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Chapter 3. Units and Calculations

Chapter 3. Units and Calculations. All measurements have three parts: 1. Number (value, quantity) 2. Uncertainty (error, shown by sig figs) 3. Unit (nature of quantity, label) Units must always be shown with numbers!. The Metric System.

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Chapter 3. Units and Calculations

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  1. Chapter 3.Units and Calculations All measurements have three parts: 1. Number (value, quantity) 2. Uncertainty (error, shown by sig figs) 3. Unit (nature of quantity, label) Units must always be shown with numbers!

  2. The Metric System The metric system is a decimal system of weights and measures based on the meter as a unit of length, the kilogram as a unit of mass, the second as a unit of time, and the kelvin as a unit of temperature. Decimal: Unit conversions are factors of 10.

  3. The Metric System Basic (fundamental, defined) Units: Length; meter (m) (about 1 yard) in lab, centimeter and millimeter Mass; kilogram (kg) (about 2.2 pounds) in lab, gram (g) Time; second (s) (same as English) minutes and hours, not decimal Temperature; kelvin (K) (no negative values) also Celcius, centigrade (C)

  4. The Metric System

  5. The Metric System The fundamental units in the metric system are too large to be convenient in chemical labs. How do we get smaller units? Some of the derived units in the metric system are very small. How do we get larger units?

  6. The Metric System We multiply the unit by some power of ten, for example 103 (1000) or 10-2 (0.01). These multipliers relate to prefixes. The prefixes are combined with names of fundamental units to obtain larger or smaller units: kilogram = 1000 grams centimeter = 0.01 meter

  7. The Metric System Metric system prefixes (multipliers) to know: My king died chewing M & M's

  8. The Metric System How to use prefixes and multipliers: Name of unitValue of unit prefix unit multiplier x unit milligram 0.001 x 1 gram one thousandth of a gram

  9. The Metric System Examples: One centimeter = 1 cm = 0.01 meter One kilogram = 1 kg = 1000 gram One millisecond = 1 ms = 0.001 second One megahertz = 1 MHz = 1,000,000 Hz One microfarad = 1F = 0.000001 F

  10. The Metric System Conversions within the metric system, e.g. convert 75833 meters to kilometers 1. Set up equality: prefix unit = multiplier x unit 1 kilometer = 1000 meters 2. Convert to ratio with desired unit in numerator: 1 km 1000 m 3. Multiply ratio by given units: 1 km x 75833 m = 75.833 km 1000 m

  11. The Metric System Convert: 0.0285 kilograms to grams (kg to g) 27935 meters to kilometers (m to km) 53.8 milliseconds to seconds (ms to s) 0.084 meters to millimeters (m to mm)

  12. The Metric System Convert: 0.000850 meters to micrometers (m to m) 250 micrograms to milligrams (g to mg)

  13. The Metric System Derived units are obtained by mathematical operations on one or more basic units. Area = length squared 1 square meter = 1 m2 Volume = length cubed (space occupied) 1 cubic meter = 1 m3 The basic unit of volume in chemistry is the liter (L). 1 L = 1 dm3 = 1000 cm3

  14. The Metric System

  15. The Metric System Other derived units: Speed = distance/time, m/s Acceleration = distance/time2, m/s2 Force = mass x acceleration, kgm/s2 newton, N Pressure = force/area, kg/ms2pascal, P Energy = force x distance, kgm2/s2 joule, J

  16. Units in Math Units can be multiplied, divided, squared, canceled, etc. -- just like numbers! 102 x 10 = 103m2 x m = m3 Pressure = force/area = kgm x 1 = kg sec2 m2 msec2 Energy = force x distance = kgm x m = kgm2 sec2 sec2

  17. Units in Math Conversion factors are ratios that specify how one unit of measurement is related to another unit of measurement. They can also be expressed as equalities. 2.54 cm = 1.00 inch (exact) 1.00 in 2.54 cm 2.54 cm 1.00 in

  18. Units in Math Example: How many centimeters are there in 12.0 inches? 2.54 cm x 12.0 in = 30.48 cm = 30.5 cm 1.00 in

  19. Units in Math Dimensional Analysis is a method for setting up calculations in which the units associated with numbers are used as a guide. Set up the calculation so that desired units remain in the answer, and all others cancel. Dimensions are quantitative properties such as length, time, mass. Units are defined measurements of dimensions, such as meters, seconds, and grams.

  20. Dimensional Analysis How to do it: 1a. Figure out what quantity is to be deter-mined, and what are the desired units. 1b. Identify given quantities in the problem.

  21. Dimensional Analysis 2a. Choose a given quantity or a conversion factor that has the desired units. 2b. Start an equation with this quantity. If it’s a ratio, the desired units should be in the numerator. 2c. Multiply this quantity by other given val-ues and conversion factors to make un-wanted units cancel and retain desired units.

  22. Dimensional Analysis 3a. Perform mathematical operations as indicated in the equation you created. 3b. Reality check: Does the result make sense? 3c. Clean up: Round to correct number of sig figs.

  23. Dimensional Analysis Example: A premature infant weighs 1703 grams. What is its weight in pounds? 454 g = 1.00 lb (inexact)

  24. Dimensional Analysis Example: At room temperature, 1.00 L of water has a mass of 1.00 kilograms. What is its mass in grams?

  25. Dimensional Analysis Example: I can ride my bicycle at 9.6 miles per hour. How long will it take me to go 23 miles?

  26. Dimensional Analysis Types of conversion factors: Equality conversion factors are ratios that interconvert different units of the same dimension. 0.454 kg = 454 g = 1.00 lb 1.00 lb 0.454 kg 454 g 1 lb

  27. Dimensional Analysis Types of conversion factors: Equivalence conversion factors are ratios that interconvert units of differ-ent dimensions. Speed = distancemiles time hour Density = mass grams volume cm3

  28. Dimensional Analysis Example: An investigator found that 50.3 cm3 of bovine fat had a mass of 45.1 gram. What is the density of the fat? The investigator also found that 49.8 cm3 of bovine lean muscle had a mass of 55.0 g. What is the density of the muscle? Which is more dense?

  29. Percentage Problems Percent is the number of items of a specified type in a group of 100 total items. Parts per hundred Percent = number of items of interest x 100% total items

  30. Percentage Problems A student answered 19 items correctly on a 23 point test. What was his score as a percentage?

  31. Percentage Problems Range as a percent of the average is a way to express precision. % of average = (highest – lowest) x 100% average = (20.50 – 19.25) units x 100 % = 6.32% 19.78 units

  32. Percentage Problems A technician measured the breaking strength of three samples of plastic. His results were: Run 1: 65.8 MPa Run 2: 72.4 MPa Run 3: 68.3 MPa What was the range of his measurements as a percent of the average? Note: 1 MPa = 145 pounds/in2

  33. Percentage Problems Percent difference is a way to express accuracy. % difference = (measured – actual) x 100% actual = (19.78 – 20.00) units x 100% = –1.1% 20.00 units

  34. Percentage Problems A student determined the density of aluminum metal to be 2.64 g/cm3. The accepted value is 2.70 g/cm3. What is the percent differ-ence between her result and the accepted value? Did she do a good job?

  35. Percentage Problems A student did three experiments to determine the density of rubbing alcohol. Her results were: 0.778 g/mL; 0.795 g/mL; 0.789 g/mL. What is her precision as % of average? The true value is 0.785 g/mL. What is her accuracy?

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