL,
but remember that the
precision of an
instrument is related to
the size of the
increment. 10 mL
increments are large.
When this same sample is poured into a more
precise graduated cylinder, we see that the beaker
was not very accurate or precise.
18.5 mL rounds to 20 mL. Because of this, we
see that the beaker is only correct to the tens
place. We would record the volume in the
beaker as 20 mL without a decimal showing
that it has only 1 significant digit.
This graduated cylinder measures
in increments of 1 mL. When a
measurement of twenty mL is
measured with this graduated
cylinder, we can be certain that it
is correct to the ones place. We
would record it as 20. mL
showing that it measures to
2 significant figures.
THE DECIMAL SHOWS PRECISION!
This burette measures in
increments of 0.1 mL. When a
measurement of twenty mL is
measured with this burette, we
can be certain that it is correct
to the tenths place. We would
record it as 20.0 mL showing
that it has 3 significant digits.
Just like many grammar rules from English class, this rule feels
awkward to use. That is why it is so important that you follow
the rule and don’t do what “feels” correct.
Both pipettes and
burettes measure to
the 0.1 mL. This
makes them very
precise methods of
adding a volume of
liquid to a container.
Pipette
Burette
The scale on the left shows us that this piece of wood is 8.0 g.
This has 2 sig figs. The scale on the right is much more precise
and shows us that the mass is 8.000 g. This has 4 sig figs. Using
the second scale allows us to report the mass as eight grams
with a much higher degree of confidence.
THE DECIMAL
SHOWS
PRECISION!
22 000 mL has how many sig figs?
When you write a number and are afraid people might
misinterpret the precision, you can clear up any confusion by
placing a number in scientific notation.
22 000 becomes 2.2 × 104
2.2 × 104 mL has how many sig figs?
According to our rules, only the numbers in the coefficient
(also called the significand) are considered significant.
2.2 × 104 has 2 significant figures.
2
6
3
2
1
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3
3
4
3
4
2
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3
4
2
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2
3
2
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2
1
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3
5
2
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2
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