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Chapter 2

Chapter 2. Measurements and Calculations. Measurements. All measurements have a number part (quantitative) and a units part (qualitative) 7 cm 27.2 in 300 calories Notice the difference in the numbers . ALWAYS WRITE THE UNITS. SI Measurement System.

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Chapter 2

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  1. Chapter 2 Measurements and Calculations

  2. Measurements • All measurements have a number part (quantitative) and a units part (qualitative) • 7 cm • 27.2 in • 300 calories • Notice the difference in the numbers. • ALWAYS WRITE THE UNITS

  3. SI Measurement System • A system of measurement agreed on by scientist all over the world • However, the US is the only country to continue to use their own measuring system (lbs, miles, cups, etc…) • There are 7 SI base units • Look on page 34 Table 1 • Prefixes added to the names of the SI units are used to represent quantities smaller or larger than the base unit • Look on page 35 Table 2

  4. Difference in Mass & Weight • Mass is the amount of matter in an object • Use a balance to measure • Weight is the measure of the gravitational pull on matter. It changes based on gravity. • Use a spring scale to measure People often get these confused because weight is often expressed in grams

  5. Practice: Which SI unit would you use to measure the following? You may use any form of that SI unit • The weight of a silver dollar • The length of a football field • The size of a farm • The thickness of a dime • The temperature of an oven • The depth of a lake • The width of a ski • The temperature outside

  6. Derived SI units • A combination of SI Units • Look on page 36 at Table 3 • Volume-the amount of space occupied by an object • Solids= m3 • Liquids & Gases= liters In chemistry, the most objects are too small to uses these units. Scientist use cm3 and mL for most volume measurements. ***1 cm3= 1 mL

  7. Density • Density= mass/volume • Units • SI= kg/m3 • Scientist= g/mL or g/cm3 • Intensive Physical Property • Doesn’t depend on amount • Can be used to identify the object. • Density is dependant on temperature. Most objects’ density will decrease as the temperature gets larger. • Look on page 38 Table 2 • Those with a smaller density will float in substance with a larger density. • Those with a larger density will sink in a substance with a smaller density.

  8. Density = mass / unit volumeD = m / V • Calculate the density with units • 35.0 g occupies 25.0 mL • 2.75 kg occupies 175 cm3 • 2.80 g occupies 2.00 L

  9. Using D = m / V 1. Calculate the volume that 35.2 g of carbon tetrachloride will occupy if it has a density of 1.6 g/cm3 2. The density of ethanol is 0.789 g/mL. What is the mass of 150 mL? 3. A block of lead measures 2.000 cm x 3.000 cm x 4.500 cm. What is the mass of the block if the density of lead is 11.34 g/mL?

  10. Specific Heat *not in book, but in our lab • q = mcDt • q is the quantity of heat in Joules • m is the mass in grams • c is the specific heat (intensive physical property like density) measured in J/goC • Dt is the change in temperature (subtract two temperatures) • Heat in = Heat Out ***Law of conservation of Energy

  11. Specific Heat • How much heat is required to raise the temperature of 14.0 g lead (c = 0.1276 J/goC) from 22 oC to 95 oC? • How much heat is needed to raise the temperature of 15 g of water (c = 4.184 J/goC) from 22 oC to 95 oC?

  12. Specific Heat • A piece of unknown metal with mass 14.9 g is heated to 100.0 oC and dropped in 75.0 g of water at 20.0 oC. The final temperature of the system is 28.5 oC. What is the specific heat of the metal?

  13. Homework • Page 42 • Numbers 1, 2, 5

  14. Question • Would you be breaking the speed limit in a 40 mi/h zone if you were traveling 60 km/h? • How can we compare the two if our units are not the same?

  15. Conversion • We are going to create conversion factors and use the dimensional analysis (factor label method) • I know this sounds scary, but it isn’t that bad. We are just going to cancel out units!!!

  16. Examples • Some of these you may can do in your head, but show the factor label method. • Also, there may be one conversion factor. • How many quarters are in 5 dollars? • How many seconds are in one day?

  17. Units ConversionsFactor-Label Method • Convert 0. 75 kg to mg • Convert 1500 mm to km • Convert 750 micrograms to grams • Convert 25 miles to km • Convert 3.20 m to inches • Convert 5 lb to grams

  18. Back to our question…. • Would you be breaking the speed limit in a 40 mi/h zone if you were traveling 60 km/h?

  19. Derived units conversions • 55 mi / hr to km / s • 13.2 g / mL to oz / qt

  20. Homework • Page 42 • Numbers 3, 4, 6

  21. Using the Factor Label Method Convert the following… • 35 mL to dL • 950 g to kg • 275 mm to cm • 1,000 L to kL • 1,000 mL to L • 4,500 mg to g • 25 cm to mm • 0.005 kg to dag • 0.075 m to cm • 15 g to mg

  22. Section 3 Using Scientific Measurements • Accuracy and Precision • Accuracy-how close to the correct or accepted value • Precision-closeness of the numbers measured made in the same way (Has nothing to do with correct answer) • Percent Error • (Valueexperimental- Valueaccepted)/ Valueaccepted then X 100 • Look at practice problems on page 45

  23. Error in Measurement • There will always be some form of error in measurement. • Some include +- in their answer to express uncertainity

  24. Significant digits • The accuracy of the measurements taken and are dependent of the instrument used to measure them. • The digits allowed in an answer can imply no more accuracy than the “worst” measurement taken.

  25. Significant digits Rules • All numbers 1-9 are significant ALWAYS • Zeroes • 0 between digits are significant ALWAYS (107 = 3SD) • 0 before any digits are NEVER significant (0.005 = 1SD) • 0 at the end • NEVER without a decimal (2000 = 1 SD) • ALWAYS with a decimal (2000.0 = 5 SD)

  26. Practice: How many SD? • 2420 • 0.0025 • 1.200 • 1020 • 0.005 800 (your book uses a space in long decimal numbers) • 2000 • What if 2000 needed to have 4 SD?

  27. Scientific Notation • If the number 2000 was an exact number, put it in scientific notation…. • 2.000 x 103 for 4 SD • 2.00 x 103 for 3 SD • 2.0 x 103 for 2 SD • + exponents indicate large numbers • - exponents indicate small numbers

  28. Add and Subtract SD • Line of numbers with decimal. • Round to the least # of significant decimal places. • 135.6 g + 2.85 g (How many decimal places?) • 250 mi – 14.87 mi (Round to which place value?) • Don’t forget your units of measure!

  29. What about rounding 5? • Rounding 5 (EXACTLY) doesn’t always round UP! • To round EXACTLY 5-----Round EVEN • 250 – 15 = 235 Final answer = 240 • 250 – 25 = 225 Final answer = 220 • To see why---average your answers.

  30. Multiply and Divide • Because the decimal moves, we will round to the LEAST # of SD • 2.888 cm* 0.086 cm = • 500 g / 0.1247 m = • 24.0 km / 13468 s = • 40.0 m* 2.0 m =

  31. Practice • 16.5 cm + 8 cm + 4.37 cm = • 350.0 m – 200 m = • 6.54 m * 0.37 m = • 39 cm2 / 24.2 cm =

  32. Graphs • Direct proportional • Forms a straight line • Inverse Proportions • Forms a curve

  33. Homework • Page 57 • 1-10

  34. Review • Page 59-61 • 1, 4-5, 8-9, 11-14, 16-23, 25, 27, 35, 37-44 • ***Do this by yourself. This is a great test review.

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