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CH. 2 - MEASUREMENT

CH. 2 - MEASUREMENT. I. Using Measurements. 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. A. Accuracy vs. Precision. Not Precise

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CH. 2 - MEASUREMENT

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  1. CH. 2 - MEASUREMENT I. Using Measurements

  2. 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

  3. A. Accuracy vs. Precision Not Precise Not Accurate Precise Accurate Precise Not Accurate

  4. your value accepted value B. Percent Error • Indicates accuracy of a measurement

  5. % error = 2.9 % 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.

  6. 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

  7. C. Significant Figures 6.34 cm

  8. C. Significant Figures • Rule 1- any # other than 0 is significant • Example 123.45 has 5 SF

  9. C. Significant Figures • Rule 2- any 0’s between significant figures are significant • Example- 1002.05 has 6 SF • 1002 has 4 SF

  10. C. Significant Figures • Rule 3- All final 0s after a decimal are significant • Example- 12.740 has 5 SF • 92100. has 6 SF

  11. C. Significant Figures • Rule 4- )s used for spacing are not significant • Example- 921000 has 3 SF • .00000233 has 3 SF

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

  13. C. Significant Figures • Rounding • If the digit to be removed is: • < 5, round down • > 5, round up • = 5, round to the nearest even number

  14. C. Significant Figures Rounding Examples Round each number to 3 sig figs 42.7 g 1. 42.68 g 17.3 m 2. 17.32 m 2.78 cm 3. 2.7851 cm 78.6 kg 4. 78.5543 kg

  15. 3 SF 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/cm3)(23.3cm3) = 324.103g 4 SF 3 SF 324g

  16. 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. 224 g + 130 g 354 g 224 g + 130 g 354 g 3.75 mL + 4.1 mL 7.85 mL 3.76 mL + 4.1 mL 7.86 mL  350 g  7.9 mL

  17. 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

  18. 5. (15.30 g) ÷ (6.4 mL)  2.4 g/mL 2 SF C. Significant Figures Practice Problems 4 SF 2 SF = 2.390625 g/mL 6. 18.9 g - 0.84 g  18.1 g 18.06 g

  19. D. Scientific Notation • Converting into Sci. Notation: • Move decimal until there’s 1 digit to its left. Places moved = exponent. • Large # (>1)  positive exponentSmall # (<1)  negative exponent • Only include sig figs. 65,000 kg  6.5 × 104 kg

  20. 7. 2,400,000 g 8. 0.00256 kg 9. 7  10-5 km 10. 6.2  104 mm D. Scientific Notation Practice Problems 2.4  106 g 2.56  10-3 kg 0.00007 km 62,000 mm

  21. EXE EXP EXP ENTER EE EE D. Scientific Notation • Calculating with Sci. Notation (5.44 × 107 g) ÷ (8.1 × 104 mol) = Type on your calculator: 5.44 7 8.1 4 ÷ = 671.6049383 = 670 g/mol = 6.7 × 102 g/mol

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