Chemistry 2410: Problem Sheet 2
1. A chemist obtained the following data for the alcohol content of a sample of blood.
% C
2
H
5
OH = 0.084, 0.089, and 0.079.
Calculate the 95% confidence limits for the mean assuming no additional knowledge about the precision of the method is available.
2. The mercury in samples of seven fish taken from the Oldman River was determined by a method based upon the absorption of radiation by gaseous elemental mercury. Calculate a pooled standard deviation for the method based on the data supplied.
Fish Specimen
Number of
Samples Measured Hg Content ppm
3
4
1
2
2
6
3
4
1.80, 1.58, 1.64
0.96, 0.98, 1.02, 1.10
3.13, 3.35
2.06, 1.93, 2.12, 2.16, 1.89, 1.95
5
6
4
5
0.57, 0.58, 0.64, 0.49
2.35, 2.44, 2.70, 2.48, 2.44
7 __4 _ 1.11, 1.15, 1.22, 1.04
28 measurements
3. The analysis of a calcite sample yielded CaO percentages of;
55.95, 56.00, 56.04, 56.08 and 56.23 % CaO
The last value appears anomalous; should it be retained or rejected?
4. Hair found at the scene of a crime is analyzed and compared with hair belonging to a suspect. Using atomic absorption analysis, a technique known to have a true (or population) standard deviation of 12.00 ppm zinc, the following results were obtained.
Hair at crime:
Hair from suspect:
Using the method of comparison of means can you deduce that the suspect is guilty of the crime?
196, 204 ppm Zn
193, 188, 195, 199 ppm Zn

5. As the instructor of an analytical chemistry course, you have just prepared a new batch of calcium samples. If your lab technique is such that your standard deviation is 0.12% CaO and you wish to be at least 95% confident that the true value is within 0.04% CaO of your analyzed value of 41.37% CaO, how many trials should you run?
6. A volumetric flask manufacturer offers a money back guarantee that her 50.000 mL volumetric flasks are accurate to ± 0.006 mL. For a random sample of 500 50.000 mL flasks the mean volume was determined to be 50.002 mL with a standard deviation of 0.002 mL. Assuming that the volume distribution of these flasks is well represented by a Gaussian curve, what fraction of flasks fall out side of the guarantee range?
7. Samples of limestone give the following analytical results for % CaO: 43.41, 42.16, 45.37 and
45.16. One of the samples was then ground to a fine powder and carefully mixed. Replicate values for this sample were: 43.17, 42.88, 43.26 and 43.42 % CaO. Determine the standard deviation associated with the sample heterogeneity and the standard deviation associated with the analytical procedure.