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Worksheet: Significant Digits & Rounding - Key 1. How many estimated digits are usually in a measurement? One 2. When is it not necessary to make an estimation when taking a measurement? When you are using an electronic (non-scaled) measuring device. 3. When are nonzero digits significant in a measurement? Always 4. A zero in a measurement is significant if it is a trailing zero after the decimal point or it is a zero with a bar placed over it or it is a zero between significant digits. 5. What is the difference between a counting number and a measurement? Counting numbers are exact whereas measurements are only as precise as the device being used to measure them. 6. How many significant digits are in each of the following values? a) b) c) d) e) f) g) h) i) j) k) l) 5.002 g four 5000 dL one 0.000689 mg three 0.3870 mL four 140ºC two 0.0006008 μm four 140.0 K four 1030 mol three 1.030 cm four 820 ms two 0.030040 cg five 23 cars this is a counting number and therefore would have an infinite number of significant digits m) 2500 0 00 mmol five 7. Rewrite the answers so that they are rounded to the correct precision. a) 1.25 g x 8.6 ºC = 10.75 gºC = 11 gºC b) 100.00 g ÷ 25.0 mL = 4 g/mL = 4.00 g/mL _ c) 500.00 cm x 40.00 cm = 20000 cm2 = 20 000cm2 d) 38 cm + 5.100 cm + 4.13 cm = 47.23 cm = 47 cm e) 716.55 g − 0.005 g = 716.545 g = 716.55 g f) 28.00 g ÷ 85.2 cm3 = 0.3286385 g/cm3 = 0.329 g/cm3 g) 8.000 km − 0.54 km = 7.46 km = 7.46 km h) 23.18 m + 6.819 m = 29.999 m = 30.00m i) _ 10.0 mm x 100.0 mm = 1000 mm2 = 10 0 0 mm2