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Chemistry I

Chemistry I. Measurements and Calculations. For a short lab:. Bring in the following: 40 pennies dated before 1982 40 pennies dated after 1982. Scientific Method. There is no ONE scientific method. A sample: State a problem or purpose

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Chemistry I

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  1. Chemistry I Measurements and Calculations

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

  3. 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.

  4. 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

  5. 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, m3 • Density, D  mass/volume, kg/m3 or g/cm3 D = m / V

  6. 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

  7. 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 • 106 mm • = 0.003292 or 3.292x10– 3 km

  8. English to Metric Conversion • Convert 2,493 mi to cm • 2,493 mi x 1.61 km x 105 cm • 1 mi 1 km • Ans: 401373000 or 4.014 x 108 cm

  9. 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 = Valueobserved – Valueacceptedx 100 • Valueaccepted

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

  11. Rules for Determining Significant Figures #1 40.7 L 3 87 009 5 #2 0.095 23 4 0.0004 1 #3 85.00 4 9.000 000 7 #4 2000 1 But, 2000. 4 The decimal point indicates all zeroes are significant Zeros appearing between nonzero digits are significant. Zeroes appearing in front of all nonzero digits are not significant/ Zeros at the end of a number and to the right of a decimal point are significant. 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.

  12. Samples • 23,000 has 2 sig figs • 23,000. has 5 • 0.09 1 • 0.090000 5 • 0.0609 3

  13. 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

  14. 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)

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

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

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

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