Chem 2113 Test 4 Fall 2000

Questions are worth 25 points each. OMIT ONE QUESTION by clearly writing OMIT in the space provided for your work. If you fail to mark OMIT on a question I will omit the last question of the test. Show your work and circle your answers for full credit.

1. The following data has been partially analyzed for the determination of gallium based on the fluorescence of a sample of the chelate formed by gallium and morin. The concentration of gallium for the standards was measured in ppm. Five standards were made, with the following values calculated:

x = 85.0; y = 327.0; x2 = 3025; y2 = 45078.62; xy = 11676; n = 5

Calculate the slope, y-intercept, and correlation coefficient for the data.

Calculate the standard deviation about the regression (sr), the standard deviation of the slope (sm), and the standard deviation of the intercept (sb).

The fluorescence of an unknown sample, determined from three replicates, is 102.7. Calculate the concentration of the unknown in ppm and the standard deviation for this result.

2. a) A potentiometric cell is used to determine the concentration of silver in a sample. The potential of a solution that is 2.50 x 10-4 M is measured as 0.345V using the cell:

SCE||Ag+(?)|Ag

What is the concentration of the silver ion if the cell potential is determined to be 0.256V when an unknown solution is added to the silver cell? What is the junction potential for this cell? [For the SCE, E = 0.244V; for Ag+ + e- Ag, E = 0.799V]

b) Calculate Ksp for AgSCN given the following cell has a potential of 0.043V. (Assume the junction potential is equal to 0 for this cell.)SCE|| SCN- (5.0 x 10-4M), AgSCN (saturated) | Ag

3. Calibration curve data was obtained to determine the concentration of sulfate using a sulfate specific-ion electrode.

[SO42-] Potential, mV
1.0 x 10-6 -274
1.0 x 10-5 -213
1.0 x 10-4 -156
1.0 x 10-3 -101

Using the above information, calculate the following values for the linearly related data. [HINT: Do you have to manipulate any of these data?]

x = ____________ y = _____________ x2 = ____________

y2 = ____________ xy = ____________ n = ______________

Sxx = ____________ Syy = ____________ Sxy = ____________

m = ____________ b = ____________ r = ____________

What is the percent Na2SO4 (MW = 142.043) in an unknown if 0.705 g of unknown is dissolved in 250.0 mL of water, with a 15.00 mL aliquot of this solution being drawn off and diluted to 1.000L. The potential of the 'second' solution was -138 mV.

4. One of the many tests that has to be done to wastewater is a test for mercury. The standard method for this determination is 'cold vapor', in which a sample is placed under a vacuum, and the amount of Hg is determined by measurement at 213 nm. If the mercury content is greater than 0.002 ppm, the company is out of compliance, and subject to a fine of $1700 for the first offense. The following data was obtained for a determination of wastewater from the RIDCO manufacturing plant in Rapid City, SD.



Sample		 mL

wastewater

 mL 0.010 ppm

standard

 Final

Volume



A
1 25.00 0.00 50.00 0.251
2 25.00 10.00 50.00 0.507

Based on these data, should RIDCO be fined $1,700 since they have not had a violation before this date?

5. Two components (A and B) are found to be present in a mixture. When the mixture was measured at 426 nm, it was found to have 20.6%T, while at 560 nm the same mixture was measured as 19.8%T. At 426 nm, A = 15,800, while B = 1,200. For the same species at 560 nm, A = 650, while B = 4,550. What is the concentration of A and B in the mixture? Assume all measurements were made in a 1.00 cm cell.

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