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Measurement

Measurement. Measurement. Determined magnitude of a property Based on a standard Must have a unit (which is based on the standard) Number of digits in a measurement depends on the device used

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Measurement

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  1. Measurement CHM 1010 PGCC Barbara A. Gage

  2. Measurement • Determined magnitude of a property • Based on a standard • Must have a unit (which is based on the standard) • Number of digits in a measurement depends on the device used • Values must be expressed in scientific notation if they are 1000 or more or in the thousandths (0.00X) CHM 1010 PGCC Barbara A. Gage

  3. Significant Figures • When making a measurement you must record all digits you are sure of and one that is a reasonable estimate (regardless of where the decimal place falls) • The object below is 4.86 cm. You can be certain of the 4 and 8. The 6 is an estimate and can vary +/- 1 between measurements. CHM 1010 PGCC Barbara A. Gage

  4. Significant figures • The graduated cylinder at the right contains 38.34 mL of liquid. It is assumed that you can visually divide the space between lines into ten parts. CHM 1010 PGCC Barbara A. Gage

  5. Significant Figures • When you encounter a measurement assume that all non-zero digits or zeros between other digits are significant. 23.789 dm 5 sig figs 2.04 gal 3 sig figs 52 kg 2 sig figs CHM 1010 PGCC Barbara A. Gage

  6. Significant Figures • Zeros may serve as actual measured values or place holders in estimated measurements. Place holder zeros are NOT significant because they are really not measured values. • Place holder zeros can be removed by expressing the number in scientific notation. CHM 1010 PGCC Barbara A. Gage

  7. Significant Figures 120,000 2 significant figures This is an estimate that can be written as 1.2 x 105. 0.0040500 5 significant figures The zeros before and immediately after the decimal point can be eliminated using scientific notation. The last zeros remain because they are actual measurements. 4.0500 x 10-3 CHM 1010 PGCC Barbara A. Gage

  8. Significant Figures –Addition and Subtraction • When you are adding and subtracting numbers you only count the columns where you are sure of all the values in that column. CHM 1010 PGCC Barbara A. Gage

  9. Significant Figures –Multiplication and Division • When multiplying or dividing two or more values, the answer should contain the number of digits in the value with the least number of significant figures. 0.003570 4 sig figs X 23.4 3 sig figs 0.083538 5 sig figs (calculator answer) You can only trust the answer to 3 sig figs so the correct answer is 8.35 x 10-2. CHM 1010 PGCC Barbara A. Gage

  10. Measurement Systems • Everyday measurements in the USA are generally made using the English system. • The scientific community and most other countries use the metric system. CHM 1010 PGCC Barbara A. Gage

  11. Units • The units in the English system do not have a common • conversion factor. • 12 in = 1 ft 3 ft = 1 yd 1760 yd = 1 mi • The units in the metric system have a common • factor which is 10. The metric system uses a common • base unit and prefixes to change the size of the unit. CHM 1010 PGCC Barbara A. Gage

  12. Metric Prefixes The boxed prefixes must be memorized. CHM 1010 PGCC Barbara A. Gage

  13. Converting Measurements • Often it is necessary to convert a measurement made in one unit to another unit. Ex. 2.45 cm = ? m In the metric system you can just shift the decimal point or set up a conversion factor. 2.45 cm = 0.0245 m CHM 1010 PGCC Barbara A. Gage

  14. Converting Measurements • Using a conversion factor: 100 cm = 1 m • 100 cm or 1m both ratios = 1 1 m 100 cm CHM 1010 PGCC Barbara A. Gage

  15. Converting Measurements • If Baltimore is 35 miles away, how far is it in km? • Using a conversion factor: 1.61 km = 1 mi CHM 1010 PGCC Barbara A. Gage

  16. Converting Measurements • How many mL are in 0.875 gal? 1.06 qt = 1 L 4 qt = 1 gal CHM 1010 PGCC Barbara A. Gage

  17. Problem… • A gas particle has a velocity of 752 m/s. What is its velocity in mi/hr? 1.61 km = 1 mi CHM 1010 PGCC Barbara A. Gage

  18. Problem… • A synthesis process requires 6.2 fl. oz. of activator for every 2.5 tons of starting material. What is the concentration of activator in the final product in mL/kg? 1 fl oz = 29.6 mL 2000 lb = 1 ton 2.20 lb = 1 kg CHM 1010 PGCC Barbara A. Gage

  19. Density • Property derived from two measurements, mass and volume • Density = Mass/Volume • Will have a unit that contains both mass and volume such as g/cm3, lb/gal, kg/L • Does not depend on the size of the sample CHM 1010 PGCC Barbara A. Gage

  20. Density • What is the density of a sample of metal that has a mass of 34.58 g and when placed in 15.0 mL of water causes the level to rise to 22.4 mL? CHM 1010 PGCC Barbara A. Gage

  21. CHM 1010 PGCC Barbara A. Gage

  22. Accuracy and Precision • Accuracy = how close a result comes to the true value • Precision = reproducibility of a measurement CHM 1010 PGCC Barbara A. Gage

  23. Precision and Accuracy Consider 3 persons shooting darts at a target… “a” is precise but not accurate “b” is accurate and precise “c” is not precise or accurate CHM 1010 PGCC Barbara A. Gage

  24. Accuracy and Precision • Measurements of the same object made by three students; actual value = 15.71 cm Student 1Student 2Student 3 14.72 cm 15.80 cm 15.72 cm 14.71 14.71 15.71 14.72 13.25 15.82 14.82 14.96 15.73 14.71 12.81 15.71 Precise not accurate Not accurate or precise Accurate and precise CHM 1010 PGCC Barbara A. Gage

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