Percentages and Percent Error

1. Percentages andPercent Error

2. Calculating Percents • Puts everything on a basis of 100. • Percent (%) = # of items of interest x 100 total number of items

3. Sample Calculation • Our class contains 33 students. Of these, 18 are men and 15 are women. • What is the percentage of women in the class? • What is the percentage of men in the class?

4. Calculations • % men = 18 x 100 = 54.5454545455 33 • % women = 15 x 100 = 45.45454545 33

5. Percents Help Us Compare • You are offered two samples of gold. • First weighs 57 g and contains 43% Au. • Second weighs 39 g and contains 66% Au • Which would you rather have?

6. 57 g @ 43% Au • 57 g sample x 43 g Au . = 25 g 100 g sample

7. 39 g @ 66% Au • 39 g sample x . 66 g Au = 26 g Au 100 g sample

8. Choice of units is arbitrary • 77% = . 77 oz Au . = . 77 g Au . 100 oz sample 100 g sample

9. % as a Conversion Factor • Try Problem 3.106 in text • Try Problem 3.108 in text

10. Percent Error • % er = measured error - accepted value x 100 accepted value

11. Try It! • Sue measured the density of Pb as 11.26. The generally accepted value is 11.34. What was Sue’s percent error?

12. Assignment • Read 3.10 • Do Assigned Problems ONLY in the 3.97-3.112 range