I. Using Measurements (pp. 28 - 32) CH. 2 - MEASUREMENT I. Using Measurements (pp. 28 - 32)

A. Accuracy vs. Precision p. 10 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

Not Accurate Nor precise. The average is close to the center but the individual values are not similar

Precise, but not accurate All measurements are close, but not at the “correct” value

Accurate AND Precise All measurements are close, AND at the “correct” value

Identify each High Accuracy, high precision. High accuracy, low precision Low accuracy, high precision Low accuracy, low precision

B. Percent Error p. 9 Indicates accuracy of a measurement your value Have students calculate % Error using my measurements vs. their measurements. accepted value

B. Percent Error % error = 2.9 % 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 %

C. Significant Figures p. 5 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

C. Significant Figures Counting Sig Figs (p. 4 of Unit 1 packet) Count all numbers EXCEPT: Leading zeros -- 0.0025 Trailing zeros without a decimal point -- 2,500

Atlantic Pacific Rule Pacific Atlantic If the decimal is ABSENT, start from the ATLANTIC side, go to the first non-zero digit and start counting. If the decimal is PRESENT, start from the PACIFIC side, go to the first non-zero digit and start counting.

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

C. Significant Figures 3.75 mL + 4.1 mL 7.85 mL 3.75 mL + 4.1 mL Calculating with Sig Figs 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 3.75 mL + 4.1 mL 7.85 mL 224 g + 130 g 354 g  7.9 mL  354 g

C. Significant Figures (13.91g/cm3)(23.3cm3) = 324.103g 324 g Calculating with Sig Figs (con’t) 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 3 SF 324 g

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

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

D. Scientific Notation p. 3 65,000 kg  6.5 × 104 kg 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.

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

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