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The SI units

The SI units. Scientists all over the world use the SI units to express measurements. Why SI ?. It is easy to use. It is based on powers of ten. Example: mega bytes = 10 6 bytes kilo gram = 10 3 grams centi meter = 10 -2 meter milli liter = 10 -3 liters. SI Prefixes.

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The SI units

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  1. The SI units Scientists all over the world use the SI unitsto express measurements.

  2. Why SI ? • It is easy to use. • It is based on powers of ten. Example: megabytes = 106 bytes kilogram = 103 grams centimeter = 10-2 meter milliliter = 10-3 liters

  3. SI Prefixes

  4. Check for Understanding Answer-Pair-Share Which SI unit will you use to express each measurement? 1) volume of water in a graduated cylinder 2.) mass of a spoonful of salt 3.) mass of a sack of rice 4.) temperature of cold water 5.) time it takes a marble to roll down a ramp 6.) density of a wooden cube

  5. Factor Label Method ofConverting Units What is 0.5 kg in grams? Step 1: Write the relationship between the two units. Step 2: Write the possible conversion factors. Step 3: Multiply the quantity by the correct conversion factor. 1 kg = 1000 g 1kg 1000 g 1000 g 1kg 0.5 kg x 1 kg = doesn’t work! 1000 g 0.5 kg x 1000 g = 500 g 1kg

  6. Factor Label Method ofConverting Units What is 50 mL in L? Step 1: Write the relationship between the two units. Step 2: Write the possible conversion factors. Step 3: Multiply the quantity by the correct conversion factor. 1 L = 1000 mL 1L 1000 mL 1000 mL 1L 50 mL x 1 L = 0.05 L 1000mL 50 mL x 1000 mL = doesn’t work! 1 L

  7. Check for Understanding Answer-Pair-Share Do the following conversions. Show your work. 1) 0.75 mL = ________L 2.) 2.0 m = ________ mm 3.) 2000 ms = ________ s 4.) 3.5 g = ________ cg 5.) 0.25 kg = _________ mg

  8. Objective: Accuracy vs. Precision Calculate % error.

  9. Accuracy vs. Precision Read p. 34, paragraphs 1-3. Find out what accuracy and precision mean.

  10. Accuracy and Precision Precise but inaccurate Imprecise but accurate Precise and accurate Accuracy – refers to how close a measurement is to the true or literature value Precision– refers to how close measurements are to each other

  11. Whose measurement is more accurate? True value = 1.000 g/mL Student A:1.003 g/mLStudent B: 1.015 g/mL Which set of measurements is more precise? A. 2.315 g, 2.317 g, 2.318 g B. 2.32 g, 2.33g, 2.31 g

  12. Check for Understanding • Think-Pair-Share • Accuracy or Precision? • May be determined by comparing a measured value to the true (literature) value. • May be determined by comparing several measurements.

  13. Percent Error • expresses the accuracy of a measurement % error = /measured value – literature value/ x 100 literature value The boiling point of water was measured to be 98.6oC. If the true (literature) value is 100oC, what is the percent error? % error = /98.6 – 100/ x 100 = 1.4 % 100

  14. Check for Understanding • Answer-Pair-Share • The melting point of gold was measured to be 1325oC. What is the % error of this measurement if the literature value is 1338oC?

  15. Significant Digits • Significant digits are used to express how precise measurements are. • The number of significant digits depends on the kind of measuring device used. • Significant digits include all the certain digits and 1 uncertain digit in a measurement.

