EQUATIONS AND CONSTANTS
A=abc   A = -log T   Kw = 1.01 x 10-14 = KaKb
Ecell = Ered - Eox = Eright - Eleft    Eeqv, soln = (n1E1° + n2E2°)/(n1 + n2) 
G = -nFE   G° = -RT ln Keq   E° = (0.05916/n) log Keq 
E = E° - (RT/nF) ln Q = E° - (0.05916/n) log Q     Esce =0.241 V    EAg/AgCl = 0.197 V 
A = coul/s    J = coul·V)    F = 96487 coul/mole e-   R = 8.314 J/mole·K 

GENERAL DIRECTIONS
Do the following problems on the paper provided. Use only ONE SIDE of each page.
SHOW YOUR WORK. NO WORK, NO CREDIT.
LABELS AND PROPER NUMBER OF SIG FIGS.
Up to 80% of the credit for a problem will be given for correctly setting it up,
including labels on all numbers.

Chemistry 242 Exam 1 March 3, 1999

• (10 pts) How many milliliters of 0.100 M KI are needed to react with 40.00 mL of 0.4000 M Hg₂(NO₃)₂ if the product of the reaction is Hg₂I₂ (s)?
• (5 pts) 500 mL of water is saturated with zinc carbonate, an insoluble salt. If sodium carbonate, a soluble salt, is added to this solution, will the concentration of zinc ion increase, decrease, or stay the same? Explain your reasoning.
• (15 pts) Nickel hydroxide is an insoluble salt with K_sp = 6 x 10 ^-16. What is the nickel concentration in parts per billion in a saturated solution of nickel hydroxide that is also 5.17 x 10 ^-5 M sodium hydroxide? Sodium hydroxide is completely soluble in water.
• (15 pts) An unknown amount of a compound with a molecular weight of 292.16 g/mole was dissolved in 5.00 mL of water in a volumetric flask. A 1.00 mL aliquot of this solution was pipetted into a 10.00 mL volumetric flask and diluted to the mark with water. The absorbance of this solution at 340 nm in a 1.000 cm cuvet was 0.427. The molar absorptivity for this compound at 340 nm is ₃₄₀ = 6100 M ^-1 cm ^-1. How many milligrams of the compound were used to make the original solution?

Work on your own, not in groups. Use any books, notes, calculators, and/or computers. You may consult with the instructor for clarification, hints or help in using the computers, but I may not answer all your questions. Show your work in a logical sequence so I can follow it. Write down enough steps so I can see what you did and locate where you may have made errors. You don't have to use a computer for these problems, but if you do: If you use QuattroPro, you may turn in the disk: document what you did in the spreadsheet, indicate which file(s) to look at, also turn in any hard copy requested in the problem. If you use other spreadsheets, you may turn in a hard copy of your results: document what you did in the spreadsheet, reference the program you used, print out or somehow show the formulas that you used as you entered them in the spreadsheet so I can follow your calculations.

The problems are stated as you might encounter them in the "real world." That means that all the steps needed to arrive at the final answer may not be explicitly stated. Consider all the pertinent aspects of statistical analysis for each problem and don't forget units and significant figures.

• (15 pts) A student was sent into the lab to standardize a sodium hydroxide solution by titrating it with KHP. The student ran seven titrations which gave concentrations of: 0.1429 M, 0.1428 M, 0.1422 M, 0.1426 M, 0.1428 M, 0.1427 M, and 0.1429 M. What are the absolute AND relative 90% confidence intervals for the NaOH concentration?
• The Sampling Constant for the analysis of protein content in a semi load of wheat is 10 kg (2 sig figs).
• (5 pts) In the lab the maximum sample size that can be analyzed is 300 g (3 sig fig). What is the sampling percent relative standard deviation expected for this 300 g sample size?
• (10 pts) To procure the best price for the wheat, the grain elevator owner must have 95% confidence that the uncertainty on the protein analysis is ±5% (relative) or less. How many 300 g samples must the lab run on each truckload of wheat to provide this level of uncertainty?
• The Gibbs Free Energy (G) is a function of the enthalpy (H) and the entropy (S)

Using the data from 0°C to 100°C for the autoionization of water in Table 6-1 (p129) in your text and equation 4:

• (5 pts) Calculate the regression line equation, then graph the data and the linear regression line. Turn in a hard copy of the graph and the spreadsheet.
• (10 pts) Calculate the absolute 95% confidence interval for the H for the autoionization reaction of water.
• (10 pts) Calculate the relative 95% confidence interval (express as a percent) for the S of that reaction.

