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CHEMISTRY

CHEMISTRY. August 8, 2011. CHEM BRAIN SAMPLER August 8, 2011. Your group has 5 minutes!! Place homework on your desk. 1 . Why do we have an SI (system International) system for units chemistry? 2. How many significant figures are in:

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CHEMISTRY

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  1. CHEMISTRY August 8, 2011

  2. CHEM BRAIN SAMPLER August 8, 2011 Your group has 5 minutes!! Place homework on your desk 1 . Why do we have an SI (system International) system for units chemistry? 2. How many significant figures are in: 0.00236 2.030 50.00 102,000 3. What is the SI unit for time? mass? temperature? 4. Refer to the diagram on the board. Report the measurement. (include an appropriate unit)

  3. Significant Figures Calculations August 8, 2011

  4. Significant Figures in Calculations What are Significant Digits? Examples Triple Beam Balance Graduated Cylinder All the certain digits plus the estimated digit in a measurement. How many decimal places can we count

  5. Significant Figures in Calculations Multiplication and Division Example The number with the smallest number of significant digits determines how many significant digits are allowed in the final answer. Volume of a box L x W x H (3.05m)(2.10m)(0.75m) 2 sig figs 4.8m3

  6. Significant Figures in Calculations Example Density of a penny M = 2.53g V = 0.3mL D = M / V # significant figures allowed D = 8g/mL

  7. Significant Figures in Calculations Addition and Subtraction Example The number of significant digits depends on the number with the largest uncertainty. (you may be using different scales) Shoes 951.0 g Clothes 1407 g Ring 23.911 g Glasses 158.18 g Total 2540. g

  8. Significant Figures in Calculations Example What is the mass of a penny if, the weighing paper alone has a mass 0.67 g and weighing paper plus the penny has a mass of 3.2 g. 3.2 g -0.67 g 2.5 g

  9. Significant Figures in Calculations Remember A calculated number can only be as precise as the least precise measurement in the calculation.

  10. Practice Calculate each of the following to the correct number of significant figures. Include units on your answer. (25 g/mol)(4.0 mol) = (3.48 in)(1.28 in)(0.010 in) = 2.06 cm + 1.8 cm + 0.004 cm = If the mass of a lead cube is 176.91 g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead?

  11. Practice Calculate each of the following to the correct number of significant figures. Include units on your answer. (25 g/mol)(4.0 mol) =1.0 x 102 (3.48 in)(1.28 in)(0.010 in) = .045 in3 2.06 cm + 1.8 cm + 0.004 cm = 3.9 cm If the mass of a lead cube is 176.91 g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead? 11.3 g/cm3

  12. Dimensional Analysis August 8, 2011

  13. Dimensional Analysis Example A tool. An organized technique of converting between units. How many eggs are in two cases? 1 case = 24 cartons 1 carton = 12 eggs Dimensional Analysis

  14. Equivalence Statement Examples Conversion Factors Examples Relates the same quantity in two different units An equation that is always equal. 1 gallon = 3.785 L 2.54 cm = 1 inch Ratio between units in the equivalence statement Dimensional Analysis

  15. How to use Dimensional Analysis (DA) Identify the unknown. What is the problem asking you to find? Start with the given (often the first # in the problem) Identify and set up the conversion factors for each unit of change Make sure all units cancel EXCEPT the one you are looking for. Check that you have the correct number of significant figures. Ask whether your answer makes sense. Dimensional Analysis

  16. Brain Teaser • Usain Bolt, a Jamaican Olympian, just ran a world record for a 100 meter dash sprint. He ran this race in 9.68 seconds beating the world record by 1-one hundredth of a second (9.69s). • How fast was he running? • How fast was he running in miles per hour? • 1 mile = 1609.34 meter • 60 seconds = 1 minute • 60 minutes = 1 hour

  17. How many kilograms are in 125 g of salt? Chloroform is a liquid once used as and anesthetic. What is the volume in mL of 5.0 g of chloroform. Density = 1.49 g/mL? The length of a marathon race is approximately 26.2 miles. What is the distance of the race in feet? 5280. feet = 1 mile Convert the freeway speed limit of 65 miles per hour to kilometers per second. 1 km = 0.62 mi Sample Problems

  18. Start with the given, end with what you need!

  19. 1. How many pieces of paper are in 3 boxes of copy paper if each box contains 16 reams and each ream is 500 pieces of paper? Practice

  20. TEAMThe Perfect Cup of Kool-Aid It is a hot summer day and you are extremely thirsty. You decide that a cup of Kool-aid would be perfect to quench your thirst. You find the following recipe for the perfect cup of Kool-aid: • 0.05 cups of Kool-aid powder should be mixed with 7.5 fluid ounces of water. However, in your kitchen you only have teaspoon with which to measure. Using dimensional analysis and the given equivalence statements, determine the number of teaspoons of Kool-aid and water you need to make your drink. 5 mL = 1 tsp 17 Tbsp = 1 cup 1 Tbsp = 15 mL 8 fl oz = 1 cup

  21. Summary • What is dimensional analysis used for? • Why is it useful? • What must accompany every number (measurement) in chemistry?

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