  16. Counting Significant Digits • Non-zero digits are significant. 65 g – 2 significant digits • Zeros after a decimal point but before a non-zero digit are not significant. 0.065 g– 2 significant digits • Zeros between two non-zero digits are significant. 6.05 g – 3 significant digits • Zeros after a decimal point and a non-zero digit are significant 65.0 g – 3 significant digits 650 g – 2 significant digits

  17. Check for Understanding Answer-Pair-Share Tell the number of significant digits: 1). 12.0 mL 2.) 0.007 L 3.) 15.05 g 4.) 1200 cars 5. ) 500 kg 6.) 0.0305oC

  18. Counting Significant Digits • A calculated value cannot be more precise than the measurement from which it is based. • Example: 5.0 mL x 1.25 g/mL = • What is the best way to record the answer? • 6.25 g or 6.3 g or 6 g? • Rules to remember: 1. When multiplying or dividing, the answer should have the least number of significant digits. 2. When adding or subtracting, the answer should have the least number of decimal places.

  19. Check for Understanding Answer-Pair-Share Perform the following operations and express answer in correct significant digits. 1) 15.2 g – 3.50 g = 2.) 1.0 g/mL x 9.00 mL = 3.) 5.0 g / 2.50 cm3 4.) 4.6 g + 11.2 g + 6.15 g = 5. 2.5 g / 0.789 g/mL =

  20. Objective: What is density?

  21. Density • measure of the “compactness” of a material A B Which material is more dense?

  22. Uses of Density Data • Identification of unknown substances • Calculation of molecular mass of substances • Explains floating/sinking of object

  23. Calculating Density • amount of mass in a given space • D = m/V D = density m = mass V = Volume What is the density of ethanol if 10.0 mL of this liquid has a mass of 7.89 g? D = 7.89g /10.0mL = 0.789 g/mL

  24. Problem Solving Where am I? • Identify the given information • Identify what is asked for • Develop possible solutions • Analyze the solutions and choose the correct one • Develop the steps to arrive at the answer • Solve the problem • Evaluate the result Where do I want to be? What paths will I take? Which path is most likely the correct one? Plan the trip. Travel along the selected path. Did I reach the place I expected?

  25. Density What is the mass of 5.0 mL of ethanol if its density is 0.789 g/mL? Given: V = 5.0 mL D = 0.789 g/mL m = ? D = m/V m = DV = 0.789 g/mL x 5.0 mL = 3.9 g

  26. Check for Understanding Answer-Pair-Share: • A block of wood has a mass of 23.45 g and a volume of 20.15 cm3. What is its density? • The density of lead is 11.3 g/cm3. What is the volume of 25.0 g of lead?

  27. Objective: Human vs. Experimental Error Systematic vs. Random Error How can we eliminate/minimize experimental errors?

  28. Experimental Errors “All experimental data is imperfect”. Types of Experimental Errors: • Random • Systematic

  29. Random vs. Systematic Systematic • cause: faulty experimental design or measuring device • can be eliminated by changing the experimental design/measuring device • can not be minimized by averaging • Effect: data is consistently higher or lower Random • cause: unpredictable/uncontrollable factors • cannot be eliminated • can be minimized by averaging • Effect: data may be higher or lower

  30. Example Determining the Mass of Alcohol Materials: digital electronic balance that reads up to 0.01 g 100 mL graduated cylinder, marked by 1 mL alcohol Procedure: • Find and record the mass of an empty graduated cylinder. • Fill the cylinder about ¾ full of alcohol. Record the volume. • Get the mass of the filled graduated cylinder

  31. Systematic • Electronic balance is not working properly (not calibrated). • Some of the alcohol is lost (evaporates) as its mass is being read. Random • Wind disturbs the balance causing the readings to fluctuate • Eye level of the experimenter moves a bit while reading the volume

  32. Check for Understanding Answer-Pair-Share: Random or Systematic? 1. may be minimized by averaging 2. may be eliminated by changing the experimental design

  33. Human Errors • mistakes or blunders • may be avoided by careful experimentation • should not be included in a lab report • Examples: • Wrong calculations • Sloppiness/Spilling chemicals • Reading an instrument incorrectly • Not following procedures • Using wrong chemical

  34. Check for Understanding Answer-Pair-Share: What should you do once you have realized you have made a “mistake” or human error in your measurement? • Report the data anyway • Discard the measurement and redo it • Include the wrong measurement in calculating the average of several trials

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