Chemistry 3.1 Uncertainty in Measurements

I. Accuracy, Precision, & Error A. Accuracy – how close a measurement comes to the “true value”. 1. Ex: Throwing Darts true value = bull's-eye

B. Precision – how close a series of measurements are together. 1. Ex: Throwing Darts

C. Pg. 64 Explanation Poor Accuracy Good Precision Good Accuracy Good Precision Poor Accuracy Poor Precision

D. Error – the difference between the accepted value and the experimental value. 1. Formula – 2. Error = ex. value – accepted value | error | accepted value

E. Examples 1. In class you determine the melting point of salt is 755 deg C. The actual value is 805 deg C. What is your percent error?  [| | / 805] x 100 =  6.2% error

II. Significant Figures A. Def – all digits known plus one estimated one. 1. Measurements must be recorded with significant figures.

2. Rules (pg.66) -All other numbers are significant -zeros may or may not be significant -leading zeros are not significant  (sig fig) -captive zeros are significant  (sig figs) -trailing zeros following the decimal point are significant  ? (sig figs)  200 ? (sig figs)  200.0? (sig figs) 4 1 4

3. Rounding with Sig Figs -Express the following #’s to 3 sig figs  = 422,000  =  1 = 1.00  8222 = 8,220  0.42 =.420

4. Scientific Notation + Sig figs A. All #’s in scientific notation are counted as significant figures. B. Ex: x 10 3 = sig figs 2.77 x 10 6 = sig figs 5 3

5. Adding and subtracted A. The answer must not contain any sig figs beyond the place value common to all #’s B. Ex: (not 6.815)

6. Multiplication and Division A. The answer must not contain more sig figs than the least # of sig figs. B. Ex:3.1 x (not )

In Class Problems 1. How many sig figs? -123 meters-30.0 meters -40,506 kg x 10 3 kg and x and 10.7 / x x 10 4