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Chapter 2

Chapter 2. Quantitative measurements. give results in a definite form, usually a number. Example. Thermometer. Qualitative measurements. gives results in a descriptive form. non-numeric value. Example. Color of a reaction. Accuracy. How close a measurement comes to the true value.

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Chapter 2

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  1. Chapter 2

  2. Quantitative measurements give results in a definite form, usually a number Example Thermometer

  3. Qualitative measurements gives results in a descriptive form non-numeric value Example Color of a reaction

  4. Accuracy How close a measurement comes to the true value Reproducibility of the measurement Precision

  5. Significant Figures include all digits that can be known accurately plus a last digit that is estimated

  6. Rules for Significant Figures (All have 3 Sig. Figs.) • Every nonzero digit in a recorded measurement is significant Examples 65.2 0.268 126

  7. 2. Zeros between nonzero digits are significant Example 2004 60.39 8.602 (All have 4 Sig. Figs.)

  8. 3. Zeros in front of all nonzero digits are NOT significant. They act as place-holders. Examples 0.0062 0.58 0.00027 (2 Sig. Figs.)

  9. 4. Zeros at the end of a number and to the right of a decimal point are significant Examples 61.00 1.030 8.000 (4 Sig. Figs.)

  10. Zeros at the end of a measurement and to the left of the decimal can be confusing. If they are place-holders to show the magnitude of a number, they are not significant

  11. Examples 300 700 27210 ambiguous If the zeros were measured- significant 700.

  12. To avoid confusion use scientific notation 3.00 x 10 2

  13. How Many Sig. Figs.? 123  3 98,000  2 40,506  5 0.07080  4 0.123  3 0.078  2 5 9.8000 x 10 4 

  14. Sig. Figs in calculations • If digit following last sig fig is <5, all digits are dropped • If digit following last sig fig is >5, digit in last sig fig place is increased by one

  15. Round These……. 314.721 4 sig figs 314.7 0.001775 2 sig figs 0.0018 64.32 x 10 -1 1 sig fig 60 x 10 -1 or 6

  16. Addition and Subtraction The answer cannot contain any more digits to the right of the decimal point than are contained in the measurement with the least number of digits to the right of the decimal point

  17. Sample Problem 12.52 + 349.0 + 8.24 369.8 or 3.698 x 10 2

  18. Multiplication and Division Answer must contain no more sig figs than the measurement with the least number of sig figs. *Decimal point has nothing to do with determining this!

  19. Example Problems 755 x .034 = 26 3 sig. figs. 2 sig. figs 2 sig. figs 2.4526  8.4 = 0.29

  20. Metric System • SI system (International System of Units)- Developed in 1790 in France

  21. SI Unit Non-SI Length m Quantity Volume m3 Liters Mass kg grams Density g/cm³, g/ml Temperature K oC Time s Pressure pa (pascal) atmospheres, mm Hg Energy J (joules) calories

  22. Units of Measurement – Meter (m) Prefix Symbol Magnitude kiloK 1000x hecto h 100x deca da 10x unit m, L, g x deci d .1x centi c .01x milli m .001x micro u 1 x 10 -6 x nano n 1 x 10 -9 x pico p 1 x 10-12 x

  23. Units of Volume – Liter (L) • Volume of a cube-10 cm x 10 cm x 10 cm= 1000 cm 3 • 1 L = 1 dm 3 = 1000 cm 3 1 cm 3 = 1 mL

  24. Units of Mass – (g) • Mass- quantity of matter • Weight- a force • Unit of mass – gram (g) = mass of 1 cm 3 of H2O at 4 ºC

  25. Density- ratio of mass of an object to volume D= M = g/mL or g/cm 3 V

  26. Specific Gravity • Specific Gravity- comparison of the density of a substance to the density of a reference substance

  27. Specific Gravity Cont’d • Sp. Gravity = density of substance g/cm 3 density of H2O g/cm 3 • Sp. Gravity of a liquid is measured with a hydrometer

  28. Temperature- Degree of hotness or coldness of an object • Determined direction of heat transfer • Heat transfer- occurs when two objects at different temperatures contact each other • Heat goes from high temperature to low temp

  29. Temperature Scales Fahrenheit 32ºF - FP of H2O 212ºF - BP of H2O Celsius (centigrade) 0ºC - FP of H2O 100ºC - BP of H2O Kelvin 273 K - FP of H2O K = oC + 273 373 K - BP of H2O

  30. Heat • Heat- measured in joule (J) or calories (cal) • One calorie is the quantity of heat that raises the temperature of 1 g of H2O 1ºC • 1 cal = 4.18 J • Calorie in nutritional terms means kilocalorie

  31. Heat Cont’d • Heat Capacity- quantity of heat required to change an objects temperature by exactly 1ºC • Depends on mass and type of substance

  32. Specific Heat • The quantity of heat required to raise 1g of a substance 1ºC • Sp. Heat = heat = q mass x T (m) (T) Units of Specific Heat: J/g°C or cal/g°C

  33. Chapter 2B Problem Solving in Chemistry

  34. Three easy steps to problem solving. . .

  35. Step 1 : Analyze • Identify a known • Determine where you are starting from. • What information do you already have to work with?

  36. Step 1 cont’d… • Identify an unknown • Where are you going? • What are you looking for? • Plan a solution • How are you going to get there?

  37. Step 2: Calculate • May involve substituting known quantities and doing the arithmetic needed to solve for unknown. • You may also need to convert.

  38. Step 3: Evaluate • Go over your answers • Does the answer make sense? • Did you use correct units? • Check your work • Make sure you copied down the given information correctly.

  39. Sample Problem • What is the mass, in grams, of a piece of lead that has a volume of 19.84 cm³?

  40. Step 1 : Analyze • List Knowns and Unknowns • Knowns : • Volume of lead: 19.84 cm³ • Density of lead = 11.4 g/cm³ (according to table 3.7) • Density = mass volume • Unknowns: mass = ?g

  41. Step 2: Calculate • Density = mass -or- Mass = volume x density volume • Mass = 19.84 cm ³ x 11.4 g/cm ³ = 226.176 g Mass = 226 g

  42. Step 3: Evaluate • Has the unknown been found? • Yes, problem asks for mass • Do you have the correct units? • Yes, units canceled correctly to yield grams (g) • Is the number of sig figs correct? • Yes, answer has 3 sig figs Mass = 226 g

  43. What is the volume, in cubic centimeters of a sample of cough syrup that has a mass of 50.0g? The density of cough syrup is 0.950g/cm³.

  44. Volume = mass density Volume = 50.0g = 52.6316 cm³ 0.950g/cm³ Volume = 52.6 cm³

  45. Your school club has sold 600 tickets to a chili-supper fundraising event, and you have volunteered to make chili. You have a chili recipe that serves 10. The recipe calls for two teaspoons of chili powder. How much chili powder do you need for 600 servings?

  46. Servings Needed = 600 10 servings = 2 tsp chili powder Amount of chili powder = ? tsp 600 servings x 2 tsp chili powder = 10 servings 1200 tsp chili powder = 120 tsp chili powder 10

  47. How many cups are in 120 teaspoons of chili powder?

  48. 120 tsp x 1 tbs x 1 cup = 3 tsp 16 tbs 120 cups = 2.5 cups 48

  49. Express 750 dg in grams

  50. Mass = 750 dg 1 g = 10 dg Mass = ?g 750 dg x 1g = 75 g 10 dg

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