Objectives To learn how uncertainty in a measurement arises

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CHEMISTRY 11 TODAY’s OBJECTIVE:

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Presentation transcript:

Objectives To learn how uncertainty in a measurement arises To learn to indicate a measurement’s uncertainty by using significant figures To learn to determine the number of significant figures in a calculated result

A. Uncertainty in Measurement A measurement always has some degree of uncertainty.

Uncertainty in Measurement

A. Uncertainty in Measurement Different people estimate differently. Record all certain numbers and one estimated number.

B. Significant Figures Significant figures are the meaningful figures in our measurements and they allow us to generate meaningful conclusions Numbers recorded in a measurement are significant. All the certain numbers plus first estimated number e.g. 2.85 cm We need to be able to combine data and still produce meaningful information There are rules about combining data that depend on how many significant figures we start with………

B. Significant Figures Rules for Counting Significant Figures Nonzero integers always count as significant figures. 1457 has 4 significant figures 23.3 has 3 significant figures

B. Significant Figures Rules for Counting Significant Figures Zeros Leading zeros - never count 0.0025 2 significant figures Captive zeros - always count 1.008 4 significant figures Trailing zeros - count only if the number is written with a decimal point 100 1 significant figure 100. 3 significant figures 120.0 4 significant figures

B. Significant Figures Rules for Counting Significant Figures Exact numbers - unlimited significant figures Not obtained by measurement Determined by counting: 3 apples Determined by definition: 1 in. = 2.54 cm

How Many Significant Figures? 1422 65,321 1.004 x 105 200 435.662 50.041 102 102.0 1.02 0.00102 0.10200 1.02 x 104 1.020 x 104 60 minutes in an hour 500 laps in the race

B. Significant Figures Rules for Multiplication and Division The number of significant figures in the result is the same as in the measurement with the smallest number of significant figures.

B. Significant Figures Rules for Addition and Subtraction The number of significant figures in the result is the same as in the measurement with the smallest number of decimal places.

Rules for Combined Units Multiplication / Division When you Multiply or Divide measurements you must carry out the same operation with the units as you do with the numbers 50 cm x 150 cm = 7500 cm2 20 m / 5 s = 4 m/s or 4 ms-1 16m / 4m = 4 Addition / Subtraction When you Add or Subtract measurements they must be in the same units and the units remain the same 50 cm + 150 cm = 200 cm 20 m/s – 15 m/s = 5 m/s

Calculate the following Calculate the following. Give your answer to the correct number of significant figures and use the correct units 11.7 km x 15.02 km = 12 mm x 34 mm x 9.445 mm = 14.05 m / 7 s = 108 kg / 550 m3 = 23.2 L + 14 L = 55.3 s + 11.799 s = 16.37 cm – 4.2 cm = 350.55 km – 234.348 km =