Measurements in chemistry

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Measurements in chemistry

  1. 1. Measurements in Chemistry<br />Scientific Notation, Significant Figures, Percent Error<br />
  2. 2. Units of Measurement<br />Put the following units in order from smallest to largest.<br />Meter, centimeter, millimeter, kilometer<br />Kilogram, centigram, milligram, gram<br />Liter, microliter, picoliter, kiloliter<br />
  3. 3. SI Units<br />
  4. 4. Units of Measurement<br />Put the following units in order from smallest to largest.<br />millimeter, centimeter, meter, kilometer<br />milligram, centigram, gram, kilogram<br />picoliter, microliter, liter, kiloliter<br />What information do the prefixes centi, milli, kilo, etc. provide?<br />
  5. 5. Prefixes<br />
  6. 6. Scientific Notation<br />When studying chemistry it is common to encounter very large or very small numbers.<br />Need a system in which to shorten long number chains.<br />Ex: The number of air molecules in a liter of air at 20oC and normal barometric pressure is 25,000,000,000,000,000,000,000.<br />Ex: The distance between two hydrogen atoms in a diatomic hydrogen molecule is 0.000,000,000,074 meters.<br />In Scientific Notation, these long chains of numbers are written in the form of;<br />M x 10n<br />
  7. 7. Scientific Notation<br />M x 10n<br />Scientific notation is simply a number time 10 raised to an exponent.<br />M is a number greater than or equal to 1 and less than 10.<br />n is the exponent (the nth power or 10). It can be either positive or negative and represent the number of decimal places moved.<br />Positive (+) n means a large number so the decimal moves to the right by n places.<br />Negative (-) n means a small number so the decimal moves to the left by n places.<br />
  8. 8. Practice<br />Convert the following from scientific notation to their usual form.<br />6.39 x 10-4<br />3.275 x 10-2<br />8.019 x 10-6<br />
  9. 9. Practice<br />Convert the following from scientific notation to their usual form.<br />6.39 x 10-4= 0.000639<br />3.275 x 102= 327.5<br />8.019 x 10-6= 0.000008019<br />
  10. 10. Practice<br />Express the following numbers in scientific notation.<br />843.4<br />0.00421<br />1.54<br />
  11. 11. Practice<br />Express the following numbers in scientific notation.<br />843.4 = 8.434 x 102<br />0.00421 = 4.21 x 10-3<br />1.54 = 1.54 or 1.54 x 100<br />
  12. 12. Scientific Notation Cheat Sheet<br />When converting from standard notation to scientific notation…<br />If the number is one or greater you will have a positive exponent and move the decimal to the left.<br />If the number is less than one you will have a negative exponent and move the decimal to the right.<br /># of spaces moved by decimal = exponent<br />801236.98<br />8.0123698 x 105<br />0.0000508<br />5.08 x 10-5<br />
  13. 13. Scientific Notation Cheat Sheet<br />When converting from scientific notation to standard notation…<br />If the exponent is positive you will have a large number (>1) and move the decimal to the right.<br />If the exponent is negative you will have a small number (<1) and move the decimal to the left.<br />Exponent = # of spaces to be moved by decimal<br />801236.98<br />8.0123698 x 105<br />0.0000508<br />5.08 x 10-5<br />
  14. 14. Significant Figures<br />If you were measuring this granite block in inches, what would you determine its width to be?<br />
  15. 15. Significant figures<br />In measurements there is always some amount of uncertainty.<br />
  16. 16. Significant Figures<br />Repeating a particular measurement will usually not obtain precisely the same result.<br />The measured values vary slightly from one another.<br />Precision – refers to the closeness of a set of values obtained from identical measurements of something.<br />Accuracy – refers to the closeness of a single measurement to its true value.<br />
  17. 17. Rules for Sig Figs<br />The number of digits reported for the value of a measured quantity.<br />All nonzero numbers and zeros between are significant.<br />909 cm, 1002 cm, 100,003 cm<br />Zeros at the beginning of a number are never significant.<br />0.000912 cm, 0.01 cm, 0.000001001 cm<br />Zeros at the end of a number are significant only if a decimal is present, and to the right of the decimal.<br />900 cm, 900.0 cm<br />
  18. 18. Significant Figures in Calculations<br />Multiplication and Division<br />The answer is given with as many significant figures in the measurement with the least amount of significant figures.<br />Addition and Subtraction<br />The answer is given with as many significant figures as the measurement with the least number of decimal places.<br />
  19. 19. Percent Error<br />Sometimes it is important to calculate how far off a measured value has deviated from the true or accepted value.<br />For this we use Percent (%) Error.<br />
  20. 20. A problem to consider<br />A student measures the volume of a piece of zinc, by water displacement, to be 75.0 cm3 and the mass to be 562.5 g. <br />Now look up the accepted value for the density of zinc in Table S on your reference tables.<br />
  21. 21. A problem to consider<br />A student measures the volume of a piece of zinc, by water displacement, to be 75.0 cm3 and the mass to be 562.5 g. <br />Now look up the accepted value for the density of zinc in Table S on your reference tables.<br />
  22. 22. A Problem to consider<br />Does your calculated value agree with the scientifically accepted value?<br />
  23. 23. A Problem to consider<br />Does your calculated value agree with the scientifically accepted value?<br />
  24. 24. A Problem to consider<br />How “ far off ” is your calculated value from the accepted value?<br />
  25. 25. A Problem to consider<br />How “ far off ” is your calculated value from the accepted value?<br />

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