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Scientific Notation & Significant Figures in Measurement Dr. Sonali Saha Chemistry Honors Fall 2014

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In science, we deal with some very LARGE numbers: 1 mole = 602257000000000000000000 We also deal with some very SMALL numbers: Mass of an electron = 0.000000000000000000000000000000091 kg Scientific Notation

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Imagine the difficulty of calculating the mass of 1 mole of electrons! 0.000000000000000000000000000000091 kg x 602257000000000000000000 x 602257000000000000000000 ???????????????????????????????????

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Scientific Notation: A method of representing very large or very small numbers in the form: M x 10 n M x 10 n M is a number between 1 and 10 n is an integer For really large and really small numbers, a convenient way to represent them is by using:

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2 500 000 000 Step #1: Insert a decimal point. Step #2: Decide where the decimal must end up so that one number is to its left up so that one number is to its left Step #3: Count how many places you bounce the decimal point the decimal point 1234567 8 9 Step #4: Re-write in the form M x 10 n

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2.5 x 10 9 The exponent is the number of places we moved the decimal. Note the exponent is positive because the original number was greater than 1.

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0.0000579 Step #1: Decide where the decimal must end up so that one number is to its left up so that one number is to its left Step #2: Count how many places you bounce the decimal point the decimal point Step #3: Re-write in the form M x 10 n 12345

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5.79 x 10 -5 The exponent is negative because the original number was less than 1.

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Characteristics of Measurement Part 1 - number Part 2 – unit of measure Examples: 20 grams 20 mL Measurement - quantitative observation consisting of 2 parts consisting of 2 parts

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Why Is there Uncertainty in Measurements? Measurements are performed with instruments. No instrument can read to an infinite number of decimal places. Which of these balances has the greatest uncertainty in measurement?

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Precision and Accuracy Accuracy refers to the agreement of a particular value with the “true” value. Precision refers to the degree of agreement among several measurements made in the same manner. Neither accurate nor precise Precise but not accurate Precise AND accurate

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More on Precision… Precision also refers to the smallest calibrated markings on laboratory instruments and equipment. –T–The precision of our electronic scale is 1/10 th of a gram. –T–The precision of a graduated cylinder is 1 milliliter (1 mL), of a beaker it is 10 mL, of a burette is 0.1 mL

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Significant Figures When reporting a measurement, how many significant figures/digits should you use? The significant figures of a number are those digits that carry meaning contributing to its precision.

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Rules for using significant figures to express lab measurement The measurement represented by the smallest graduated marking

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For example you used a beaker and measured 20 mL (one sig. fig.) while the mass of the water was 18.2 g (three sig. figs) how would you report your density value? Multiplication and Division: The number of sig figs in the result equals the number in the least precise measurement used in the calculation. 18.2 (3 sig.fig) 20 (1 sig.fig.) = calculator says 0.91 18.2 (3 sig.fig) ÷ 20 (1 sig.fig.) = calculator says 0.91 REALLY IS = 0.9 g/mL

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Rules for Significant Figures in Mathematical Operations Addition and Subtraction: The number of decimal places in the result equals the number of decimal places in the least precise measurement. 6.8 + 11.934 = 18.734 18.7 (1 place after the decimal)

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Rules for Counting Significant Figures Exact numbers (definitions and counting numbers) have an infinite number of significant figures.Exact numbers (definitions and counting numbers) have an infinite number of significant figures. 1 inch = 2.54 cm, exactly

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Rules for Counting Significant Figures - Zeros - Leading zeros do not count as significant figures. 0.0486 has 3 sig figs.

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Rules for Counting Significant Figures - Zeros - Captive zeros always count as significant figures. 16.07 has 4 sig figs.

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Rules for Counting Significant Figures - Zeros Trailing zeros are significant if the number contains a decimal point. Little bit ambiguous!!!! 9.300 has 4 sig figs. 9300 has 9300 has 2 sig figs.

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Sig Fig Practice #1 How many significant figures in each of the following measurements? 1.0070 m 5 sig figs 17.10 kg 4 sig figs 100,890 L 5 sig figs 3.29 x 10 3 s 3 sig figs 0.0054 cm 2 sig figs 3,200,000 2 sig figs

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Sig Fig Practice #2 3.24 m + 7.0 m CalculationCalculator says: Correct Answer 10.24 10.2 m 100.0 g - 23.73 g 76.27 76.3 g 0.02 cm + 2.371 cm 2.391 2.39 cm 713.1 L - 3.872 L 709.228709.2 L 1818.2 lb + 3.37 lb1821.57 1821.6 lb 2.030 mL - 1.870 mL 0.16 0.160 mL

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Sig Fig Practice #3 3.24 m x 7.0 m CalculationCalculator says: Correct Answer 22.68 23 m 2 100.0 g ÷ 23.7 cm 3 4.219409283 4.22 g/cm 3 0.02 cm x 2.371 cm 0.04742 0.05 cm 2 710 m ÷ 3.0 s 236.6666667240 m/s 1818.2 lb x 3.23 ft5872.786 5870 lb·ft 1.030 g ÷ 2.87 mL 0.3588850170.359 g/mL

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