chemistry
DESCRIPTION
Chemistry. Chapter 3 Scientific Measurement. Qualitative Measurement. Gives results in a descriptive form Nonnumeric. Quantitative Measurement. Gives results in a definite form Usually as numbers and units. Scientific Notation. Shorthand way to express very large and very small numbers. - PowerPoint PPT PresentationTRANSCRIPT
Chemistry
Chapter 3
Scientific Measurement
Qualitative Qualitative MeasurementMeasurement
•Gives results in a descriptive form
•Nonnumeric
Quantitative Quantitative MeasurementMeasurement
►Gives results in a Gives results in a definite formdefinite form
►Usually as numbers Usually as numbers and unitsand units
Scientific Notation
•Shorthand way to express very large and very small numbers
Example3.6 x 104
= 3.6 x 10 x 10 x 10 x 10
= 36 000
0.0081 =8.1 x 10-3
Direction of decimal movement
To the left is +
To the right is -
–Operations with numbers in scientific notation
MultiplicationMultiplication
–Multiply the numbers and then add the exponents
DivisionDivision
–Divide the numbers and subtract the exponents
Addition and subtraction
Exponent must be the same to proceed
Must move the decimal appropriately and then adjust the exponent
Then you can solve the problem
Measured values only as reliable as the instrument used to take the measurement!
Uncertainty in measurement
AccuracyMeasure of how close a measurement comes to the actual or true value
PrecisionPrecision–Measure of how close a Measure of how close a series of measurements series of measurements are to one anotherare to one another
Pg. 64 Dartboard example
In class: Pg. 97 #80
Evaluating the accuracy of a Evaluating the accuracy of a measurementmeasurement
• Percent error
• Percent error = [error] X 100
accepted value
Error - the difference between the accepted value and the experimental value (absolute value)
• Experimental value – measured in the lab
• Accepted value – correct value based on reliable references
• Pg. 65 example
Everyone understand so far?
Good!!!
Significant figures in measurements (sig figs)
Rules page 66-67
Sample problems
Pg. 68
Sig Figs in Calculations
Rules for rounding Pg. 68Page 69 Sample
Solving problems with sig figs
Multiplying and dividing with sig figs
The answer you get must be rounded to the same number of sig figs as the measurement with the lowest number of sig figs (that you multiplied or divided)
Example
Multiply 4.610 feet by 1.7 feet. Express your answer in correct sig figs
4.610 x 1.7 = 7.837 How do you round it? 4.610 has 4 sig figs 1.7 has 2 sig figs Round answer to 2 sig figs Answer = 7.8 square feet
Adding and Subtracting with sig figs
When adding or subtracting measurements, the answer cannot have more certainly than the least certain measurement.
Answer must have the same number of sig figs to the right of the decimal point as the measurement with the fewest sig figs to the right of the decimal point
Example
4.271 grams (3 sig figs to the right of decimal)
2 grams (0 sig figs to the right of decimal)
+ 10.0 grams (1 sig fig to the right of decimal)
16.271 grams round 16 grams
Handout practice – work with a partner!
Grab a calculator
SI System of UnitsSI System of Units
•Page 73 Units of measurement
•Table 3.1
• Metric system established in France in 1790
• SI Adopted by international agreement in 1960
PrefixesPage 74 Table 3.2
Length
SI unit - meter (m)
Pg. 74 Table 3.3
VolumeSpace occupied by any sample of matter
L x W x H“Derived” unitPg. 75 Table 3.4
•Volume of a cube 1m on each side
•SI unit = m3 •More common to use Liter (L) = dm3
1 Literthe volume occupied by a cube 10 cm on each side
10 cm x 10 cm x 10 cm = 1000 cm3
1000 cm3 = 1 L
1 dm = 10 cm
1 L = 1 dm3
1 mL = 0.001 L1000 mL = 1 L1000 cm3 = 1000 mL = 1 L
•Volumes for solids, liquids, gases change with change in temperature
Much more dramatic with gases
Measuring devices calibrated at 20oC
Room temperature
MassMass•Difference between mass and weight
•SI unit = Kilogram (kg)•1 g = 0.001 kg•1000 g = 1 kg•Pg. 76 Table 3.5
• Will show on board something special about H2O
Temperature Scales
CelsiusKelvin
Absolute zero
Kelvin scale explanation
Heat measurement
calorie Joule
1 cal = 4.184 J 1J = 0.2390 cal
Unit Conversions
Also called “factor labeling”How many inches in 2 feet?How many feet in 36 inches?You just did a unit
conversion!!!!!!Look at board
Must use correct “conversion factor”
• 230 cm = ? m• Must know that 100 cm = 1m• Write possible conversion
factors• 1m or 100 cm
100 cm 1 m
Write the number you are converting first
Multiply it by the conversion factor that has the unit you want your answer to be in on the TOP
This guarantees that you will divide or multiply when you are supposed to.
• 230 cm x 1 m = 2.3 m
100 cm
The top and bottom units cancel out and the only unit left is the one you want you answer to be in!!!!!
Groups!!
Pg. 84-85 # 32-35
Two step conversions
4500 cm = ? km
Derived units
What does “derived” mean?
A derived unit is a measurement unit created by multiplying or dividing other units
Miles per hour words per minute
Area Area Length x width ft x ft = ft2 ft2 is a derived unit (derived from
two length units) m x m = m2 m2 is a derived unit (derived from
two length units)
Volume
Length x width x height
ft x ft x ft = ft3
m x m x m = m3
cm x cm x cm = cm3
Density
Describes how dense something is How heavy it is for its size
Density = mass divided by volume D = M V M = D x V
V = M D
Since you are dividing two different measurements, the unit for density is a DERIVED UNIT.
Derived from a mass measurement and a volume measurement
g/mL g/L
Density problem
Calculate the density of a substance with a mass of 24.3 g and a volume of 32.9 mL. Use the correct unit and the correct number of sig figs in your answer.
D = M V D = 24.3 g 32.9 mL Ans. = 0.739 g/mL
Problem
What is the volume of an object with a density of 1.25 g/mL and a mass of 281 g?
V = M D V = 281 g 1.25 g/mL g cancels, so units are mL for answer V = 225 mL
Volume of irregularly shaped object Water displacement
Go over hw