Chemistry Chapter Two

1
Chemistry Chapter Two
Measurement andCalculations
2
Units of Measure Section 2-2

• A quantity is something that has magnitude, size,
  or amount.
• Measurements represents quantities.
• All measurements have a unit that goes with a
  numerical value.
• In Chemistry the SI unit is the standard of
  measurement

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Section 2-2

• A standard is an agreed unit of measure that is
  practical, reproducible, and constant in value.
• There are seven base units of measurement in the
  SI (System International ) or metric system.
• Prefixes are used to represent larger or smaller
  quantity of a base unit.
• This allows for accurate and precise measurement,
  regardless of the measurement being observed.

4
What is Metrology?

• The science of measurement is called metrology.
• The units of measurement must be an agreed unit
  or standard.
• All measurements have an uncertainty and ALL
  units must clearly indicate the level of
  uncertainty.
• Primary standards have the lowest amount of
  uncertainty and have the highest precision.

5
Accuracy and Precision

• Accuracy is the exactness, correctness, or
  closeness to an accepted value of a measurement.
• Precision is the repeatability or reproducibility
  of the measurements with each other.
• Results must be precise and accurate in
  Chemistry.
• Measurements usually made in groups of three and
  the results are averaged.

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Factors that Influence Accuracy

• The instrument being used-
• How precise can the measurement be?
• How many decimal places can be measured?
• What is the range that the instrument is
  accurate?
• Is calibration of the instrument required?
• The skill of the operator
• Are special skills required by the operator?
• Does this require specialized training?
• How does the operator of the measurement tool
  know that their measurements are correct?

7
Percent Error

• Percent error calculations are used to determine
  how close to the true values, or how accurate,
  the experimental values really are to the
  accepted value.
• A negative value for percent error means the
  observed value was less than the true value.
• A positive value for percent error means the
  observed value was larger than the true value.
• (Observed Value - True
  Value)
• Percent Error ----------------------------------
  ------ x 100                                 
             True Value

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The Seven SI Base Units
9
SI Prefixes
10
Metric Conversions
11
Using the 7-Step Metric Conversion
12
Derived Units

• Some quantities cannot be directly measured, but
  are based on a calculation.
• Combinations of SI base units form derived units.
• Examples of derived units AREA
  VOLUME DENSITY ENERGY
  MOLAR MASS

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Table 2-3 Derived Units
14
Volume
15
Volume relationships

• One liter is the same as 1000 cubic centimeters
  (cm3).
• One milliliter (mL) is equal to one cubic
  centimeter (cc) of water.
• One mL of water has a mass of one gram at 4
  degrees Celsius.

16
Density

• Density is the amount of matter in a given space
  or volume.
• Density is a ratio of mass to volume.
• Density is a physical property of matter that is
  constant at a given temperature.
• Density is referenced to a temperature of 20
  degrees C.

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Common Densities
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Density Formula
M Mass in grams D Density in grams per
cc V Volume in cm3
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Density Problems
Mass 250 g Volume 31.25 cc D
____?????___ D M 250 g
V 31.25 cc D 8.00 g/cc
20
Density Problems
Mass 250 g Density 1.0 g/cc V
____?????___ V M 250 g
D 1.0 g/cc V 250 cc
21
Density Problems
Mass ?????? g Density 2.7 g/cc V ____100
cc___ M D V 2.70 g/cc 100 cc M
270 g
22
Heat and Temperature

• Temperature is
• a measure of the average kinetic energy of the
  particles in a sample.
• NOT the same quantity as heat.
• based on the freezing and boiling points of water
  at standard temperature and pressure. (STP)

23
Common Temperature Scales

• Fahrenheit Based on water freezing at 32 degrees
  and boiling at 212 degrees. (180 divisions).
  (STP)
• Celsius Based on water freezing at 0 degrees,
  and boiling at 100 degrees. (100 divisions).
  (STP)
• Kelvin Based on energy content of a material,
  where 0 degrees or absolute zero, there is NO
  kinetic energy. (degree divisions C degree)

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Temperature Scale Comparisons
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HEAT OR HEAT ENERGY

• The sum total of the kinetic energy contained in
  a substance and is measured in joules or
  calories.
• Heat energy always flows from a material of
  higher temperature to one of colder temperature.
• Heat energy is found based on the change in
  temperature, mass of the material and the type of
  material being measured
• The Kelvin scale is the only temperature scale
  that is directly linked to kinetic energy.

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Heat Formula

• where
• q heat in calories or joules
• m mass in grams
• c specific heat in joules or calories per
  gram oC
• DT change in temperature

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Specific Heat Table

• Specific heat is based on the type of material.
• Use the second column when the answer must be in
  joules.
• Use the third column when the answer must be in
  calories.

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Heat Problems

• How much heat in joules needs to be added to
  water to raise 200 grams of water from 20 to 60
  degrees Celsius?
• DT _________ m _________
• c _________ q ?????????
• Formula q m c DT

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Heat Problems

• How much heat in joules needs to be added to
  water to raise 200 grams of water from 20 to 60
  degrees Celsius?
• DT 40 .C m ___200 g___
• c __4.184 J/g .C q ?????????
• Formula q 200 g 4.184 J/g .C 40
  .C
• q 33 472 J

30
Heat Problems

• How much heat in joules needs to be added to iron
  to raise 20 grams of iron from 20 to 80 degrees
  Celsius?
• DT _________ m _________
• c __0.444 J/ g .C q ?????????
• Formula q m c DT

31
Heat Problems

• How much heat in joules needs to be added to iron
  to raise 20 grams of iron from 20 to 80 degrees
  Celsius?
• DT 60 .C m 20 g ______
• c 0.444 J/ g .C q ?????????
• Formula q 20 g 0.444 J/g .C 60
  .C
• q 532.8 J

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Heat Problems

• How many grams of water can be heated from 20 to
  90 Celsius by the absorption of 25 000 J of heat?
  
• DT __________ m ___???????_____
• c q
• Formula m q
• c DT

33
Heat Problems

• How many grams of water can be heated from 20 to
  90 Celsius by the absorption of 25 000 J of heat?
  
• DT 70 .C m ??????
• c 4.184 J/g .C q 25 000 J
• Formula
• m q 25 000 J 85.4 g
• c DT 4.184 J/g .C 70 .C

34
Heat Problems

• What temperature change in Celsius is produced
  when 300 grams of water give off 30 000 J of
  heat?
• DT ??????? m 300 g
• c 4.184 J/g .C q 30 000 J
• Formula
• DT q 30 000 J 23.9 .C
• c m 4.184 J/g .C 300 g

35
Heat Problems

• What temperature change in Celsius is produced
  when 300 grams of water gives off 30 000 J of
  heat?
• DT 70 C m ??????
• c 4.184 J/g C q 25 000 J
• Formula
• m q 25 000 J 85.4 g
• c DT 4.184 J/g C 70 C