1. Chemistry 381 Advanced Module in Physical Chemistry Objectives:
Learn how to do experiments
Learn how to interpret experimental data
Learn how to report experimental data

3. Coincidence mass spectrum K +(CH3NO2, CH3Br, air
oriented + random O-
random
Mass2

4. CH3NO2 & CH3Br
NO2- e-
O2- ??
CH3NO2-
Br- O-

5. Oriented + random
random X X

6. S/N 28 65
NO2- e- 17
O2- No!
CH3NO2- 18
Br- 48 4 O-

7. Errors
Always present. At best we reduce to a tolerable level
How to reduce?
Improve methods
Calibrate equipment
Alternate methods
Replicate runs
Types of errors: Precision vs Accuracy
Precision: how reproducible
Accuracy: how close to right answer

8. Accuracy vs Precision True value Accuracy & Precision
Accuracy No Precision
Precision No Accuracy
Neither
True value

9. Determinate (Systematic)--Definite value. Accuracy
Kinds of errors
Determinate (Systematic)--Definite value. Accuracy
Can be measured & corrected
Personal
Ignorance, carelessness
Improper technique
Arithmetic
Prejudices--trying to get “right” answer
Physical limits
Instrumental
Improper calibration
Procedure
These can be corrected for
Nonideality
Equilibrium not attained
Side reactions
Indeterminate (Random) can’t be controlled. Tend to average out.

10. Treatment of Systematic Errors
A. Write down all possibilities for error
Be imaginative. It’s easier to throw them out that to dream them up.
B. Estimate the accuracy with which these are known.
C. Using the chain rule of differentiation, evaluate the effect of each of these on the final result
D. Retain those sources of error which are significant (and not stupid.)

11. Propagation of systematic errors
Does not average out
Measure X with error X
How does it affect final result, R?
a) Addition & subtraction
R=(X1+ X1) + (X2+ X1)
R=(X1+ X2) + (X1+ X2)
RX1+X2

12. b) Multiplication & Division
Now divide by R
Fractional errors add & subtract

13. Propagation of random errors
These have random sign
How does it affect final result, R?
Magnitude difficult mathematically, so square each term
a) Addition & subtraction
R=x-y
If x≈y, R≈x2

14. b) Multiplication & Division
Random errors

15. See Shoemaker, et al, Chapter 1 & 2
Reports
See Shoemaker, et al, Chapter 1 & 2
Reports must be typed (except for data & equations)
Introduction
Legibility
II. Experimental
Clarity
III Results
Brevity
Accuracy
IV Discussion
Proper English & spelling
Meaning
Accuracy (including error discussion