I II III I. Using Measurements CH. 2 - MEASUREMENT

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

Accurate but not 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

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

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 %

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

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

Atlantic Pacific Rule  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. Atlantic Pacific

, C. Significant Figures Counting Sig Fig Examples , sig figs 3 sig figs 2 sig figs

C. Significant Figures  Calculating with Sig Figs  Add/Subtract - The # with the lowest decimal value determines the place of the last sig fig in the answer mL mL 7.85 mL 224 g g 354 g  7.9 mL  354 g 3.75 mL mL 7.85 mL

C. Significant Figures  Calculating with Sig Figs (con’t)  Multiply/Divide - The # with the fewest sig figs determines the # of sig figs in the answer. (13.91g/cm 3 )(23.3cm 3 ) = g 324 g 4 SF3 SF

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 5. (15.30 g) ÷ (6.4 mL) Practice Problems = g/mL  18.1 g g g g 4 SF2 SF  2.4 g/mL 2 SF

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

D. Scientific Notation 7. 2,400,000  g kg 9.7  km  10 4 mm Practice Problems 2.4  10 6  g 2.56  kg km 62,000 mm

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 = = 670 g/mol= 6.7 × 10 2 g/mol Type on your calculator: