Range, concentration studentized

Scenario A student determined that the optimal wavelength for the absorbance of FeSCN2+ experiment was 445 nm. Then the student prepared samples of known concentrations of FeSCN2+ ranging from 4.0 x 10 5 M to 1.4 x 10 4 M. The samples were then examined by means of a spectrophotometer and their transmittances recorded. From the transmittance, the absorbance was calculated and graphed. Next, he mixed 5.0 mL of 2.0 x 10 3 M Fe(N03)3 with 5.0 mL of 2.0 x 10 3 M KSCN. This solution was then analyzed in the spectrophotometer and through extrapolation, he was able to determine that the concentration of FeSCN2+ at equilibrium was 1.3 x 10" M. 

The intent of the problem is to have students understand the definition of pH and to use the fact that, in water and dilute aqueous solutions and within the ordinary range of room temperatures, the ion product [H30+] x [OH ] of 1 x 10 14 is commonly used as a constant. If you know the concentration of either the hydronium ion or the hydroxide ion, you can calculate the concentration of the other. But apart... 

The estimation of fluoride concentration in human body fluids has been widely performed. However, data are expressed in various units. Kissa [87] reported that the mean plasma fluoride level in healthy subjects is between 20 to 60 pg L-1 (1-3 pmol L 1). Others [88, 89] present L-l plasma fluoride concentrations in the range of 38.8-88.6 mol. The average fluoride concentration in plasma of 8-16-year-old students from Winterthur (Switzerland) was 12.7 3.8 pg L-1 (0.65 pmol L-1) [90]. Human breast milk (samples were collected from 57 lactating mothers) contains 0.019 0.004 ppm of fluoride [91]. 

Fig. 3.12. A Protective effect of (-)-BPAP in the high micromolar concentration range, with a peak effect at 10 6M concentration, against serum-free condition induced cell death in low-cell-density culture of the cerebral cortex from E17 rats. B Lack of a protective effect of (-)-BPAP under the same conditions in the low nanomolar concentration range. Experiments were carried out in triplicate. Data are the mean SEM from six independent experiments. The data were analyzed using Student s f-test after multiple comparisons of ANOVA. P value was < 0.05 compared with the results in the vehicle-treated culture. See Hamabe et al. (2000) for methodology...

Mean baseline PEFR declined by about 2% over a 10-week period in a group of 24 physical therapy students who dissected cadavers for 3-hour periods per week (Kriebel et al. 1993). Estimates of breathing zone fomialdehyde concentrations ranged from 0.49 to 0.93 ppm (geometric mean 0.73 1.22 ppm). PEFR, the only pulmonary function variable measured in this study, was measured before and after each exposure period. Postexposure PEFR means were 1-3% lower than preexposure PEFR means during the first 4 weeks, but this difference was not apparent during the last 6 weeks. Fourteen weeks after the end of the 10-week period, the mean PEFR for the group returned to the preexposure baseline value. 

Formaldehyde-specific IgE was not detected in a group of 45 medical students, before or after the students attended a 4-week anatomy dissecting course (Wantke et al. 1996b). Estimates of laboratory air concentrations of formaldehyde ranged from 0.059 to 0.219 ppm (mean 0.124 0.05 ppm). Surveys revealed frequencies of irritation symptoms consistent with other studies (e.g., itching of the skin in 33/45 students, headache in 15/45, and burning eyes in 13/45). 

Determinate error usually results from experimental equipment which is faulty. Students usually first meet this concept in the analytical chemistry laboratory in determinations of weight and volume. The quality of the equipment used is reflected in the accuracy of the results obtained. Accuracy is a measure of how close the experimental result is to the truth. For example, if one wishes to make up a solution of accurately known concentration in a volumetric flask of 100 mL, both the flask and the balance used must be carefully calibrated. The flask is calibrated by weighing it empty and then filled with distilled water at a known temperature. On the basis of the weight of water, and the known density of the water, one may calculate an accurate volume for the flask when it is properly filled to the mark. Calibration of the balance is based on the use of standard weights which do not corrode and which cover the range in mass used in the experiment. The accuracy of the standard weights and the quality of the volumetric flask determine the accuracy of the concentration of the solution which is made. 

In this equation the confidence interval Xupper,xu,u,er) for a predicted value of jto is calculated. The parameter is the slope of the regression line, t is the Student t statistic, s is the residual mean square error, Sxx is the sum of squares of the X values from the mean of the calibration x values, and n is the total number of calibration samples. The values from these confidence limits place upper and lower bounds for the analyte concentration, jtq. at all points along the calibration range. The best confidence is found near the mean of the data, whereas lower confidence is found at the extremes. 

Unknown hard-water solution. Prepared by dissolving ca. 20 g of dried CaCOs in a minimum volume of 1 M HCl. To this solution is added a few drops of methyl red indicator, followed by dropwise addition of 1 M NaOH until the methyl red end point is noted (red to yellow). The resulting solution is then quantitatively transferred to a 2.000-L volumetric flask and diluted to the mark followed by thorough mixing. Student unknowns may then be dispensed from a buret into individual 100.00-mL volumetric flasks. Typical ahquots range from 10.00 to 20.00 mL, resulting in diluted unknowns with concentrations ranging from ca. 400 to 800 ppm Ca ". ... 

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