Chapter 2 Measurements and Calculations Or It all adds up!

Accuracy and Precision Accuracy is how close your results are to the accepted value. If the accepted value is 12, then is accurate.

Accuracy and Precision Precision is how close the number are to each other. 7.99, 8.01 and 8.00 are precise. But what if the accepted value is 12?

Accuracy and Precision Section 3 Using Scientific Measurements Chapter 2

Remember !! Precision Bombing is not necessarily Accurate.

Error Error = accepted value – experimental value. Or: What you want – what you got. %Error = (absolute value of error/accepted value)x100 OR [(What you want – what you got) divided by what you want] times 100

Accuracy and Precision, continued Sample Problem C A student measures the mass and volume of a substance and calculates its density as 1.40 g/mL. The correct, or accepted, value of the density is 1.30 g/mL. What is the percentage error of the student’s measurement? Section 3 Using Scientific Measurements Chapter 2

Accuracy and Precision, continued Error in Measurement Some error or uncertainty always exists in any measurement. skill of the measurer conditions of measurement measuring instruments Section 3 Using Scientific Measurements Chapter 2

Accuracy and Precision, continued Sample Problem C Solution Section 3 Using Scientific Measurements Chapter 2

Significant Figures Significant figures in a measurement consist of all the digits known with certainty plus one final digit, which is somewhat uncertain or is estimated. The term significant does not mean certain. Section 3 Using Scientific Measurements Chapter 2

Reporting Measurements Using Significant Figures Section 3 Using Scientific Measurements Chapter 2

Significant Figures Every nonzero reported is a significant figure. Example: How many significant figures? 7 sig figs

Significant Figures Zeros between sig figs are significant. Example: How many sig figs? 5

Significant Figures Leftmost zeros appearing in front of nonzero digits are not significant. They are placeholders. Example: How many sig figs? 2

Significant Figures Zeros at the end of a number and to the right of a decimal place are always significant. (the money rule) 4.00, how many sig figs? 3

Significant Figures Zeros at the rightmost end of a measurement that lie to the left of an understood decimal are not significant. They are placeholders. 560,000 sig figs? 2

Significant Figures In an exact count, there are an unlimited number of significant figures. Example: 60 minutes in an hour. 2 sig figs

Significant Figures, continued Determining the Number of Significant Figures Section 3 Using Scientific Measurements Chapter 2

Significant Figures, continued Sample Problem D How many significant figures are in each of the following measurements? a g b cm c. 910 m d L e kg Section 3 Using Scientific Measurements Chapter 2

Significant Figures, continued Rounding Section 3 Using Scientific Measurements Chapter 2