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) It is assumed that you can divide the space between any two lines into 10 segments visually. CHM 1010 PGCC Barbara A. Gage

4. Significant figures The graduated cylinder at the right contains 38.34 mL of liquid. The 4 is an estimate and can vary +/- 1 between measurements. 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

5. What is the reading on the 1 st graduated cylinder? On the thermometer? On the last graduated cylinder? CHM 1010 PGCC Barbara A. Gage

6. 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

7. 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

8. Significant Figures 120,000 2 significant figures This is an estimate that can be written as 1.2 x 10 5. 0.00405005 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

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

10. 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. 0.0035704 sig figs X 23.43 sig figs 0.0835385 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

11. 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

12. Units English SystemMetric System Lengthinch, foot, yard, milemeter (or metre) (m) Volumeteaspoon, cup, gallonliter (or litre) (L) Massounce, pound, tongram (g) CHM 1010 PGCC Barbara A. Gage 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.

13. The boxed prefixes must be memorized. Metric Prefixes

14. CHM 1010 PGCC Barbara A. Gage 1 cm 3 = 1 mL

15. 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

16. 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

18. 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

19. 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

20. 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

21. 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

22. 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/cm 3, lb/gal, kg/L Does not depend on the size of the sample CHM 1010 PGCC Barbara A. Gage

23. 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

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

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

27. Accuracy and Precision Measurements of the same object made by three students; actual value = 15.71 cm Student 1Student 2Student 3 14.72 cm15.80 cm15.72 cm 14.7114.7115.71 14.7213.2515.82 14.8214.9615.73 14.7112.8115.71 Precise not accurate Not accurate or precise Accurate and precise CHM 1010 PGCC Barbara A. Gage