2. I II III III. Using Measurements (p. 44 - 57) CH. 2 - MEASUREMENT

3. A. Accuracy vs. Precision  Accuracy - how close a measurement is to the accepted value  Precision - how close a series of measurements are to each other ACCURATE = CORRECT PRECISE = CONSISTENT

4. Let’s use a golf analogy

5. Accurate?No Precise?Yes

6. Accurate?Yes Precise?Yes

7. Precise?No Accurate? No

8. Accurate?Yes Precise?We can’t say!

9. Dart Example

10. B. Percent Error  Indicates accuracy of a measurement your value accepted value

11. B. Percent Error  A student determines the density of a substance to be 1.40 g/mL. Find the % error if the accepted value of the density is 1.36 g/mL. % error = 2.9 %

12. Your Turn  What is the percent error for a mass measurement of 17.7 g, if the correct value is 21.2 g?  A volume is measured experimentally as 4.26 mL. What is the percent error, given the correct value is 4.15 mL?

13. Error in Measurement  Some error or uncertainty always exists in any measurement.  skill of the measurer  conditions of measurement  measuring instruments

14. C. Significant Figures  Indicate precision of a measurement.  Recording Sig Figs  Sig figs in a measurement include the known digits plus a final estimated digit 2.35 cm

15. C. Significant Figures  Counting Sig Figs (Table 2-5, p.47)  Count all numbers EXCEPT:  Leading zeros -- 0.0025  Trailing zeros without a decimal point -- 2,500

16. 4. 0.080 3. 5,280 2. 402 1. 23.50 C. Significant Figures Counting Sig Fig Examples 1. 23.50 2. 402 3. 5,280 4. 0.080 4 sig figs 3 sig figs 2 sig figs

17. C. Significant Figures  Calculating with Sig Figs  Multiply/Divide - The # with the fewest sig figs determines the # of sig figs in the answer. (13.91g/cm 3 )(23.3cm 3 ) = 324.103g 324 g 4 SF3 SF

18. C. Significant Figures  Calculating with Sig Figs (con’t)  Add/Subtract - The # with the lowest decimal value determines the place of the last sig fig in the answer. 3.75 mL + 4.1 mL 7.85 mL 224 g + 130 g 354 g  7.9 mL  350 g 3.75 mL + 4.1 mL 7.85 mL 224 g + 130 g 354 g

19. C. Significant Figures  Calculating with Sig Figs (con’t)  Exact Numbers do not limit the # of sig figs in the answer.  Counting numbers: 12 students  Exact conversions: 1 m = 100 cm  “1” in any conversion: 1 in = 2.54 cm

20. C. Significant Figures 5. (15.30 g) ÷ (6.4 mL) Practice Problems = 2.390625 g/mL  18.1 g 6. 18.9g - 0.84 g 18.06 g 4 SF2 SF  2.4 g/mL 2 SF

21. D. Scientific Notation  Converting into Sci. Notation:  Move decimal until there’s 1 digit to its left. Places moved = exponent.  Large # (>1)  positive exponent Small # (<1)  negative exponent  Only include sig figs. 65,000 kg  6.5 × 10 4 kg

22. D. Scientific Notation 7. 2,400,000  g 8. 0.00256 kg 9.7  10 -5 km 10.6.2  10 4 mm Practice Problems 2.4  10 6  g 2.56  10 -3 kg 0.00007 km 62,000 mm

23. D. Scientific Notation  Calculating with Sci. Notation (5.44 × 10 7 g) ÷ (8.1 × 10 4 mol) = 5.44 EXP EE ÷ ÷ EXP EE ENTER EXE 78.1 4 = 671.6049383= 670 g/mol= 6.7 × 10 2 g/mol Type on your calculator: