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 2. What two things does a quantity consist of? • a number and an unit
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. 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
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
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
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
The largest drawing represents a cube that has sides 1 m in length and a volume of 1 m3.
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
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
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
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
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
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!
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.
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!
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
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
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
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?
How would you record this? • definitely between 3 and 4 • between 3.1 and 3.2 • 3.13 maybe?
Definitely 2 • Definitely 2.5 • How about 2.50? • Maybe 2.51
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??
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.
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)
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
25 246.31 409 20.06 29.200 1.050 .12 .030 100 100. Example: How many sig digs
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
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?
When describing very small distances, such as the diameter of a swine flu virus, it is convenient to use scientific notation.
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
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
HOMEWORK • 5,7,35,41,51,55,139
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
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
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?