Transcript How Sweet It Is Chemistry Lab

Quantitative vs. Qualitative
• Make a quantitative observation about your
textbook
• Make a qualitative observation about your
textbook
Quantitative vs. Qualitative
• Quantitative observation:
• Qualitative observation:
Precision vs. Accuracy…
• Archery Activity
Precision vs. Accuracy
• Which is more precise for measuring volume,
a beaker or a graduated cylinder?
Precision vs. Accuracy
• Accuracy: refers to the closeness of
measurements to the correct or accepted
value of the quantity measured.
• Precision: refers to the closeness of a set of
measurements of the same quantitiy made in
the same way.
Precision vs. Accuracy
• Measured values that are accurate are close
to the accepted value
• Measured values that are precise are close to
one another but not necessarily close to the
accepted value
Darts within
small area =
High precision
Area covered
on bull’s-eye =
High accuracy
Darts within
small area =
High precision
Area far from
bull’s-eye =
Low accuracy
Darts within
large area =
Low precision
Area far from
bull’s-eye =
Low accuracy
Darts within
large area =
Low precision
Area centered
around bull’s-eye
= High accuracy
(on average)
Unit conversions
• Copy metric conversion from book
Unit Conversions
• Practice problems:
750 km = __________m?
283 m = __________km
112 Mwatt = __________Kwatt?
112 Mwatt = __________Gwatt
Scientific Notation & Significant
Figures
Unit Estimation
Scientific Notation
• Used to make numbers more usable
• 1,000,000,000 = 1x109
• 0.0000000001=1x10-10
How do you figure this out?
• You move the decimal until you have only
one digit in front of the decimal.
• If you move right, then the exponent will
be NEGATIVE based on the number of
places your decimal moved.
• If you move left, then the exponent will be
POSITIVE based on the number of places
your decimal moved.
Practice
• Give the following in scientific notation
– 6,289,030,987
– 0.004500678
– 5.60987
– 568.2365400
– 35.98340002
– 0.23476
Give the following in
scientific notation…
Practice
– 6,289,030,987 =
– 0.004500678 =
– 5.60987 =
– 568.2365400 =
– 35.98340002 =
– 0.23476 =
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6.289030987x109
4.500678x10-3
5.60987
5.682365400x102
3.59834002x10
2.3476x10-1
Going the other way…
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1.3487x105 =
4.9800456x104 =
2.345x101 =
5.6789x10-3 =
3.591x10-1 =
2.0080x10-2 =
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134,870
49,800.456
23.45
0.0056789
0.3591
0.020080
Try For Yourself
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7.234x10-5=?
8.234x103=?
5.000x10-4=?
9.99998x10-2=?
8.555x106=?
ANSWERS
7.234x10-5 = 0.000 072 34
8.234x103 = 8,234
5.000x10-4 = 0.000 500 0
9.99998x10-2 = 0.099 999 8
8.555x106 = 8,555,000
Significant Digits - What is it?
• When we take measurements in science, we
can only be sure of our numbers to a certain
point
• The numbers we are sure of are called
significant digits or significant figures (“sig
figs”)
Sig Figs - How do we use them?
• Two types
– Measured
• You actually measure and record your answer to a
certain digit
– Calculated
• You use already measured numbers to compute an
answer
Measured Sig Figs
• Questions you can answer:
– How long is your book?
• Measure it with a meterstick and read the length.
– What is the mass of an orange?
• Put it on a scale and read the mass.
– How much milk is in the carton?
• Pour the milk into a graduated cylinder and read the
volume.
Calculated Sig Figs
• Sometimes, you’ve collected the data and you
need to calculate a final answer
• Example - you find the length, width and
height of your book and you want to find the
volume.
– You need to multiply the three numbers together
to get an answer.
Determining what counts…
Sig Fig Rules!
• All non-zero numbers are significant
– Example 1,2,3,…,9
• All zeros between non-zero numbers are
significant
– Example
1080.305
• All zeros before a written decimal are
significant
– Example
600.
More Rules…
• All zeros following non-zero numbers, after a
decimal are significant
– Example
1.00
0.003470030
• These rules are to determine what counts
when you are looking at a number.
Practice
• How many sig figs are in the following
numbers?
– 2.341
– 0.0004580
– 560
– 560.
– 560.0003
Answers
2.341 has 4 sig figs
All the numbers are non-zero digits, so they all
count!
Answers
0.0004580 has 4 sig figs
The three non-zero numbers 458 and the zero
following this set
The first four zeros are place holders - they get
the 4 into ten thousands place
Answers
Another way to think about 0.0004580 having four
sig figs is to write it in scientific notation
0.0004580=4.580x10-4
When you write in scientific notation, you only write
the sig figs before you write the x10whatever
So here you see that you wrote the 4, 5, 8, and 0.
Those are the sig figs!
