Measurement

1
Measurement
2
Common laboratory devices
3
In the chemistry laboratory, the balance is
commonly used to measure mass
Central Carolina Scale. Metler Toledo PL Series
Toploading Balances. Online. Available
http//mettler-toledo.centralcarolinascale.com/PL-
Balance.htm. Accessed 28 June 2010.
4
In the chemistry laboratory, the ruler is
commonly used to measure length
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
5
In the chemistry laboratory, the timepiece and
stopwatch are commonly used to measure time
Timex. Online. Available http//www.timex.com/.
Accessed 28 June 2010. Holabird Sports. Timex
Marathon 50-Lap Stopwatch. Online.
Available http//www.holabirdsports.com/m/Running
-Gear/Pedometers-Stopwatches/Timex/p1/870196.htm.
Accessed 28 June 2010.
6
In the chemistry laboratory, the thermometer is
commonly used to measure temperature
The Beer Keg. Online. Available
http//thebeerkeg.co.za/catalog/index.php?cPath30
. Accessed 28 June 2010.
7
In the chemistry laboratory, different devices
are used to measure volume
Brown, , E. LeMay, and B. Bursten. 2000.
Chemistry The Central Science. 8th ed. Phils
Pearson Education Asia Pte. Ltd.
8
What did the thermometer say to the graduated
cylinder?
You may have graduated, but Ive got many degrees
http//www.123rf.com/photo_3394414_thermometer--sm
iling-cartoon-illustration-as-vector.html http//2
7melody27.deviantart.com/art/Graduated-Cylinder-13
4411332
9
Units
10
Units are essential in stating measurements
correctly
Total Distance
miles 718.44
kilometers 1156.18
Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
Online.
Available http//www.mapcrow.info/cgi-bin/cities_
distance_airpt2.cgi?city3-33717352CQcity491700
302CT. Accessed 26 June 2010.
11
The Cubit

• length of the arm from the tip of the finger
  to the elbow

National Physical Laboratory. History of Length
Measurement. Online. Available
http//www.npl.co.uk/educate-explore/posters/histo
ry-of-length-measurement/history-of-length-measure
ment-(poster). Accessed 17 June, 2009.
12
The Yard

• "the distance from the tip of the King's
  King Henry I nose to the end of his
  outstretched thumb"

National Physical Laboratory. History of Length
Measurement. Online. Available
http//www.npl.co.uk/educate-explore/posters/histo
ry-of-length-measurement/history-of-length-measure
ment-(poster). Accessed 17 June, 2009.
13
The SI system has seven base units from which all
other units are derived
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
14
The temperature scales that are currently in use
differ in the size of the unit and/or the zero
point temperature
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
15
It is possible to convert from one temperature
scale to another
Fahrenheit to Celsius
Celsius to Fahrenheit
Kelvin to Celsius
K 0C 273.15
Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
16

• Solder, an alloy made of tin and lead, is
  used in electronic circuits. A certain solder has
  a melting point of 224C. What is its melting
  point in F?

Answer 435F
Ebateable Blog. Solder On, Soldier! Online.
Available http//blog.ebates.com/ebates/2009/08/
solder-on-soldier.html. Accessed 26 June
2010. Howard Electronic Instruments, Inc.
No-Clean Solder Paste and NC Core Wire Solder.
Online. Available http//www.howardelectronics.c
om/amtech/nc500.html. Accessed 26 June 2010.
17
SI units are modified in decimal fashion by a
series of prefixes
Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
18
Scientific notation
19
To manage numbers more conveniently, scientific
notation is used

• N x 10n
• For N,
• 1 N lt 10
• n exponent
• positive or negative whole number

Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
20
If N lt 1 or N 10, count the number of places
that the decimal point must be moved to give the
proper N

• If the decimal point has to be moved to the left
• add number of times from the original n
• If the decimal point has to be moved to the right
• subtract number of times from the original n

Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
21
Convert the following into correct scientific
notation

• 20568
• Answer 2.0568 x 104
• 0.2193
• Answer 2.193 x 10-1
• 25 x 103
• Answer 2.5 x 104
• 0.25 x 10-3
• Answer 2.5 x 10-4

22
How long is the metal strip shown below?
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
23
Significant figures
24
The number of significant figures in a
measurement depends on the measuring device
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
25
Rules of Significant Figures

• Any non-zero digit is significant
• Zeros between nonzero digits are significant
• Zeros to the left of the first nonzero digit are
  not significant

Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
26
Rules of Significant Figures

• For numbers with a decimal point, zeros after the
  last non-zero digit are significant
• For numbers without a decimal point, zeros after
  the last non-zero digit may or may not be
  significant
• Scientific notation removes the ambiguity
• Counting numbers and conversion factors have
  unlimited number of significant figures

Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
27
How many significant figures does each quantity
have?

• 5.35 g
• Answer 3 sf
• 3.80 moles
• Answer 3 sf
• 607 K
• 3 sf

28
How many significant figures does each quantity
have?

• 0.0080 L
• 2 sf
• 26 letters in the English alphabet
• Unlimited number of sf
• 1 km 1000 m
• Unlimited number of sf

29
In rounding off numbers, look at the leftmost
digit to be dropped

• If the leftmost digit to be removed is less than
  5, the preceding number is left unchanged
• If the leftmost digit to be removed is 5 or
  greater, the preceding number is increased by 1

Example 7.248 g Rounded to two significant
figures 7.2 g Rounded to three significant
figures 7.25 g
30
Rules for significant figures in answers
1. For addition and subtraction. The answer has
the same number of decimal places as there are in
the measurement with the fewest decimal places.
Example adding two volumes
106.78 mL 106.8 mL
Example subtracting two volumes
863.0879 mL 863.1 mL
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
31
Rules for significant figures in answers
2. For multiplication and division. The answer
contains the same number of significant figures
as there are in the measurement with the fewest
significant figures.
Example multiplying numbers
23.4225 cm3 23 cm3
9.2 cm x 6.8 cm x 0.3744 cm
Silberberg, M. 2010. Principles of General
Chemistry. 2nd ed. New York McGraw-Hill.
32
Rules for significant figures in answers

• Counting numbers and conversion factors have
  unlimited number of significant figures
• For multiple-step problems, carry on some
  insignificant numbers

33
Perform the following calculations and express
the final answer with the correct number of
significant figures
Answer 2.104 cm
Answer 4.16 g/cm3
34
Terms used in discussing uncertainties
35
Precision is a measure of how closely individual
measurements agree with one another
Brown, , E. LeMay, and B. Bursten. 2000.
Chemistry The Central Science. 8th ed. Phils
Pearson Education Asia Pte. Ltd.
36
Accuracy refers to how closely individual
measurements agree with the correct, or true
value
Brown, , E. LeMay, and B. Bursten. 2000.
Chemistry The Central Science. 8th ed. Phils
Pearson Education Asia Pte. Ltd.
37
Factor-label method
38
The factor-label method is used to convert from
one unit to another
1 peso 4 25-centavos
Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
39
SI units are modified in decimal fashion by a
series of prefixes
Chang, R. 2002. Chemistry 7th ed. Singapore
McGraw-Hill.
40
Perform the following calculations and express
the final answer with the correct number of
significant figures

• 234 pesos to 25-centavos
• Answer 936 25-centavos
• 57.8 m to cm
• Answer 5.78 x 103 cm
• The density of silver is 10.5 g/cm3. Convert the
  density to units of kg/m3.
• Answer 1.05 x 104 kg/m3