1. Ch. 3 Scientific Measurement Ch. 3 Scientific Measurement

2. Measurement  Quantitative information  Need a number and a unit (most of time)  Represents a quantity  For example: 2 meters 2 is number 2 is number Meters is unit Meters is unit Length is quantity Length is quantity  Units compare what is being measured to a defined measurement standard

3. SI Measurement  Le Systeme International d’Unites : SI  System of measurement agreed on all over the world in 1960  Contains 7 base units  We still use some non-SI units

4. Important SI Base Units QuantitySymbolUnitAbbreviation Lengthlmeterm Massmkilogramkg Timetseconds TemperatureTKelvinK Amountnmolemol

5. Prefixes  Prefixes are added to the base unit names to represent quantities smaller or larger Mmega10 6 1,000,000larger kkilo10 3 1,000larger ccenti10 -2 1/100smaller mmilli10 -3 1/1000smaller μmicro10 -6 1/1,000,000smaller

6. Length  SI unit: m  use cm a lot too cm  km is used instead of miles for highway distances and car speeds in most countries km

7. Mass  Measure of the quantity of matter  SI unit: kg  use g a lot too g  mass vs. weight weight is the measure of gravitational pull on matter weight is the measure of gravitational pull on matter mass does not depend on gravity mass does not depend on gravity on a new planet, mass would be same but weight could change on a new planet, mass would be same but weight could change

8. Temperature Conversions Fahrenheit to Celsius

9. Celsius to Kelvin conversions +273.15→ CK ←-273.15

10. Temperature Conversions

11. Example  What is 32°F in Kelvin?  freezing point of water!

12. Example  What is 298 K in Fahrenheit?

13. Derived SI Units  come from combining base units  combine using multiplication or division Example:Area: A = length x width = m x m = m x m = m 2 = m 2

14. Volume  amount of space occupied by object  SI: m 3 = m x m x m  use cm 3 in lab a lot cm 3cm 3  non-SI: 1 liter = 1dm 3 = 1000cm 3 1 liter = 1000 mL 1cm 3 = 1mL

15. Density  ratio of mass to volume  SI:  Other units: g/ cm 3 or g/ mL  characteristic property of substance (doesn’t change with amount ) because as volume increases, mass also increases  density usually decreases as T increases exception: ice is less dense than liquid water so it floats

16. Example A sample of aluminum metal has a mass of 8.4 g. The volume is 3.1 cm 3. Find the density. KnownUnknown m = 8.4 gD = ? V = 3.1 cm 3

18. Conversion Factors  ratio that comes from a statement of equality between 2 different units  every conversion factor is equal to 1 Example: statement of equality conversion factor Dimensional Analysis

19. Conversion Factors  can be multiplied by other numbers without changing the value of the number  since you are just multiplying by 1

20. Example 1 Convert 5.2 cm to mm cm to mmcm to mm 5.2 cm= 5.2 x 10 1 mm = 52 mm cm= 5.2 x 10 1 mmcm= 5.2 x 10 1 mm  Known:100 cm = 1 m 1000 mm = 1 m  Must use m as an intermediate

21. Example 2 Convert 0.020 kg to mg kg to mgkg to mg 0.020 kg = 0.020 x 10 6 mg= 20,000 mg kg = 0.020 x 10 6 mgkg = 0.020 x 10 6 mg  Known:1 kg = 1000 g 1000 mg = 1 g  Must use g as an intermediate

22. Example 3 Convert 500,000 μg to kg μg to kgμg to kg 500,000 μg = 500,000 x 10 -9 kg= 0.0005 kg μg = 500,000 x 10 -9 kgμg = 500,000 x 10 -9 kg  Known:1,000,000 μg = 1 g 1 kg = 1000 g  Must use g as an intermediate

23. Advanced Conversions  One difficult type of conversion deals with squared or cubed units Example  Convert 3 dm 3 to cm 3 1dm =10 cm 3 dm 3 = 3 x 10 cm x 10 cm x 10 cm= 3000 cm 3

24. Example  Convert: 2000 cm 3 to m 3 OR Known: 100 cm = 1 m cm 3 = cm x cm x cm m 3 = m x m x m

25. Advanced Conversions  Another difficult type of conversion deals units that are fractions themselves  Be sure convert one unit at a time; don’t try to do both at once  Work on the unit on top first; then work on the unit on the bottom

26. Example Convert 350 g/ mL to kg/L  Top first 350 g to kg 350 g= 350 x 10 -3 kg=.35 kg  Bottom part after 1mL to L 1mL= 1x 10 -3 L= 0.001 L  Result: 350 g/ mL =.35 kg/ 0.001L= 350kg/ L

27. Combination Example Convert 7634 mg/m 3 to Mg/L  Top 7634 mg= 7634 x 10 -9 Mg= 0.000007634 Mg  Bottom 1m= 10 dm 1 m 3 = 10 dm x 10 dm x10 dm= 1000 dm 3 = 1000 L  Result: 7634 mg/m 3 = 0.000007634 Mg/ 1000 L= 0.000000007634 Mg/L = 7.634 x 10 -9 Mg/L

28. Accuracy vs. Precision  Accuracy- closeness of measurement to correct or accepted value  Precision- closeness of a set of measurements

29. Accuracy vs. Precision

30. Percent Error vs. Percent Difference  Percent Error: Measures the accuracy of an experiment Measures the accuracy of an experiment Can have + or – value Can have + or – value

31. Percent Error vs. Percent Difference  Percent Difference: Used when one isn’t “right” Used when one isn’t “right” Compare two values Compare two values Measures precision Measures precision

32. Example  Measured density from lab experiment is 1.40 g/mL. The correct density is 1.36 g/mL.  Find the percent error.

33. Example  Two students measured the density of a substance. Sally got 1.40 g/mL and Bob got 1.36 g/mL.  Find the percent difference.