Answers
560 has 2 sig figs
This one is tricky. Notice that there is no
decimal, so the zero is just a place holder to
get the 6 into the tens spot.
Answers
560. Has 3 sig figs.
This time the zero counts because the decimal
means it was actually measured.
Answers
560.0003 has 7 sig figs
All zeros are between non-zero digits, so they
are all significant.
How do you know when
to stop?
• When you’re measuring, you know when to
stop based on your equipment.
• If your equipment reads to the tens, then you
can guess up to one more place. You can read
to the ones…
• Let’s look at it.
Multi step calculations
• Keep One Extra Digit in Intermediate Answers
• When doing multi-step calculations, keep at least one
more significant digit in intermediate results than
needed in your final answer.
• For instance, if a final answer requires two significant
digits, then carry at least three significant digits in
calculations. If you round-off all your intermediate
answers to only two digits, you are discarding the
information contained in the third digit, and as a result
the second digit in your final answer might be
incorrect. (This phenomenon is known as "round-off
error.")
2 Greatest Sins in Sig Figs
• Writing more digits in an answer
(intermediate or final) than justified by the
number of digits in the data.
• Rounding-off, say, to two digits in an
intermediate answer, and then writing three
digits in the final answer.
Reading the right number
of digits.
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Ruler/Meterstick
Graduated Cylinder
Beaker
Scale
Calculations - The rules!!!
• Addition/Subtraction
– Your answer should have the same number of
decimal places as the number with the least
number of decimal places
• Multiplication/Division
– Your answer should have the same number of sig
figs as the number with the least number of sig
figs
• Always follow the order of operations!
Practice
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2.786 + 3.5 =
0.0004 x 3001 =
65 + 45.32 x 90 =
45.6 - 34.23 =
900.3/30.2450 =
Percent Error
• Percent error determines how accurate an
experimental value is compared quantitatively
with the correct or accepted value.
• Percent error: calculated by subtracting the
experimental value from the accepted value,
dividing the difference by the accepted value,
and then multiplying by 100
Percent Error
Percent error = Valueaccepted – Valueexperimental x 100
Valueaccepted
Percent error can have a positive or negative value
Percent Error
A student measures the mass and volume of a
substance and calculates its density as 1.40
g/mL. The correct, or accepted value of the
density is 1.36 g/mL. What is the percent error
of the student’s measurement?
= 1.36g/mL – 1.40 g/mL x 100 = -2.9%
1.36 g/mL
Percent Error
What is your percent error from the lab when you
found the density of water?
Your experimental
value
= 1.00g/mL –
g/mL x 100 = -2.9%
1.00 g/mL
Percent Error – pg. 45
Two technicians independently measure the density
of a new substance.
Technician A Records: 2.000, 1.999, 2.001 g/mL
Technician B Records: 2.5, 2.9, and 2.7 g/mL
The correct value is found to be: 2.701 g/mL.
Which Technician is more precise? Which is more
accurate?
B
A
Go Through Answers on Packet
Directly Proportional
• Two quantities are directly proportional if…
– Dividing one by the other gives a constant value
• y/x = k
• k = constant
• You can rearrange above equation by saying : y = kx
– If one increases…the other increases at the same
rate (doubling one constant = doubles the othr
• 2y/2x = k (constant)
Directly Proportional
All directly
proportional
relationships
produce linear
graphs that pass
through the origin
Inverse Proportions
• Two quantities are inversely proportional if…
– Their product is constant
• xy = k
• k = constant
– The greater the speed = less time to travel a given
distance
– Double speed (2x) = ½ required time
– Halving the speed (½) = 2 times the time
Inverse Proportional
How Sweet It Is
Chemistry Lab
How Sweet It Is Lab
• Benedicts Solution
Water Bath Test:
• Results:
– Beverages should
have tested
positive if they
had a sugar
sweetener
– Beverages should
test negative if
they had an
artificial
sweetener
How Sweet It Is Lab
• What beverages tested positive?
• What beverages tested negative?
• Evaluate against labels on Sodas
How Sweet It Is Lab
• What did you notice about the densities of the
solutions?
• Which ones had artificial sweeteners?
Densities less than one?
• Which ones had natural sugar sweeteners?
Densities more than one?
How Sweet It Is Lab
Analysis Questions
1. Evaluate the results against the labels on the
soda? Record actual sweeteners on a table in
your lab write-up.
• How accurate were your results?
How Sweet It Is Lab
• Analysis Questions:
2. Which sample do you think had the
highest/lowest sugar content? Explain why
you think this.
How Sweet It Is Lab
• Application Questions:
1. How could you prove that carbonated water
contains no sweetener?
How Sweet It Is Lab
• Application Questions:
2. How could you determine a regular/diet soda
by using density and not opening the can?
Immerse in water…which one will sink…which
one will float?