CHM 103 NAME ______

GAGE

FALL 2007 October 19, 2007

EXAM I

Show all calculations to receive credit for an answer. Be sure to indicate the final answer in the calculations. Graph paper is available if needed.

Concentration (M) / Absorbance
0 / 0
0.00200 / 0.375
0.00320 / 0.462
0.00450 / 0.855
0.00450 / 0.954
0.00680 / 1.043

1.  A red food dye has a λmax of 525 nm. The data for a series of student-generated standards are listed below. The samples were measured in a 1.20 cm cell.

a.  Determine the average molar absorptivity coefficient. (12)

b.  Calculate the concentration of a food dye sample with an absorbance of 0.720. (12)

c. Someone brings you a red food dye sample for analysis but the absorbance is so low that its reading is unreliable. What can you easily do to improve the absorbance reading so that it will be more reliable? Explain the basis for your process. (10)

2.  You have just done a back-titration of “Burp-eeze”, an antacid with aluminum hydroxide as the active ingredient. You record the following data and results in the process:

Molarity of HCl = 0.785 M Molarity of NaOH = 0.105 M

Mass of tablet 750 mg

Active Ingredient/tablet 500 mg

HCl added 25.00 mL

NaOH titrated 10.35 mL

a.  Calculate the % antacid in the tablet based on your analysis and the % error. Be sure to show all supporting calculations well labeled. (20)

b.  You needed to make 500.0 mL of the above NaOH solution. You have a stock solution that is 0.680M NaOH. Explain exactly how to make the solution. (10)

3.  Below is data (in mg) for three students’ analyses of the amount of magnesium hydroxide in Phillips antacid tablets. The manufacturer states on the label that there are 311 mg of magnesium hydroxide in each tablet.

Susie Q / Mr. T / Queen B
294 mg / 288 mg / 302 mg
325 / 286 / 312
369 / 290 / 318
338 / 291 / 314

a.  Who has the most accurate results? Justify your answer with appropriate statistics. (12)

b.  Which student is most precise? Justify your answer with appropriate statistics or graphically. (12)

c.  Do the results of any students show determinate error? Justify your answer. (6)

Whose results show indeterminate error and why? (6)

3