1. Measurements in Chemistry
The vodcast for this powerpoint is divided in to 3 parts

2. Part 1
Recording Data

3. Types of Observations and Measurements
We make QUALITATIVE observations of reactions — changes in color and physical state.
We also make QUANTITATIVE MEASUREMENTS, which involve numbers.

4. Nature of Measurement
Measurement – quantitative observation consisting of two parts:
Number
Scale (unit)
Examples:
20 grams
6.63 × joule·seconds
1.3
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5. Precision and Accuracy in Measurements
Precision – how closely repeated measurements approach one another
Accuracy – closeness of measurement to “true” (accepted) value
Darts are close together, and are “bullseyes”.
Darts are close together, but they aren’t “bullseyes”.
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

6. Precision and Accuracy in Measurements
In the real world, we never know whether the measurement we make is accurate
We make repeated measurements, and strive for precision
We hope (not always correctly) that good precision implies good accuracy
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

7. Uncertainty in Measurement
In recording measurements, the numbers should be written in a way that reflects the precision of the measuring device.
Significant figures – all known digits, plus the first uncertain (estimated) digit.
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

8. Significant Figures What is the length of the cylinder?
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

9. Significant figures The cylinder is 6.3 cm…plus a little more
The next digit is uncertain; 6.36? 6.37?
We use three significant figures to express the length of the cylinder.
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

10. Measurement of Volume Using a Buret
1.4
Copyright © Houghton Mifflin Company. All rights reserved.

11. Counting Significant Figures
Part 2
Counting Significant Figures

12. Rules for Counting Significant Figures
Nonzero integers always count as significant figures:
3456 g has 4 sig figs
1.5
Copyright © Houghton Mifflin Company. All rights reserved.

13. Counting Significant Figures
Number of Significant Figures
38.15 cm ___
5.6 ft ___
65.6 lb ___
m ___

14. Rules for Counting Significant Figures (continued)
Leading zeros do not count as significant figures:
0.048 g has 2 sig figs
1.5
Copyright © Houghton Mifflin Company. All rights reserved.

15. Leading Zeros Number of Significant Figures 0.008 mm ____
oz ____
lb ____
mL ____

16. Rules for Counting Significant Figures (continued)
Captive zeros always count as significant figures:
16.07 has 4 sig figs
1.5
Copyright © Houghton Mifflin Company. All rights reserved.

17. Captured Zeros 50.8 mm ____ 2001 min ____ 0.702 lb ____
Number of Significant Figures
50.8 mm ____
2001 min ____
0.702 lb ____
m ____ ____

18. Rules for Counting Significant Figures (continued)
Trailing zeros are significant only if the number contains a decimal point:
9.300 m has 4 sig figs
150 m has 2 sig figs
1.5
Copyright © Houghton Mifflin Company. All rights reserved.

19. Trailing Zeros 25,000 in. ____ 200. yr ____ 48,600 gal ____
Number of Significant Figures
25,000 in. ____
200. yr ____
48,600 gal ____
25,005,000 g ____

20. Learning Check A. Which answers contain 3 significant figures?
1) ) ) 4760
B. All the zeros are significant in
1) ) ) x 103
C. 534,675 rounded to 3 significant figures is
1) ) 535, ) 5.35 x 105

21. Learning Check
In which set(s) do both numbers contain the same number of significant figures?
1) and 22.00
2) and 40
3) and 150,000

22. Significant figures are for measurements
Defined and counting numbers do not have uncertainty.
14 people
1000 m = 1 km
7 beakers
These are exact numbers.
They have as many figures as are needed.
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

23. Calculations with Significant Figures
Part 3
Calculations with Significant Figures

24. Significant Numbers in Calculations
A calculated answer cannot be more precise than the measuring tool.
A calculated answer must match the least precise measurement.
Significant figures are needed for final answers from
1) adding or subtracting
2) multiplying or dividing

25. Significant figures in calculated results
Addition and Subtraction
Use the same number of decimal places in the result as the data with the fewest decimal places.
m m – m = ?
= m (calculator)
= m (two decimal places)
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

26. Adding and Subtracting
The answer has the same number of decimal places as the measurement with the fewest decimal places.
one decimal place
two decimal places
26.54
answer one decimal place

27. Learning Check
In each calculation, round the answer to the correct number of significant figures.
A =
1) ) ) 257
B =
1) ) ) 40.7

28. Significant figures in calculated results
Multiplication and division
Use the same number of significant figures in the result as the data with the fewest significant figures.
1.827 m x m = m2 (calculator)
= 1.39 m2 (three sig. fig.)
453.6 g / 21 people = 21.6 g/person (calculator)
= g/person (four sig. fig.)
(Question: why didn’t we round to 22 g/person?)
Chemistry: An Integrated Approach, 3rd ed., Hill & Petrucci. ©2002 Prentice Hall. All rights reserved

29. Learning Check
A X 4.2 =
1) ) )
B ÷ =
1) ) ) 60
C X =
X 0.060
1) ) )

30. The End