Transcript Intro to Chem

Scientific Measurement
Chapter 3
Wilbraham, Antony C., Dennis D. Staley, Michael S. Matta, and Edward L. Waterman. Prentice Hall Chemistry. 1st ed. Upper Saddle River, NJ: Pearson Prentice Hall, 2008.
3.1 Measurements and their
Uncertainty

Using and Expressing Measurements
◦ Measurement – a quantity that has both a
number and a unit.
◦ Measurements are fundamental to the
experimental sciences.
 It is important to be able to make measurements
and to decide whether a measurement is correct.
◦ Scientific notation:
 602,000,000,000 = 6.02 x 1011
 0.0000059 = 5.9 x 10-6

Accuracy, Precision, and Error
◦ Accuracy – measure of how close a
measurement come to the actual or true
value of whatever is measured.
◦ Precision – is a measure of how close a series
of measurements are to one another.
 To evaluate the accuracy of a measurement, the
measured value must be compared to the correct
value. To evaluate the precision of a measurement,
you must compare the values of two or more
repeated measurements.
Darts precision & accuracy
Daily Exit Slip 9/6/2011
1.
Target score is 100, who is A & P, P, not A
or P
◦ Mark scored 97, 88, 95
◦ Tim scored 90, 91, 90
◦ Joe scored 98, 99, 100
2.
3.
Convert 500 mL to L
Convert 73 m to cm
Significant Figures in Measurements
Significant figures – in a measurement
include all of the digits that are known,
plus a last digit that is estimated.
 Measurements must always be reported
to the correct number of significant
figures because calculated answers often
depend on the number of significant
figures in the values used in the
calculation.

Sig Fig Rules (Zeros)
Zeros between nonzero are always significant
 Zeros at the beginning of a # are never
significant (place holders)
 Zeros that fall both at the end of a number and
after the decimal point are always significant
(0.02000, 1.010, 9.000 = 4)
 When a number ends in zeros but contains no
decimal point, the zeros are not significant (130
= 2)

Uncertainty in Measurement

Exact Numbers: (values know exactly)
◦ 12 eggs in a dozen, 1000 g in 1 kg
◦ Count the exact number of people in a group

Inexact numbers: (values that have
uncertainty)
◦ Numbers obtained by measurements
Sig Figs
2.003
1.
5.
◦ four sig figs
2.2
2.
◦
2.345
3.
◦
4.
two sig figs
4 sig figs
0.002345
◦ 4 sig figs
200
◦ 1 sig fig
6.
200.
◦ 3 sig figs
Sig Fig Rules Calculation
Least certain measurement used in a
calculation limits the certainty of the
calculated quantity
 Final answer for any calculation should be
reported w/only one uncertain digit.

Sig Fig Multiplication/Division
The result must be reported with the
same number of sig figs as the
measurement w/the fewest sig figs
 6.221 cm x 5.2 cm = 32.3492 =32cm2

Sig Fig Rules Addition/Subtraction

The result can have no more decimal
places than the measurement w/the
fewest number of decimal places.
20.4
1.322
83_____
104.722
105
3.2 International System of Units

Measuring with SI Units
◦ SI units is a revised version of the metric
system.
 From the 7 base units all other measurements can
be derived.
 5 SI units used by Chemist
 Meter (m), kilogram (kg), Kelvin (K), second (s), and mole
(mol)
Prefix
Factor
mega (M)
106
kilo (k)
103
deci (d)
10-1
centi (c)
10-2
milli (m)
10-3
micro (µ)
10-6
nano (n)
10-9
pico (p)
10--12
Units and Quantities

Units of length
◦ Meter, kilometer, centimeter
Unit
Relationship
Example
Kilometer (km)
1 km = 1000 m
Length of about 5
city blocks
Meter (m)
Base unit
Height of doorknob
from the floor
Decimeter (dm)
10 dm = 1 m
Diameter of large
orange
Centimeter (cm)
100 cm = 1 m
Width of shirt
button
Millimeter (mm)
1000 mm = 1 m
Thickness of a dime
Micrometer (µm)
106 = 1 m
Diameter of
bacterial cell
Units and Quantities

Units of volume
◦ Liter, milliliter, cubic centimeter, and microliter
Unit
Relationship
Example
Liter (L)
Base Unit
Quart of milk
Milliliter (mL)
1000 mL = 1 L
20 drops of water
Cubic centimeter
(cm3)
1 cm3 = 1 mL
Cube of sugar
Microliter (µL)
106 µL = 1 m
Crystal of table salt
Units and Quantities

Units of Mass
◦ Kilogram, gram, milligram, and microgram
Unit
Relationship
Example
Kilogram (kg)
Base Unit
Small textbook
Gram (g)
1000 g = 1 kg
Dollar bill
Milligram (mg)
1000 mg = 1 g
10 salt grains
Microliter (µg)
106 µg = 1 g
Particle of baking
powder

How does weight differ from mass?
◦ Mass – is measure of the amount of matter an
object contains.
◦ Weight – is a force that measures the pull on
a given mass by gravity.
 Weight can change by location, but mass remains
the same.
Units and Quantities

Temperature
◦ Measure of how hot or cold and object is.

Units of Temperature
◦ Degree Celsius and Kelvin
 Celsius water freezes at 0°C and boils at 100°C
 Kelvin water freezes at 273.15K and boils at
373.15K,
 0K or Absolute Zero = - 273.15°C
◦ Converting between the two.
 K = °C + 273
 °C = K – 273
Units and Quantities

Energy
◦ Capacity to do work

Units of Energy
◦ Joule (j) and calorie (cal)
 1 cal is the quantity of heat that raises the
temperature of 1 g of pure water by 1°C.
◦ Converting between the two.
 1J = 0.2390 cal
 1 cal = 4.184 J
3.2 Section Assessment
Page 79
 #s: 18, 19, 23 – 26.

C 3.1 – 3.2 Test Review
Page 96,
 #s: 57 - 61, 63, 66, 79, 80, 82, 84

3.3 Dimensional analysis

Conversion factor – is a ration of
equivalent measurements.
◦ 1000 m = 1 km