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The SI units
Scientists all over the world use the SI units to express measurements.
Quantity Unit Symbol
Length meter m
Mass kilogram kg
Time second s
Temperature Kelvin K
Amount of Substance
mole mol
Why SI ?
It is easy to use. It is based on powers of ten.Example:megabytes = 106 byteskilogram = 103 gramscentimeter = 10-2 metermilliliter = 10-3 liters
SI Prefixes
Prefix Symbol Meaning Multiplier
giga G billion 109
mega M million 106
kilo k kilo 103
deci d tenth 10-1
centi c hundredth 10-2
milli m thousandth 10-3
micro µ millionth 10-6
nano n billionth 10-9
Check for Understanding
Answer-Pair-Share
Which SI unit will you use to express each measurement?
1) volume of water in a graduated cylinder
2.) mass of a spoonful of salt
3.) mass of a sack of rice
4.) temperature of cold water
5.) time it takes a marble to roll down a ramp
6.) density of a wooden cube
Factor Label Method ofConverting UnitsWhat is 0.5 kg in grams?
Step 1: Write the relationship between the two units.
Step 2: Write the possible conversion factors.
Step 3: Multiply the quantity by the correct conversion factor.
1kg1000 g
1000 g1kg
1 kg = 1000 g
0.5 kg x 1 kg = doesn’t work!1000 g
0.5 kg x 1000 g = 500 g 1kg
Factor Label Method ofConverting UnitsWhat is 50 mL in L?
Step 1: Write the relationship between the two units.
Step 2: Write the possible conversion factors.
Step 3: Multiply the quantity by the correct conversion factor.
1L1000 mL
1000 mL1L
1 L = 1000 mL
50 mL x 1 L = 0.05 L 1000mL
50 mL x 1000 mL = doesn’t work! 1 L
Check for Understanding
Answer-Pair-Share
Do the following conversions. Show your work.
1) 0.75 mL = ________L
2.) 2.0 m = ________ mm
3.) 2000 ms = ________ s
4.) 3.5 g = ________ cg
5.) 0.25 kg = _________ mg
Objective:
Accuracy vs. PrecisionCalculate % error.
Read p. 34, paragraphs 1-3. Find out what accuracy and precision
mean.
Accuracy vs. Precision
Accuracy and Precision
Precise but inaccurate
Imprecise but accurate
Precise andaccurate
Accuracy – refers to how close a measurement is to the true or literature value
Precision – refers to how close measurements are to each other
Whose measurement is more accurate? True value = 1.000 g/mL
Student A:1.003 g/mL Student B: 1.015 g/mL
Which set of measurements is more precise?
A. 2.315 g, 2.317 g, 2.318 g
B. 2.32 g, 2.33g, 2.31 g
Check for Understanding
Think-Pair-Share
Accuracy or Precision?
1. May be determined by comparing a measured value to the true (literature) value.
2. May be determined by comparing several measurements.
Percent Error
expresses the accuracy of a measurement
% error = /measured value – literature value/ x 100
literature value
The boiling point of water was measured to be 98.6oC. If the true (literature) value is 100oC, what is the percent error?
% error = /98.6 – 100/ x 100 = 1.4 %
100
Check for Understanding
Answer-Pair-Share
The melting point of gold was measured to be 1325oC. What is the % error of this measurement if the literature value is 1338oC?
Significant Digits
Significant digits are used to express how precise measurements are.
The number of significant digits depends on the kind of measuring device used.
Significant digits include all the certain digits and 1 uncertain digit in a measurement.
Counting Significant Digits
Non-zero digits are significant.
65 g – 2 significant digits Zeros after a decimal point but before a non-zero digit are
not significant.
0.065 g– 2 significant digits Zeros between two non-zero digits are significant.
6.05 g – 3 significant digits Zeros after a decimal point and a non-zero digit are
significant
65.0 g – 3 significant digits
650 g – 2 significant digits
Check for Understanding
Answer-Pair-Share
Tell the number of significant digits:
1). 12.0 mL
2.) 0.007 L
3.) 15.05 g
4.) 1200 cars
5. ) 500 kg
6.) 0.0305oC
Counting Significant Digits
A calculated value cannot be more precise than the measurement from which it is based.
