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Lecture

# Significant figures, why do we care?, precision, accuracy, error

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School
Department
Chemistry
Course
CHEM 111
Professor
Dana Johnson
Semester
Fall

Description
27 August Significant Figures (Sig Figs) No measurement is ever perfect Always a degree of uncertainty Sometimes shown with “±” as in 1.5 ± 0.1 mL If there’s no “±”, uncertainty assumed in the last digit The more digits, the more certain the measurement Determining Sig Figs #1: all non – zero digits 89.6 µm has 3 sig figs #2: all zeroes between non – zero digits 10,345 lb has 5 sig figs #3: leading zeroes are NOT 0.00035 sec has 2 sig figs #4: ending zeroes depend #4a: no decimal → no 1500 atm has 2 sig figs #4b: between non – zero and decimal on right → yes 300. MHz has 3 sig figs #4c: decimal on left, after non – zero →yes 0.0400 mL has 3 sig figs #5: scientific notation always shows sig figs 20,000 mi looks like 1 sig fig vs. 2.0 x 10 mi shows 2 sig figs 1.2 x 10 vs. 1.20 x 10 2 Practice How many sig figs? 1. 32.010 a. 3 b. 4 c. 5 * 2. 0.0032 a. 4 b. 2 * c. 5 3. 30.0 a. 1 b. 2 c. 3 * Significant zeroes? 50,400 a. 1 * b. 2 c. 3 Why do we care? Your calculated answer cannot be more certain than your measurements → initial sig figs inform your answer Multiplication and division: the answer must contain the same # of sig figs as that of the LEAST certain number Volume of a cube 3.4 in to a side V = r = (3.4 in) = 39.304 in Must
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