2 Objectives Understand how to calculate percentage error. Understand the concepts of accuracy and precision.Explain how precision lead to accuracy.
3 Accuracy accuracy: closeness to accepted value (true, actual) percentage error: represents accuracy
4 Sample ProblemsAn automobile scale is used to measure the weight of a car. The true weight of the car is 3245 lbs, but the scale says 3238 lbs. What is the percentage error?The same scale is used to measure the weight of a cat. The cat actually weighs 12 lbs, but the scale says 5 lbs. What is the % E?
5 Precision precision: consistency of results precise results: mL, 45.8 mL, 45.9 mL, 45.9 mLnot as precise: 46 mL, 48 mL, 51 mL, 45 mLprecision: number of significant figures in a measurement;depends on the quality of the measuring toolmore precise: cm less precise: 3.6 cm
6 Accuracy and Precision A tool with better precision usually results in greater accuracy…less estimation is required, so…greater certainty = more accurate
7 ObjectivesBe able to determine the number of significant figures in a measurement.Be able to report the correct number of significant figures for a calculation.
8 Significant Figures What is the world’s population? 7,052,155,751 people = very precise value7,052,000,000 people = not as precisesignificant figures: measured values that imply precisionWhich zeros are significant andwhich ones are not?50 mL or 50.0 mL?
9 Sig Fig Rules Non-zero digits are always significant: 25.6 m = 3 SF Zeros between SF are significant: 1026 g = 4 SFLeft-most zeros are not significant: mL = 2 SFEnding zeros after decimal are significant: g = 4 SFPlaceholder zeros are not significant: people = 2 SFEasily counted whole items, defined values, and whole #s in equations have unlimited significant figures:13 cats min = 60 sec C = 2pr (all are exact!)
10 How many significant figures? 49.4 cm0.023 s7000 bison22 students9.08 g1.030 mL0.40 mkg1 ft = 12 in321unlimited4
11 Calculations with Sig Figs Multiplication and division: the reported answer should have the same number of SFs as the least precise measurement13.4 m x 2.7 m = m2 = 36 m245.67 g / 12.8 mL = g/mL = 3.57 g/mLYou can use theremoved digits toround if necessary.
12 Calculations with Sig Figs Addition and subtraction: the answer should be rounded to the least precise decimal position4.5 m m = ?4.5+9.1813.68 = 13.7 mg – 9.25 g = ?14.832-9.255.582 = 5.58 g
13 ObjectivesBe able to define commonly used types of measurements used in chemistry.Be able to identify the units used in chemistry.Be able to identify the types of instruments used to make measurements.
14 Common Metric Measurements Which of the followingare measurements?12 gramspounds4.5 mL3.1415measurement: includes value and unitmass: amount of mattergrams (g)use a balancenearest 0.01 gMASS IS NOT WEIGHT!weight is the force of gravitylength: measure of distancecentimeters (cm)nearest 0.1 cm with ruler42.3 cm
15 Volume volume: amount of 3-D space cubic centimeters (cm3) for solids milliliters (mL) for liquids# cm3 = # mLuse a graduated cylinder or graduated flask+/- 0.1 mL (using meniscus)volume = 52.7 mL# mL water = # g water ONLY TRUE FOR WATER!1.00 L water = 1.00 kg water
16 Temperature temperature: measures particle vibrational motion (faster = hotter)degrees Celsius (oC) or Kelvin (K)measure to nearest 1oC# oC = # K# K = # oCabsolute zero: coldesttemperature, all particlemotion stops0 K = -273oC
17 Objectives Be familiar with Celsius and Kelvin temperature. Be able to make conversions between common prefixes used in the metric system.Understand the meaning of accuracy.Be able to calculate percentage error.
18 Metric Prefixes M mega = 106 k kilo = 103 c centi = 10-2 m milli = 10-3m micro = 10-6Use the prefix scale to move the decimal point…M ● ● k ● ● unit ● c m ● ● m145 mm14.5 cm = ? mm34.6 mL = ? LL4.57 kg = ? g4570 g
19 ObjectivesUnderstand the concept of density in terms of a physical property.Be able to use the density equation to calculate density, mass, or volume.
20 Density density: mass per volume (g/cm3 or g/mL) All samples of a substance (at sameconditions) will have an identical density.If 3.00 cm3 of lead has a mass of 34.0 g,then 6.00 cm3 will have a mass of 68.0 g.What are their densities?
21 Density If the density and either the mass or volume is known, then the other variable can be calculated.What is the volume of a 45.8 g piece of copper?d = 8.96 g/cm3V = 5.11 cm3
22 Density What is the mass of a 12.5 cm3 piece of iron? Look up the density on your periodic table!m = 98.3 g
23 Thickness of Aluminum Foil Lab Question: How much thicker is “heavy duty” aluminumfoil than “regular” aluminum foil?We can use density tohelp answer this question.Measure the mass, length, and width of a sheet ofeach type of foil. The density will be the same for each.The thickness (h) can be calculated and compared.
24 Vernier Calipers = 1.23 cm This line shows that the length is a little more than1.2 cmThe 3rd line on vernier scalealigns with scale above