1. CHEMISTRY
August 2, 2012

2. Place Homework on your desk (2 mins)
CHEMISTRY BRAIN SAMPLER
August 2, 2012
Place Homework on your desk (2 mins)
1. How many sig figs are in
,000
2. What is the SI unit for mass? temperature?
4. Refer to the diagram on the board. Report the measurement?
(include an appropriate unit)

3. Agenda Measurement Lab and Post Lab Brain Teaser Demo Grade Homework
Measurement Worksheet 2
Measurement Lab and Post Lab
Notes:
Accuracy and Precision
Significant Figures Calculations
Homework
Accuracy and Precision Worksheet
Post Lab Questions

4. Accuracy and Precision

5. Data Terms Quantitative Measurements Examples
Give results in a definite form, usually values
24L, 10 cm, 14 ºC 5

6. Data Terms Qualitative Measurements
Examples
Give results in a descriptive, non-numeric form.
The beaker was warm.
The density was greater than that of water. 6

7. Data Terms Accuracy Examples
How close a measurement comes to the actual value of whatever is being measured
Water freezes at 0º C, and boils at 100º C. How close is the measurement to the values. 7

8. Data Terms Precision Examples Reproducibility of the measurement
9 out of 10 lab groups report the temperature of boiling water to be 95º C.
A basketball player shoots 20 free throws, 18 of which bounce off the right side of the rim. 8

9. Accuracy vs. Precision Target Practice
Accurate Precise Accurate & Precise 9

10. An archery target illustrates the difference between accuracy and precision.10

11. Scale Reading and Uncertainty
Uncertainty: Limit of precision of the reading (based on ability to guess the final digit).
Existed in measured quantities versus counted quantities
Addition and substraction(no. of sig. figs)
Division and Multiplication (no. of sig. figs) 11

12. Percent error Theoretical – Experimental x 100 = % error Theoretical12

13. Significant Figures in Calculations

14. Significant Figures in Calculations
What are Significant Digits?
Examples
Triple Beam Balance
Graduated Cylinder
All the certain digits plus the estimated digit in a measurement.
How many decimal places can we count

15. Significant Figures in Calculations
Exact Numbers
Examples
Infinite # of sig figs
Do not affect the number of significant digits in the final answer. They are not measurements!!
1000m = 1 km
12 in = 1 foot

16. Significant Figures in Calculations
Multiplication and Division
Example
The number with the smallest number of significant digits determines how many significant digits are allowed in the final answer.
Volume of a box
L x W x H
(3.05m)(2.10m)(0.75m)
2 sig figs
4.8m3

17. Significant Figures in Calculations
Example
Density of a penny
M = 2.53g V = 0.3mL
D = M / V
# significant figures allowed
D = 8g/mL

18. Significant Figures in Calculations
Addition and Subtraction
Example
The number of significant digits depends on the number with the largest uncertainty. (you may be using different scales)
Shoes g
Clothes g
Ring g
Glasses g
Total g

19. Significant Figures in Calculations
Example
What is the mass of a penny if, the weighing paper alone has a mass 0.67 g and weighing paper plus the penny has a mass of 3.2 g.
3.2 g
-0.67 g
2.5 g

20. Significant Figures in Calculations
Remember
A calculated number can only be as precise as the least precise measurement in the calculation.

21. Practice
Calculate each of the following to the correct number of significant figures. Include units on your answer.
(25 g/mol)(4.0 mol) =
(3.48 in)(1.28 in)(0.010 in) =
2.06 cm cm cm =
If the mass of a lead cube is g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead?

22. Practice
Calculate each of the following to the correct number of significant figures. Include units on your answer.
(25 g/mol)(4.0 mol) =1.0 x 102
(3.48 in)(1.28 in)(0.010 in) = .045 in3
2.06 cm cm cm = 3.9 cm
If the mass of a lead cube is g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead? 11.3 g/cm3
Period 1 Stop Here 22

23. Rally rows Sig figs in Calculations
12 cm cm cm =
( g g) ÷ 8.01 mL=
17.3 cm x 6.2 cm cm2 =
m2  m =
What is the mass of Salt (NaCl) if the sodium has a mass of g and the Cl a mass of 35.5g?