Chem 2113 Test 4 Summer 1996

Questions are worth 25 points each. Show your work and circle your answers for full credit. Omit one question by leaving completely blank or by writing OMIT in the space provided for your work. Good Luck.

1. Before the glass pH electrode became a standard piece of equipment for the analytical laboratory, the pH for an unknown solution could be determined by measurement of the potential for a solution which was saturated in quinhydrone. Quinhydrone is an equimolar mixture of quinone (Q) and hydroquinone (H2Q). The reaction between the two species is:

Q + 2H+ + 2e- <==> H2Q E = 0.699V

Calculate the pH of a solution that is saturated in quinhydrone given the potential of a platinum electrode immersed in the solution was 0.054 vs the SCE (ESCE = 0.242V).

2. The nitrate ion concentration in a sample can be determined by using a nitrate specific ion electrode. A series of standards was prepared in a constant ionic strength buffer, and the data are reported below:

[NO3-] ; Potential (mV)

1.00 x 10-4 ; -105

3.16 x 10-4 ; -83

1.00 x 10-3 ; -44

3.16 x 10-3 ; -24

Calculate the following values, and determine the slope and the y-intercept for the linearly-related data.

An 5.00 g sample containing nitrate is dissolved and the nitrate is extracted into 250 mL. 10.00 mL of this solution is diluted to 200 mL in a constant ionic strength buffer, and the potential is determined to be -55.2 mV. Assuming all of the nitrate is present as sodium nitrate (FW = 84.99) calculate the percent sodium nitrate in the original sample.

3. Amines form salts with picric acid, and all amine picrates have an absorbance maximum at 359 nm with = 12,500. A 0.200 g sample of aniline (an amine with a formula weight of 93.13) is dissolved in 500.0 mL of water. A 25.0 mL aliquot is reacted with picric acid in a 250.0 mL volumetric flask and diluted to volume. A 10.0 mL aliquot of this solution is diluted to 100.0 mL and the absorbance is read in a 1.00 cm cell. If the absorbance was 0.425, what is the percent purity of the aniline?

4. The following data were obtained for solutions containing potassium permanganate, KMnO4:

[KMnO4] ; %T

0.0 x 10-4 ; 100.0

1.0 x 10-4 ; 62.8

2.0 x 10-4 ; 39.4

4.0 x 10-4 ; 15.5

Calculate the following values, and determine the slope and the y-intercept for the linearly-related data. Also, determine the molar absorptivity for potassium permanganate, assuming all measurements were made in a 1.00 cm cell.

A 1.500 g sample of steel is dissolved in nitric acid and diluted to 250.0 mL. A 25.0 mL aliquot of this solution is reacted with KIO4 to convert any manganese to permanganate, diluted to 50.0 mL, and the %T for the solution was 35.1%. Calculate the %Mn in to original steel sample.

5. Define the following terms:

Membrane electrode

Reference electrode

Chromophore

Alkaline Error (for the glass pH electrode)

Analytical Chemistry

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