SELECTED K's

acid	pKa	acid	pKa	solid	pKsp
acetic acid	4.757	ammonium ion	9.224	silver bromide	12.30
hydrofluoric acid	3.17	hypochlorous acid	7.53	silver carbonate	11.09
methyl ammonium	10.64	barium oxalate	6.0

SELECTED INDICATOR pKa's

Phenolphthalein	Bromthymol Blue	Methyl Red
8.8	6.8	5.4

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.

1. (20 pts) 100.0 mL of water from a certain lake was found to contain 592.7 µg of mercury(II) chloride. What is the concentration of mercury in a) parts per million, and b) molarity?
2. (15 pts) 0.3157 g of an unknown monoprotic acid required 45.72 mL of 0.1036 M NaOH to titrate to a phenolphthalein endpoint. What is the molecular weight of the acid?
3. (15 pts) 1.3540 g of a stainless steel sample is dissolved in aqua regia (HCl + HNO₃) and then the iron is precipitated as iron(III) oxide. The precipitate, after heating to constant weight, weighed 0.7058 g. What is the percent of iron in the stainless steel?

1. (15 pts) As a chemist in an analytical lab, you have been given an ore sample to analyze for nickel content. Gravimetric analysis of 6 aliquots of the ore yielded the following percentages of Ni: 6.35%, 6.20%, 6.34%, 6.37%, 6.36%, 6.35%. What is the 99% confidence interval for this analysis?
2. The following calibration data were collected in a polarographic determination of lead in a ground water sample. (In polarography current is a linear function of the concentration.)

Calibration Curve Data

[Pb+2] mM	Current µA	Unknown run #	Current µA
0.000	1.32	1	26.32
0.510	5.65	2	26.30
1.020	10.07	3	26.35
2.040	19.08	4	26.32
3.060	27.91	5	26.34
4.080	36.08	6	26.33
5.100	45.82

1. (7 pts) Plot the calibration curve.
2. (15 pts) 6 runs were made on the same unknown sample. Using linear least squares, calculate the absolute 95% confidence interval for the lead concentration (mM) in the sample.
3. (8 pts) Convert the lead concentration and absolute 95% CI of the unknown sample to units of parts per million.
4. (5 pts) What is the relative 95% confidence interval for the unknown sample?

1. (10 pts) Which of the following compounds are soluble in water? Cu(CH₃COO)₂, Al(OH)₃, BaSO₄, Hg₂Cl₂, Pb(NO₃)₂
2. (15 pts) Will the following form acidic, basic, or neutral aqueous solutions? For those that form acidic or basic solutions, what is the conjugate of each?

HBrO3   NH3   SO3 -2   NO2   CH3OH 

N₂(g) + 3 H₂(g) = 2 NH₃(g)                  K = 760

3. (25 pts) Commercially ammonia is produced by the Haber process (above). This reaction is exothermic, but it proceeds very slowly at 25°C and 1 atm pressure even with a catalyst. What changes in these reaction conditions would you make to optimize the yield from the reaction? Explain your reasoning for each change.
4. (25 pts) The absorbance of a saturated solution of silver chromate at 370 nm in a 1.000 cm cell is 0.321. The molar absorptivity of chromate at 370 nm is 4.79 x 103 M-1cm-1 and silver does not absorb at 370 nm. What is the Ksp of silver chromate?
5. (25 pts) What is the pH of a saturated solution of calcium hydroxide? (Ksp = 6.5 x 10^-6).

1. (15 pts) Will the following produce acidic, basic, or neutral solutions in water? Write the reactions that justify your prediction. P₂O₅, NH₄Cl, CH₃NH₂, Ca(NO₃)₂, Ca(NO₂)₂
2. (10 pts) What are the strong acids and bases?
3. (20 pts) For a solution containing 0.10 M KCl and 0.10 M Na₂C₂O₄, what is:
1. the charge balance equation
2. the oxalate mass balance equation
4. (15 pts) A 0.25 M Na₂S solution is saturated with MnS. Write the complete reactions and the equilibrium reactions for this system making no assumptions.
5. (20 pts) What is the pH of 0.1 M ammonium sulfate? (for the conjugate acid/base pair: Ka = 5.6 x 10^-10; Kb =1.8 x 10^-5)
6. (20 pts) What is the pH of a solution that is 0.10 M acetic acid and 0.11 M aluminum acetate? (for the conjugate acid/base pair: Ka = 1.8 x 10^-5; Kb = 5.6 x 10^-10)

Chemistry 242 Exam 4 May 5, 1993

1. (10 pts) Which of the indicators in the table above would work the best for the titration of 0.1 M sodium hydroxide with 0.1 M hydrochloric acid? Explain your reasoning.
2. (15ts) Balance the following reaction in basic solution.
(CH₃)₂C=O + Cr₂O₇ ^-2 = Cr⁺³ + CO₂
3. (5 pts) Which reactant in the above reaction is the oxidizing agent? Which element is oxidized?
4. (15 pts) Give an ion or molecule that will reduce Br₂ but will not reduce Ag⁺. Briefly explain how you chose the species.
5. (20 pts) What is the E_cell and Keq for Pt/Fe⁺², Fe⁺³//I ^-, I₃ ^-/Pt
6. (15 pts) Gold plating is often used to provide corrosion resistance for metals. If 285 coulombs of electricity is used to plate a 250 cm² cone,
1. what weight of gold is there on each square centimeter of the cone?
2. what weight of NaAuCl₄ is required to plate the cone?
7. Permanganate (MnO₄ ^-) in basic solution has an E° of 0.60 V (1/2 reaction on the table).
1. (15 pts) What is the potential for the reaction if the pH is changed to 7.00?
2. (5 pts) Will permanganate at pH 7.00 be a stronger or a weaker oxidizing agent than under standard conditions? Explain your reasoning.

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