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Chemistry I Measurements and Calculations

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For a short lab: Bring in the following: 40 pennies dated before 1982 40 pennies dated after 1982

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Scientific Method There is no ONE scientific method. A sample: – State a problem or purpose – Observe by collecting data using equipment and materials – Organize and interpret the data collected through observation – Interpret data and and show with tables, graphs – Conclude what your research shows.

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Terms used in the scientific method Hypothesis – a testable statement – Control - a constant condition – Variable – a changing condition Theory – an explanation of the facts (accepted true without proof) Model – a method used to explain observations to support a theory

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Measurement in Science SI (international system) base units are Length, l meter, m Mass, m kilogram, kg Time, t second, s Derived units: Volume, V meter cubed, m 3 Density, D mass/volume, kg/m 3 or g/cm 3 D = m / V

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Common English Metric Conversions LENGTH 1 inch = 2.54 cm 1 mile = 1.61 km WEIGHT 1 ounce = 28 g 1 pound = 454 g 2.2 pound = 1kg VOLUME 1.06 quart = 1 liter

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Making conversions Use information from the prior charts to make conversions in chemistry (and other sciences) Problem sample: Convert 3,294 mm to km. 3292 mm x 1 km = 3292 10 6 mm 10 6 = 0.003292 or 3.292x10 – 3 km

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English to Metric Conversion Convert 2,493 mi to cm 2,493 mi x 1.61 km x 10 5 cm 1 mi 1 km Ans: 401373000 or 4.014 x 10 8 cm

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Using the Scientific Measurements that You have Collected Accuracy – numbers near or close to the accepted value Precision – readings are close to one another Percent Error to determine accuracy % Error = Value observed – Value accepted x 100 Value accepted

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% Error Problem The accepted boiling point of a chemical is 32.1 o C. You determine the bp to be 29.7 o C. What is your percent error? % Error = V o – V a x 100 = 29.7 – 32.1 x 100 V a 32.1 Ans. 7.48 %

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Rules for Determining Significant Figures #1 40.7 L3 87 0095 #2 0.095 234 0.00041 #3 85.004 9.000 0007 #4 20001 But, 2000.4 The decimal point indicates all zeroes are significant 1.Zeros appearing between nonzero digits are significant. 2.Zeroes appearing in front of all nonzero digits are not significant/ 3.Zeros at the end of a number and to the right of a decimal point are significant. 4.Zeros at the end of a number but to the left of a decimal point may or may not be significant. A decimal point placed after zeros indicates that they are significant.

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Samples 23,000has2 sig figs 23,000. has 5 0.091 0.0900005 0.06093

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Rounding – rule #3 may be slightly different than the book. Rules for Rounding numbers to the correct number of significant figures. If the digit following the last digit to be retained is 1. greater than 5, then add one to the last digit retained. 2. less than 5, then leave the last digit retained alone. 3. is exactly 5, then add one to the last digit retained only if it is odd. Let's look at some examples. Round the numbers 9.473, 9.437, 9.450, and 9.750 to two significant figures. For 9.473 the last digit retained is 4, and the decimal fraction is 0.73. So we use rule #1 above and 9.473 is rounded to 9.5 For 9.437 the last digit retained is 4, and the decimal fraction is 0.37. So we use rule #2 above and 9.437 is rounded to 9.4 For 9.450 the last digit retained is 4, and the decimal fraction is 0.50. So we use rule #3 above and 9.450 is rounded to 9.4 For 9.750 the last digit retained is 7, and the decimal fraction is 0.50. So we use rule #3 above and 9.750 is rounded to 9.8

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Math with sig figs Addition and Subtraction Involving Significant Figures In addition or subtraction, the arithmetic result should be rounded off so that the final digit is in the same place as the leftmost uncertain digit. Multiplication & Division Involving Significant Figures The arithmetic product or quotient should be rounded off to the same number of significant figures as in the measurement with the fewest significant figures. Addition: 4.09cm + 2.9cm = 6.99 --> 7.0 (to the tenths place because of 2.9) Mult.: 1.39m x 4.276m = 5.94364 --> 5.94 (3 s.f. because of the 1.39)

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See handouts for any other information about significant digits, rounding, and the math involved.

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Direct Proportions Distance (km) Time (hr) The longer time that a car moves, the further it goes. As time increases, so does distance – they are directly proportional to one another.

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Inversely Proportional As pressure acting on a gas goes up, the volume of the gas goes down.

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