1. Liquid chromatography was used to measure the concentration of caffeine in coffee. The caffeine standards used to make the calibration curve ranged from 2 ppm to 20 ppm. Analysis of the coffee sample using the curve yielded a concentration of 32 ppm.
a. Why would you not be able to use this concentration in your report?
b. What can be done in this circumstance to ensure the method and reported value is valid?
2. Why is it desirable to use a small volume of concentrated standard rather than a large volume of diluted standard in the method of standard addition?
So that the matrix is similar or nearly the same for all solutions measured.
3. Explain the difference between systematic and random error. Give three examples for sources of systematic error.
Systematic errors in experimental observations usually come from the measuring instruments.
Systematic error is a series of errors in accuracy that are consistent in a certain direction and they usually come from the measuring instruments, while random errors are those which are caused by random and unpredictable variation in an experiment.
Examples of causes of systematic errors
• Faulty Instruments (Instrumental) - there is something wrong with the instrument or its data handling system.
• Observational - Wrong use or handling of the instruments by the experimenter or when the observer records a wrong reading
• Environmental - When problems in the lab surrounding lead to inaccurate results..
4. The carbohydrate content of a glycoprotein is found to be 12.6, 11.9, 13.0, 12.7, and 12.5 wt% in replicate analyses. Find the 90% and 99% confidence intervals for the carbohydrate content.
5. A standard reference material is certified to contain 51.1 ppm bismuth. Your analysis gives values of 53.1, 52.9, 53.7, 50.0, and 54.2 ppm. Do your results differ from the expected result at the 95% confidence level? (4 marks)
6. The concentration of HCl in a solution was measured by titrating with different indicators to find the endpoint.
(a) Is the difference between indicators 1 and 2 significant at the 95% confidence interval? (3 marks)
(b) What about the difference between indicators 2 and 3? (4 marks)
Hint: using 40 degrees of freedom for each →Ftable = 2.2 and ttable= 2.02
|
Indicator
|
Mean HCl concentration (M)
(± standard deviation)
|
Number of Measurements
|
|
1. Bromothymol blue
|
0.09565 ± 0.00225
|
28
|
|
2. Methyl red
|
0.08686 ± 0.00098
|
18
|
|
3. Bromocresol green
|
0.08641 ± 0.00113
|
29
|