Unit 2 Scientific Measurement

1. Unit 2Scientific Measurement And Problem Solving in Chemistry Precision and Accuracy Significant Figures % error Measurement Density

2. Accuracy vs Precision Accuracy: All the data represents the true measurement. When a measurement is accurate It is very close to the accepted value. Generally, you want to be within 5% or less of the accepted value.

3. Accuracy vs Precision Precision: All the data is very Similar to the average value. The data is very reproducible. It is not necessarily accurate. The measuring device could be consistent but not accurate. The measurements will have been taken under the same conditions.

4. To be both Accurate and Precise The data must be consistent AND near the Accepted value.

5. Precise, Accurate, or both? Water level Rf = .3 Rf = .5 Rf = .8 These are both accurate and precise Trial 1 Arrow # 1 = 0.4 cm Arrow #2 = 0.6 cm Arrow #3 = 1.0 cm Arrow #4 = 1.2 cm Trial 1 Rf1 = 0.4/1.2 = 0.3 Rf2 = 0.6/1.2 = 0.5 Rf3 = 1.0/1.2 =0.83 Trial 2 Rf1 = 0.3/1.1 = 0.3 Rf2 = 0.5/1.1 = 0.5 Rf3 = 0.9/1.1 =0.8 Trial 2 Arrow # 1 = 0.3 cm Arrow #2 = 0.5 cm Arrow #3 = 0.9 cm Arrow #4 = 1.1 cm

6. Precise, Accurate, or both? Water level Rf = .3 Rf = .8 Rf = .5 These are only precise Trial 1 Arrow # 1 = 0.6 cm Arrow #2 = 0.8 cm Arrow #3 = 1.1 cm Arrow #4 = 1.2 cm Trial 1 Rf1 = 0.6/1.2 = 0.5 Rf2 = 0.8/1.2 = 0.7 Rf3 = 1.1/1.2 =0.92 Trial 2 Rf1 = 0.6/1.1 = 0.5 Rf2 = 0.8/1.1 = 0.7 Rf3 = 1.0/1.1 =0.91 Trial 2 Arrow # 1 = 0.6cm Arrow #2 = 0.8cm Arrow #3 = 0.9 cm Arrow #4 = 1.1 cm

7. % error – instead of guessing if data is precise or accurate, you can use statistics. • Error = Accepted Value – experimental value. • In order to use % error, you must be using a substance for which there IS an accepted value. In one of our next chemistry lab, we will be using Appendix A1 to compare our values of density of elements to your experimental values. % error = x 100% Accepted Value – experimental value. Error Accepted Value Accepted Value

8. Significant Figures • Non Zero Numbers are significant • Zeros in between significant digits are significant • Place-holder zeros are NOT significant. All other zeros are. • Scientific notation is used if a zero which SHOULD be a place-holder is actually a measured value. • 1.234 = 4 sig figs • 1.01 = 3 sig figs • 10004 = 5 sig figs • 0.0001 = 1 sig fig • 10000 = 1 sig fig • 101000 = 3 sig figs • 1.0 x 108 = 2 sig figs

9. Scientific Notation 1 x 100 1 x 101 1 x 102 1 x 103 1 x 104 1 x 105 1 x 106 1 x 107 1 10 100 1 000 10 000 100 000 1 000 000 10 000 000 0.1 0.01 0.001 0.000 1 0.000 01 0.000 001 0.000 000 1 0.000 000 01 1 x 10-1 1 x 10-2 1 x 10-3 1 x 10-4 1 x 10-5 1 x 10-6 1 x 10-7 1 x 10-8

10. Test Yourself… • 9009.00 • 3000 • 0.000404 • 564 • 0.0100 • 0.000434 • 4540000 • 9.400 x 1010 • 1.90 x 10-4 9009.00 = 6 sig figs 3000 = 1 sig fig 0.000404= 3 sig figs 564 = 3 sig figs 0.0100 = 3 sig figs 0.000434 = 3 sig figs 4540000 = 3 sig figs 9.400 x 1010 = 4 sig figs 1.90 x 10-4 = 3 sig figs

11. Test Yourself….Round each number to two sig figs. • 9009.00 • 3000 • 0.000404 • 564 • 0.0100 • 0.000434 • 4540000 • 9.400 x 1010 • 1.90 x 10-4 9.0 x 103 3.0 x 103 4.0x 10-4 5.6 x 102 1.0x 10-2 4.3x 10-4 4.5x 106 9.4 x 1010 1.9 x 10-4

12. Find the density of a metal • Find the mass using a balance • Find the volume either using rulers or using measurement by difference. • Density = mass/volume • Look up the accepted value for density. % error = 100% x Accepted – Experimental Accepted