1. Chem. 31 – 9/6 Lecture

2. Announcements I Adding Turn in today
If you received an add slip, turn it into the office soon (so I know status of lab sections)
Turn in today
Corrected diagnostic quiz
Quiz 1 – Today (after announcements)
Lab Procedures Quiz – Thursday and next Monday (sect. 5, 6, and 7 are now ahead)

3. Announcements II Websites Today’s Lecture
Text homework solutions posted (class website)
SacCT site is set up (only grading columns so far; but I will post the diagnostic quiz and quiz 1 solutions soon)
Today’s Lecture
Stoichiometry (Chapter 1)
Error and Uncertainty (Chapter 3)
Definitions
Significant figures
Accuracy and precision in measurements

4. Stoichiometry
Stoichiometry refers to ratios between moles of reactants and products in chemical reactions
The ratio of moles of reactants and products is equal to the ratio of their stoichiometric coefficients
Example: aA + bB ↔ cC + dD
Moles A/moles B = a/b

5. Stoichiometry
Example problem: How many moles of H2O2 are needed to completely react with 25 mL of 0.80 M MnO4-?
Reaction:
5H2O2(aq) + 2MnO4- +6H+ ↔ 2Mn2+ + 5O2(g) + 8H2O(l)

6. Stoichiometry
Remember: there are two (common) ways to deliver a known amount (moles) of a reagent:
Mass (using formula weight)
Volume (if molarity is known)

7. Chapter 3 – Error and Uncertainty
Error is the difference between measured value and true value or
error = measured value – true value
Uncertainty
Less precise definition
The range of possible values that, within some probability, includes the true value 7

8. Measures of Uncertainty
Explicit Uncertainty:
Measurement of CO2 in the air: ppmv
The + 3 ppm comes from statistics associated with making multiple measurements (Covered in Chapter 4)
Implicit Uncertainty:
Use of significant figures (399 has a different meaning than 400 and ) 8

9. Significant Figures (review of general chem.)
Two important quantities to know:
Number of significant figures
Place of last significant figure
Example: 13.06
4 significant figures and last place is hundredths
Learn significant figures rules regarding zeros 9

10. Significant Figures - Review
Some Examples (give # of digits and place of last significant digit)
21.0
0.030
320
10.010 10

11. Significant Figures in Mathematical Operations
Addition and Subtraction:
Place of last significant digit is important (NOT number of significant figures)
Place of sum or difference is given by least well known place in numbers being added or subtracted
Example:
= 15.03
= 15
Hundredths place ones place
Least well known 11

12. Significant Figures in Mathematical Operations
Multiplication and Division
Number of sig figs is important
Number of sig figs in Product/quotient is given by the smallest # of sig figs in numbers being multiplied or divided
Example: 3.2 x =
= 520 = 5.2 x 102
2 places 5 places 12

13. Significant Figures in Mathematical Operations
Multi-step Calculations
Follow rules for each step
Keep track of # of and place of last significant digits, but retain more sig figs than needed until final step
Example: (27.31 – 22.4)2.51 = ?
Step 1 (subtraction): (4.91)2.51
Step 2 multiplication = = 12
Note: 4.91 only has 2 sig figs, more digits listed (and used in next step) 13

14. Significant Figures More Rules
Separate rules for logarithms and powers (Covering, know for homework, but not tests)
logarithms: # sig figs in result to the right of decimal point = # sig figs in operand
example: log(107)
Powers: # sig figs in results = # sig figs in operand to the right of decimal point
example: =
= 2.029
107 = operand
3 sig fig
results need 3 sig figs past decimal point
= 2.51 x = 3 x 10-12
1 sig fig past decimal point 14

15. Types of Errors Systematic Errors Random Errors
True Volume
Systematic Errors
Always off in one direction
Examples: using a “stretched” plastic ruler to make length measurements (true length is always greater than measured length); reading buret without moving eye to correct height
Random Errors
Equally likely in any direction
Present in any (continuously varying type) measurement
Examples: 1) fluctuation in readings of a balance with window open, 2) errors in interpolating (reading between markings) buret readings
eye
Meas. Volume

16. Accuracy and Precision
Accuracy is a measure of how close a measured value is to a true value
Precision is a measure of the variability of measured values
Precise, but not accurate
Poor precision (Accuracy also not great)
Precise and Accurate

17. Accuracy and Precision
Accuracy is affected by systematic and random errors
Precision is affected mainly by random errors
Precision is easier to measure

18. Accuracy and Precision
Both imprecise and inaccurate measurements can be improved
Accounting for errors improves inaccurate measurements (if shot is above and right aim low + left)
Averaging improves imprecise measurements
rough ave of imprecise shots
aim here