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Unit 2

Unit 2. Measurement and Calculations. STATE, ORHS and VM Objectives:. You Will Be Able To: Distinguish between English, Metric, and SI Systems of Measurement. Chemical Quantities. 1. What is a quantity? quantity is a term for things that can be measured

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Unit 2

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  1. Unit 2 Measurement and Calculations

  2. STATE, ORHS and VM Objectives: You Will Be Able To: • Distinguish between English, Metric, and SI Systems of Measurement

  3. Chemical Quantities 1. What is a quantity? • quantity is a term for things that can be measured 2. What two things does a quantity consist of? • a number and an unit

  4. Systeme Internationale 3. What is the systeme internationale? • An attempt to have a international and uniform way of describing quantities 4. Which are the only two countries that have resisted fully adopting SI? • US and Canada

  5. What if different states defined a gallon as being different amounts?

  6. 5. Why will we be using SI in chemistry? • Because even though the American population is slow to convert, the scientific community in America has pretty much converted completely

  7. Do you know why the Mars Lander was lost?

  8. SI-Why is it better? 6. Why is SI better than the old way of doing things? • everyone in the world understands it • meter is the same length in US as in South Africa • all subunits are based on 10, easier to remember

  9. SI Units: Length 7. What is length, what is its SI unit, what is it measured with? • Distance between two points • measured in m • measured with a meter stick or ruler

  10. Comparison of English and metricunits for length on a ruler.

  11. SI-Volume 8. What is volume, what is its SI unit, what is it measured with? • the length x width x height of an object • measured in cm3(mL) or dm3 (L) • measured with a graduated cylinder or more precisely with a buret

  12. The largest drawing represents a cube that has sides 1 m in length and a volume of 1 m3.

  13. A 100-mL graduated cylinder.

  14. Graduated Cylinder-meniscus

  15. A student performing a titration in the laboratory using a buret

  16. SI-Mass 9. What is mass, what is its SI unit, what is it measured with? • amount of matter a sample contains • measured in grams(g) • measured with a balance scale

  17. Electronic Balance Scale

  18. Triple Beam Balance

  19. An electronic analytical balance used in chemistry labs.

  20. SI-Temperature 10. What is temperature, what is its SI unit, what is it measured with? • Measure of the amount of heat in an object • Measured in °C(Celsius) or K(Kelvin) • °F(Fahrenheit) will NEVER be used in this class • Measured with a thermoprobe or thermometer

  21. SI-Time 11. What is time, what is its SI unit, what is it measured with? • interval between two occurrences • measured in seconds(s) • measured with a time piece or chronometer

  22. Other SI Units Quantity Name Symbol Pressure Pascal Pa Energy Joule J Force Newton N Speed meters/sec m/s Area square meter m2 Amount mole mol Concentration mole/L M

  23. Isaac Newton

  24. STATE, ORHS and VM Objectives: You Will Be Able To: • Be able to distinguish between accuracy and precision • Determine the uncertainty in a measurement using significant digits • Learn how to express numbers in scientific notation

  25. Accuracy v Precision 12. What does it mean to be precise in a measurement? Give an example • Precision refers to the ability to make several measurements with the same results or close to the same results • Example: if you shoot an arrow the same way everytime and you get them close together you are being precise, you may not have hit the bullseye but you were precise!

  26. Accuracy v Precision 13. What does it mean to be accurate in a measurement? Give an example of accuracy. • Accuracy refers to how close a measurement is to the true or accepted value(what is really is) • You can shoot 5 arrows at a target and hit one bulleyes, the rest were all over the place. You were accurate but not very precise.

  27. Accuracy v Precision 14. What is always our goal in lab? • You want your measurements to be very close to the same each time you do it. Of course, you want those measurements to be right on the money as well! • You will not get credit for being lucky and getting the right answer in chemistry. You have to be able to get the right answer consistently!

