CHEM 2480 Lecture Notes - Standard Deviation, Stoichiometry, Concentration

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12 Oct 2013

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Solutions for problem set 2: statistics, error analysis and standard curves. The following 10 results are from experimental measurements for the atomic weight of carbon: Calculate the mean, the average deviation from the mean, and the standard deviation for these results. If so, re-calculate an improved value for the mean and the standard deviation. mean deviation from the mean standard deviation, outlying pt: Qcrit = 0. 41 at 90% confidence level since qexp > qcrit, reject value, (also at 99%)! new mean 12. 0106, new standard deviation 0. 0008. Calculate the value of the function f and its standard deviation for f defined as: a. b. c. d. e. f. (ans. A student standardizes a sodium hydroxide solution by weighing out some potassium biphthalate, khc h o , dissolving it in water, and titrating it with the sodium hydroxide solution. 4 weight of empty beaker = 50. 8392 0. 0002 g weight of beaker + khc h o = 51. 0009 0. 0002 g.

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Q2. The homogeneity of the chloride level in a water sample from a lake was tested by analysing portions from drawn from the top and near the bottom of the lake. The results are given below, in ppm Cl^-:

Top		Bottom
26.30		26.22
26.43		26.32
26.19		26.11
26.49		26.42
26.28		26.20

Calculate (manually) the mean and standard deviation for samples from the top of the lake, and for samples from the bottom of the lake. You may make tables with columns for deviations and squared deviations.

Should the value 26.19 ppm be rejected at 95% confidence?State the null hypothesis, show calculation of the experimental statistic, and show the choice of critical statistic.

What is the 90% confidence interval for [Cl^-] at the bottom of the lake?

Find the pooled standard deviation for the chloride analyses in ppm.

Determine if there is a systematic difference between the mean values of the lake-water (top versus bottom) at the 90% confidence level. State the null hypothesis, show the calculation of the test value, and how you chose your statistical value.

Is there homogeneity at the 99% confidence level? Show working with the statistical test.

Dr. Caras was characterizing a cytochrome (an iron containing protein) from a new strain of bacteria. He obtained the following values in eight measurements of the percent iron in the protein: 0.2306%, 0.2301%, 0.2298%, 0.2294%, 0.2417%, 0.2298%, 0.2302%, 0.2294%

Calculate the mean (x) and standard deviation (s) for these results.

Use the Grubbs test to determine if one of these values is an outlier. What is the value of Gcalculated? Should this potential outlier be rejected with 95% confidence? Critical values of G can be found in the following table.

CHEM 2480 Lecture Notes - Standard Deviation, Stoichiometry, Concentration

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