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Hill College - SCIENCE 1406 Chapter 4 : Making Decisions with Data 1)Some people prefer the taste of cane sugar (sucrose: C12H22O11; mm = 342

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Hill College - SCIENCE 1406

Chapter 4 : Making Decisions with Data

1)Some people prefer the taste of cane sugar (sucrose: C₁₂H₂₂O₁₁; mm = 342.2 g/mol) over high fructose corn syrup in their soft drinks.  A particular brand of soft drink contains 0.11 moles +/- 0.02 moles of sucrose per 0.355 L +/- 0.002 L of solution.  Using the method of error propagation, report the molarity, with the absolute uncertainty, of sugar in the drink.

A fraction will have the largest value when the numerator is large and the denominator is small = (0.11 + 0 .02) / (0.355 - 0.002) = 0.13 / 0.353 = 0.3683. 

 

2.  Molar masses should be reported with their uncertainty.  For example, dilute solutions of hydrogen peroxide are often used as a disinfectant.  Using this data from a periodic table data provided and the uncertainties reported what would you report as the molar mass and uncertainty of H₂O₂?

H atomic mass = 1.007 94  +/- 7

O atomic mass = 15.999 4  +/- 3

                                              

 

 

 

3.  For each of the following lab situations involving measurements, decide if the problem is most likely a random error or systematic error (Fill in the blank with “R” (random) or “S”(systematic).

(A) Three Analytical Chemistry students use the same HCl solution to titrate identical samples of the same bleach solution, but one requires 28.45 mL to neutralize, the two others use 28.55 mL and 28.35 mL    _____

(B) A bleach label states that it is a 5.0% hypochlorite solution.  Fifteen students titrating, with the same HCl standard solution, report average values from 4.8% to 4.9% hypochlorite. ______                         

(C) Which type of error in your measurement is impossible to eliminate?              

(D) Which type of error tends to be all too high or all too low?                                         

(E) Which type of error is most likely eliminated by re-calibrating a balance?         

 

 

 

 

4. If an initial volume reading of 0.05 mL +/- 0.02 mL was recorded in a buret and a final reading of 15.45 +/- 0.02 mL was made after a liquid was delivered, what would be reported as the total volume including the uncertainty, delivered?  *Use ALL considerations for the correct significant digits and uncertainty in the answer.

 

 

5. Chocolate contains the caffeine-like stimulant theobromine.  A chemist working with a chocolate company performed five theobromine analytical tests on a chocolate sample.  The results of five determinations were found as follows:

  Trial 1 = 1.3%

  Trial 2 = 1.2%

  Trial 3 = 1.0%

  Trial 4 = 0.98%

  Trial 5 = 1.4%

(A) If the mean of the results is 1.2% what is the standard deviation? =

(B) What is the relative standard deviation?

(C) What is the confidence interval (at the 95% level) that can be reported for the trials?

 

 

 

6. Suppose another analysis of theobromine was done on the same chocolate.  However, this time a different solvent was used in the extraction procedure.  This analysis was performed three times obtaining an average value of 1.4% with a standard deviation of 0.25.  At the 95% confidence level, can the this mean and the 1.2% from question #5 means be considered statistically close enough to be the same?

 

 

7. A university Plant Science department has a project involving genetics and the amounts of amylose (unbranched starch) and amylopectin (branched starch) in peas.  (This could prove to be important for diabetics.) 

(A) The starch content of one genetic variety of pea was analyzed and found to be the following.  What is the standard deviation of the following data?

 

 

 

(B) What could you claim would be true if you increased the number of peas analyzed?

            (i) increasing number of peas would increase the accuracy

            (ii) increasing number of peas likely decreases the standard deviation

            (iii) increasing number of peas likely increases the standard deviation

(iv) increasing number of peas eliminates systematic errors

(v) increasing number of peas likely increases the mean

(C) Another pea analysis, using five pea samples, revealed an average amylose as 242 g/kg with a standard deviation of 12. Using the F-test, would you consider these two standard deviations to be significantly different (at the 95% confidence level)?

 

 

 

9. Two sets of chromium analyses were performed on water samples from a stream that runs near a chromium plating plant (they make those shiny bumpers).

             Before the spill                                              After the spill

mean:              0.95 ppm Cr                                        1.10 ppm

n                         5                                                           6

s                       0.05 ppm                                             0.08 ppm

 

 Could it be stated, at the 95% confidence level, that after a spill there was an increase in the chromium level in the stream? 

 

10. (A) t_calc = |79-81|/(standard deviation of the mean);