Chemistry 242 Exam 2 April 9, 1999

• (10 pts each) If the following compounds are dissolved in water (everything dissolves at least a little bit), would you expect the pH of the solution to be higher, lower, or the same as pure water? Clearly explain your reasoning and include reactions where appropriate.
• BaCl₂
• SO₂
• (15 pts) What is the pH of 1.00 x 10 ^-4 M strontium hydroxide?
• (10 pts) What acid/base pair from the attached table would you choose to prepare a buffer of pH 2.00? Explain your reasoning.
• (15 pts) What is the pH of 0.10 M HF?
• 25 mL of 0.10 M ammonia is titrated with 0.100 M hydrobromic acid.
• (15 pts) What is the pH of the solution after 20 mL of the hydrobromic acid is added?
• (5 pts) What indicator from the attached table would you choose for this titration? Explain your reasoning.
• (20 pts) Set up the system of mathematical equations (not just reactions) needed to solve the problem below without making assumptions. DO NOT SOLVE, JUST SET UP THE EQUATIONS.
• What is the pH of 0.0500 M lithium sulfide?

• (5 pts) Identify a compound or ion that will oxidize Ce⁺³ but not Bi⁺³ under standard conditions. Briefly explain your reasoning.
• (5 pts) Which is the strongest reducing agent: Hg⁺², Na⁺, or Ni⁰? Explain your reasoning.
• Copper dissolves in nitric acid according to the following reaction:

• (15 pts) Balance the reaction assuming acid conditions.
• (10 pts) What is the equilibrium constant for this reaction?
• (20 pts) Calculate E for the following half cell:

• (25 pts) The cell below produces -0.714 V. What is the K_sp of nickel hydroxide? (NHE = normal hydrogen electrode = 0V)

• (20 pts) One section of a bumper factory applies the chromium plating. The plating vats in this factory contain 3.0 M Cr₂(SO₄)₃. Assuming the plating operates at 100% current efficiency (all electrons are used to reduce Cr, no side reactions), what mass of Cr is plated in 1.000 hour at a constant current of 3.58 kA?

• (3 pts) Which of the above compounds will be insoluble in water?
• (3 pts) Which of the above compounds will be strong acids in water?
• (3 pts) Which of the above compounds will be weak bases in water?
• (10 pts) Glass is usually considered to be an insulator. How then does the glass membrane of a pH electrode generate a voltage and why is that voltage related to the pH of the solution?
• 20.00 mL of 0.200 M methylamine is titrated with 0.1000 M hydrochloric acid.
• (10 pts) What will the pH be at the equivalence point of this titration?
• (5 pts) What indicator from the list at the bottom of the K_a table would work the best for this titration? Explain your reasoning.
• (20 pts) 25.0 mL of 0.0250 M Sn⁺² is titrated with 0.1000 M Ce⁺⁴. The titration is followed potentiometrically using the following cell:

• (10 pts) Consider the half cell || Ag⁺ (aq) | Ag (s). Will the E for the half cell increase, decrease, or stay the same if KBr(s) is added to the solution compartment of the half cell? Explain your reasoning.
• (8 pts) PUTTING KNOWLEDGE INTO CONTEXT: Barium is considered a heavy metal and it's ion, like the ions of lead and mercury, is toxic. In spite of that, BaSO₄ is used for imaging the upper gastrointestinal tract because Ba is a good absorber of x-rays. In the process a slurry of BaSO₄ is swallowed by the patient which then coats the GI tract. The solid is not absorbed by the body, and the insolubility of BaSO₄ prevents the ion from being absorbed by the body. One particular doctor was a bit sloppy in writing directions to a technician for a particular patient and specified BaSO₃ instead of BaSO₄. The technician noticed the irregularity, but also recognized that BaSO₃, like BaSO₄, is insoluble in water. So, since the doctor had specified BaSO₃, the technician used it. The patient became ill soon after and subsequently died. An autopsy showed the cause of death to be acute barium poisoning. Why does BaSO₃ cause this terrible problem when swallowed, but BaSO₄ does not?
• (20 pts) BRINGING IT ALL TOGETHER: Set up the system of mathematical equations (not just reactions) needed to solve the problem below without making assumptions. DO NOT SOLVE, JUST SET UP THE EQUATIONS.
• What voltage would the following cell produce?

• (8 pts) Based on the class's experience in lab, you might get the impression that determination of Mg by atomic absorption spectroscopy (AA) is pretty much impossible. However, that method is capable of producing quite good results. Last summer I ran the procedure used in lab, measuring the absorbance of a series of Mg⁺² solutions with concentrations ranging from 0.09-0.45 ppm. The regression calculations on that data (template output) is presented in the table below. Using that data, calculate the absolute 95% confidence interval for the molar absorptivity for Mg at the wavelength used in the analysis (285.2 nm). The table of t values is on the bottom of the K_a table page. The path length in AA is the length of the flame, 10 cm for our instrument.

 

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