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Problem-Solving in Chemistry

Problem-Solving in Chemistry. Problems in Chemistry. Tend to fall into two categories, algebraic or conversion. In algebraic problems you are solving for an unknown. In conversion problems you are changing units. Conversions within the Metric System.

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Problem-Solving in Chemistry

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  1. Problem-Solving in Chemistry

  2. Problems in Chemistry • Tend to fall into two categories, algebraic or conversion. • In algebraic problems you are solving for an unknown. • In conversion problems you are changing units.

  3. Conversions within the Metric System Remember, it’s a decimal-based system. Conversions should be easy.

  4. Use the factors in Table C to figure out the conversion fact.

  5. Convert 129 milligrams to grams. Look for milligrams in Table C Find the row: Factor is 10-3 What does that mean? It means 1 X 10-3 g = 1 mg This is the conversion fact!

  6. Conversion Fact or Definition • Once you have your conversion fact, you can choose the ratio method or the factor label method to do the work. 1 X 10-3 g = 1 mg or .001 g = 1 mg Equivalently, 1 g = 1000 mg

  7. Ratio Method as means = 129 mg as 1 mg X g .001g (129 mg) • .001 g = 1 mg • X g X = .129 g 1 X 10-3 g = 1 mg or .001 g = 1mg

  8. Ratio Method 129 mg as 1000 mg X g 1g (129 mg) • 1 g = 1000 mg • X g X = .129 g .001 g = 1mg or 1 g = 1000 mg

  9. Factor-Label Method • In the factor label method, you always take the given and multiply it by the conversion factor. • The conversion factor is a ratio like 12 inches or 1 meter 1 foot 100 cm

  10. Factor-Label Method 129 mg .001 g = .129 g 1 mg Conversion factor Given 1 X 10-3 g = 1 mg or .001 g = 1mg

  11. Factor-Label Method 129 mg 1 g = .129 g 1000 mg Conversion factor Given Use 1 g = 1000 mg

  12. 129 milligrams = 0.129 grams Think about your answer for a moment. Does it make sense? Reality Check Yes, you switched to a larger unit, so the number should be smaller!

  13. Convert 2.765 km to meters. Kilometers to meters From Table C: the factor is 103 Or 1 X 103 meters = 1 km, 1000 m = 1km 2.765 km 1 X 103 meters = 2765 m 1 km

  14. Convert 2.765 km to meters. 2.765 km = 2765 meters Does this answer make sense?

  15. Try a few more problems! • How many liters are in 345 milliliters? • This problem is worded a little differently. Think about how you are doing the conversion. • From ________ to _________?

  16. 345 milliliters to liters 10-3 What’s the conversion fact? 1 X 10-3 liters = 1 milliliter 345 ml 1 X 10-3 l = 0.345 l 1 ml 345 milliliters = 0.345 liters Does this answer make sense?

  17. 8.93 meters 45000 grams 5.9 deciliters 0.078 meter 240000 milligrams

  18. The Factor-Label Method orDimensional Analysis • A more general conversion technique. • More powerful in multistep problems. • Much loved by chemists!

  19. Problem: Convert 3.5 years to seconds. What information do you need to solve this problem? You need to know a bunch of relationships or equalities. We use these equalities to make conversions factors.

  20. For days to seconds, the relationships are? • 1 year = 365 days • 1 day = 24 hours • 1 hour = 60 minutes • 1 min = 60 seconds Use the equalities to set up conversion factors. Conversion factors are ratios.

  21. Convert 3.5 years to seconds • In the factor-label method, you always multiply! • Let the units do the work in setting up your conversion factors.

  22. 3.5 years X 365 days X 24 hours X 60 min X 60 sec 1 year 1 day 1 hour 1 min And the answer is … 110,376,000 seconds

  23. Problem: How many quarters in $23.00 • Recall: What relationship do you need to solve this problem? • Set-up: How would you set the problem up?

  24. $23.00 X 4 quarters = the # of quarters 1 dollar $23.00  92 quarters

  25. You just have to be a little careful when you are working with less familiar units. Take your time! Try converting 192 cm to meters.

  26. Recall: 1 m = 100 cm Set-up: 192 cm X 1 meter = 1.92 meter 100 cm

  27. Use the factor-label method for these! 14 quarts 3700 milliliters 2.7 meters 0.0565 grams 23.468 kilometers

  28. K H D  d c m • “Kids have dropped over dead converting metrics.” •  = gram, meter, sec, liter, etc. • Upper-case: kilo or 1000X, hecto or 100X, deka or 10X • Lower-case: deci or 1/10, centi or 1/100, milli or 1/1000. • As the unit gets smaller, the number has to get bigger. • Convert 25 mg to grams. Move decimal 3 places to the left, or .025 grams.

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