  28. 15. If given a string measurements and the exact answer, how can you determine if you were accurate and/or precise? • Ask yourself the following questions? • Are the numbers closely packed together? If yes--> precise, if no--> not • Do the numbers average out to be the right answer? Yes--> accurate, no--> not

  29. Examples 1. You measure the mass of a block of iron to be 2.5 g, 2.6 g, and 2.7 g. Using a very precise and accurate scale the block was certified at 2.6 g? Were your measurements accurate, precise, or both? • Both, they were closely packed together and they averaged to the right answer

  30. Examples 2. You measure the temperature of a liquid to be 23 ºC, 26 ºC, 30º C, and 37º Cº? Using a very precise and accurate thermoprobe, the temperature was certified to be 29ºC. Were your measurements accurate, precise, or both? • Accurate because they avg to be 29 but not precise because they were not closely packed together

  31. Significant digits 16. What limits how accurately you can measure stuff in chemistry? • your accuracy is limited by the precision of the instrument with which you are measuring • There will always be uncertainty in measurements, that is why it is important how you record your measurements?

  32. How would you record this? • definitely between 3 and 4 • between 3.1 and 3.2 • 3.13 maybe?

  33. Definitely 2 • Definitely 2.5 • How about 2.50? • Maybe 2.51

  34. How would you record this? • definitely between 5 and 6 • no digits in the .1 range, so you will have to estimate • 5.8 maybe??

  35. Significant Digits • in chemistry you record all the units that you are sure of and you are allowed 1 digit that is uncertain or a best guess 17. What do we call the numbers that we are sure about & our estimate? • significant digits(figures) • it is sometimes difficult to figure out how many digits are measured in a recorded amount and how many are simply place holders.

  36. 18. What rules govern sig digs? • All non 0 numbers are sig(22=2, 53.6=3) • 0 between non 0 digits are sig(202=3, 3.002=4) • 0 beyond decimal at the end of a number are sig(1.00=3, 2.23000=6, 50.00=4)

  37. Sig Dig Rules • 0 preceding the first non 0 number are not sig, they are just place holders(.00054=2, 0.565=3) • 0 following integers without decimals, they are just place holders(1000=1, 50,500=3, 1000.=4

  38. 25 246.31 409 20.06 29.200 1.050 .12 .030 100 100. Example: How many sig digs

  39. 19. What do you do with Sig Digs when performing a Math operation? • adding or subtracting- answer should be equal to the lowest decimal place in the problem round up if needed • 15.3 + 2.35 = 17.65 = 17.7 • mutiply or divide- answer should be equal to the lowest number of sig digs in problem round up if needed • 2.3 x 3.05 = 7.015 = 7.0

  40. Scientific Notation • sometimes the numbers we have to deal with in chemistry can be rather...like...large or small • One of the numbers you will be using repeatedly is the numerical value for the mole: • 602200000000000000000000 • Big number, huh? Wouldn’t 6.02 x 1023 be more manageable?

  41. When describing very small distances, such as the diameter of a swine flu virus, it is convenient to use scientific notation.

  42. VIR(very important rules) 20. What are the rules for placing a large or small number in scientific notation? • only sig digs are recorded • the number in sci note must be between 1 and 9.99 • Multiply that number by 10x, where x is the number of jumps to the left(+) or to the right(-) you made to get the number just right

  43. 5.44 x 10-5 1.055 x 105 3.405 x 10 -2 3.45 x 10 -5 5.435 5.67 x 10 -9 1.055 x 10 9 3.000 x 10 2 .0000544 105,500 .03405 .0000345 5.435 .00000000567 1,055,000,000 300.0 Examples: Place the following in Sci Note

  44. HOMEWORK • 5,7,35,41,51,55,139

  45. Factor Labeling Method & Percentage Error

  46. STATE, ORHS and VM Objectives: You Will Be Able To: • Solve problems and do conversions using the Factor Labeling Method in single and multiple steps • Be able to analyze data by computing percentage error

  47. Factor Labeling Method • often it is important to be able to convert measurements from one unit to another • in algebra, the use of ratios accomplishes this task quite nicely • in chemistry, things get a little hairier • it is more useful to use the Factor Labeling Method

  48. Factor Labeling Method • Warning: the following are extremely complex algebraic relationships. You may experience a brief time of nausea! a = b a/b = 1 b/a =1 a/b = b/a Any Problems with this?

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