Name: Professor Key

Chemistry 3600
Final Examination
December 14 1998

Directions: There are 5 questions, each worth equal points. Some are more difficult than others. Read over all questions before you start to answer. Answering the harder questions last will insure that you obtain the maximum score. You must show your work and pertinent formula's to get full credit. The final number answer is only ten percent of the total score.

Useful formulas:

Useful data:

Values of Q @ 90% confidence

Values of Student's t @ 95% confidence

Values of F @ 90% confidence

1. Statistics

1. Use the Q test to determine if the 3.483 value can be dropped from the following data set

3.274, 3.258, 3.265, 3.258, 3.350, 3.483

Q_calc = gap/range

Q_calc = (3.483-3.350)/(3.483-3.258) = 0.591

Q_table = 0.56

Since Q_table< Q_calc datum can be rejected from the data set

B) Based on the above results, calculate the 95% confidence interval and report the statistically probable mean value at 95% confidence. Show your work using appropriate formulas.

The table t value for 5-1=4 degrees of freedom is 2.776 Using the mean

and standard deviation

the confidence interval is

C) Calculate the variances of the data in part A with and without the 3.483 value. Apply the F-test to determine if there is a significant difference in the two variances (@ 90% confidence). Does your F-test result agree with the Q test in part A?

Without the rejected datum, there are N=6 points, with a mean and standard deviation of 3.315 and s₁=0.08958 The calculated F is

The whole data set, used to calculate s₁, has 6-1=5 degrees of freedom. The data set with one point rejected, used to calculate s₂, has 5-1=4 degrees of freedom. F_table = 4.05 Since F_table < F_calc the two standard deviations are statistically different. This result agrees with that of the Q test. The later indicated that one datum could have come from a different measurement population. Including this datum results in an overall standard deviation that is statistically different than that obtained with the rejected datum.

D) Gravimetric analysis of nickel by precipitation with dimethylglyoxime gave the linear (y=mx+b) calibration shown below. Regression values for the slope and intercept, and their standard deviations, are given in the inset box. Calculate the weight % nickel, and estimate the error using propagation formulas, for a sample with precipitate weight 5.120± 0.001 g.

From y=mx+b, solve for x (wt. %) as a function of y (wt)

The error estimate is obtained from the slope and intercept standard deviations (0.020 and 0.35) and the weight error (0.001). Using the above equation and the error propagation rules formulas, for a sample with precipitate weight 5.120± 0.001 g.

2. Acid-Base Titrations/Advanced Acid-Base Chemistry

The amino acid, methionine, is diprotic with a carboxylic acid pK_a1=2.20 and an ammonium pK_a2=9.05. In this problem, 10 mL of a 0.1 F solution of this amino acid is titrated with 0.1 F NaOH titrant.

(a) Below is the resulting titration curve. Indicate the two points where the solution has the maximum buffer capacity by A and C, and indicate the first and second equivalence points as B and D. Label the Titrant Volume axis markers.

(b) There are simple expressions for the pH at points A, B, and C. What are the pH at these points? Give the equations below and put numbers on the pH axis on the above plot.

(c) At what pH would you want your indicator color change to occur at in order to titrate to the first equivalence point?

Around pH=6

3. Potentiometry

1. Calculate the electrode potential for the copper-copper iodide electrode, Cu/CuI/I^-// with an iodide activity of A_I= 0.001, given

   

   

   Start with the Nernst equation for copper reduction

   

   Next, solve the K_sp equation for Cu⁺

   

   Substitution into the Nernst equation yields

   

B. The ideal potential response for the solid-state fluoride ion selective electrode is given by

But the solid-state conductor also responds to hydroxide when [OH^-]» 0.1 M, and hydrogen fluoride is a moderate acid with pK_a=3.17. Give reasons for two pH-dependent sources of error associated with fluoride measurements

1. at low pH At low pH, F^- is bound in the molecular form (HF).
2. at high pH At high pH, OH^- competes with F^- at the electrode surface
3. Fluoride is known to complex with many metals to form both soluble ions and insoluble salts. How would the presence of metals affect the pF measurements? Metals that complex with F^- will lower its concentration (and thus activity). This will increase pF
4. Ionic strength also affects potential. Does an increase in ionic strength increase or decrease the potential according to the equation above? Increased ionic strength lowers activity of the fluoride. This will increase potential.

4. Spectroscopy

1. A 3.96´ 10^-4 M solution of a substance had an absorbance of 0.534 at 350 nm in a 1 cm pathlength cuvet. A blank solution containing only solvent had an absorbance of 0.025 at the same wavelength. What is the molar absorptivity of the compound?

2. What is the concentration of the substance in a sample exhibiting a transmittance of 34.7 %T in the same solvent and cuvet? (hint: transmission losses due to solvent and cuvet are both the same as in part A)

3. Define the following terms

1 - %T Percent transmission. The fraction of light passing through the sample

2 – excited state A quantum state other than the ground state.

3 – luminescence Light emission

4 – Beer’s law A=ebC relates sample absorbance to molar absorption coefficient, pathlength and concentration.

5 - absorption spectrum Wavelength or photon energy dependent absorbance. Plot of absorbance versus wavelength or photon energy.

5. Analytical Separations

1. A solute with a partition coefficient of K_D=1.0 is extracted from 10 mL of phase 1 into phase 2.

   1 - What volume of phase 2 is needed to extract 75% of the solute in a single extraction?

   

   2 - What percent of the solute is extracted upon three equal-volume extractions using 10 mL each?

   

   Since m₁=m₂, each extraction removes 50%. The amount remaining after 3 extractions is (0.5)³=0.125. The amount removed is (1-0.125)´ 100%=87.5%

2. Write the type of chromatography after the appropriate description. The choices are; Adsorption, Affinity, Ion exchange, Partition, and Size Exclusion

3. The Van Deemter equation describes the "height equivalent to a theoretical plate" in terms of mobile phase flow rate and three phenomenological constants

1 - Give the names for and/or describe the effects thought to be responsible for A, B, and C.

A: Eddy Diffusion: due to different paths through column

B: Longitudinal Diffusion: due to mass diffusion along the length of the column

C: Dis-equilibrium: due to dynamic effects between stationary and mobile phases

2 - Is it better to minimize or maximize H.E.T.P. when performing chromatographic separations?

Minimizing the HETP increases the number of theoretical plates resulting in better separations.