Example: 5.0 mL x 1.25 g/mL = What is the best way to record the answer? 6.25 g or 6.3 g or 6 g? Rules to remember:
1. When multiplying or dividing, the answer should have the least number of significant digits.
2. When adding or subtracting, the answer should have the least number of decimal places.
Check for Understanding
Answer-Pair-Share
Perform the following operations and express answer in correct significant digits.
1) 15.2 g – 3.50 g =
2.) 1.0 g/mL x 9.00 mL =
3.) 5.0 g / 2.50 cm3
4.) 4.6 g + 11.2 g + 6.15 g =
5. 2.5 g / 0.789 g/mL =
Objective:
What is density?
Density
measure of the “compactness” of a material
A B
Which material is more dense?
Uses of Density Data
Identification of unknown substances Calculation of molecular mass of
substances Explains floating/sinking of object
Calculating Density
amount of mass in a given space D = m/V
D = densitym = massV = Volume
What is the density of ethanol if 10.0 mL of this liquid has a mass of 7.89 g?
D = 7.89g /10.0mL = 0.789 g/mL
Problem Solving Identify the given information
Identify what is asked for
Develop possible solutions
Analyze the solutions and choose the correct one
Develop the steps to arrive at the answer
Solve the problem
Evaluate the result
Where am I?
What paths will I take?
Which path is most likely the correct one?
Plan the trip.
Travel along the selected path.
Did I reach the place I expected?
Where do I want to be?
Density
What is the mass of 5.0 mL of ethanol if its density is 0.789 g/mL?
Given: V = 5.0 mLD = 0.789 g/mLm = ?D = m/Vm = DV = 0.789 g/mL x 5.0 mL= 3.9 g
Check for Understanding
Answer-Pair-Share:1. A block of wood has a mass of 23.45 g and
a volume of 20.15 cm3. What is its density?
2. The density of lead is 11.3 g/cm3. What is the volume of 25.0 g of lead?
Objective:
Human vs. Experimental ErrorSystematic vs. Random Error
How can we eliminate/minimize experimental errors?
Experimental Errors
“All experimental data is imperfect”.
Types of Experimental Errors:
1. Random
2. Systematic
Random vs. Systematic
Random- cause:
unpredictable/uncontrollable factors
- cannot be eliminated
- can be minimized by averaging
- Effect: data may be higher or lower
Systematic- cause: faulty experimental
design or measuring device
- can be eliminated by changing the experimental design/measuring device
- can not be minimized by averaging
- Effect: data is consistently higher or lower
Determining the Mass of Alcohol
Materials:
digital electronic balance that reads up to 0.01 g
100 mL graduated cylinder, marked by 1 mL
alcohol
Procedure:
1. Find and record the mass of an empty graduated cylinder.
2. Fill the cylinder about ¾ full of alcohol. Record the volume.
3. Get the mass of the filled graduated cylinder
Example
Random- Wind disturbs the
balance causing the readings to fluctuate
- Eye level of the experimenter moves a bit while reading the volume
Systematic- Electronic balance is not
working properly (not calibrated).
- Some of the alcohol is lost (evaporates) as its mass is being read.
Check for Understanding
Answer-Pair-Share:
Random or Systematic?1. may be minimized by averaging
2. may be eliminated by changing the experimental design
Human Errors
- mistakes or blunders- may be avoided by careful experimentation- should not be included in a lab report- Examples:1. Wrong calculations2. Sloppiness/Spilling chemicals3. Reading an instrument incorrectly4. Not following procedures5. Using wrong chemical
Check for Understanding
Answer-Pair-Share:What should you do once you have realized
you have made a “mistake” or human error in your measurement?
A. Report the data anywayB. Discard the measurement and redo itC. Include the wrong measurement in
calculating the average of several trials