What Is the Cell Potential for The Galvanic Cell Exam Practice

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Show your work, answers alone will not receive partial credit. Where indicated, please enter your answer the box beneath the question. There are blank pages near the end of the exam to use as scratch paper. Answers without the units indicated (when necessary) will not be graded.

The last page of the exam contains a selection of data tables and equations for your use – feel free to remove the page from the exam.

 

Question

Value

Score

1

10

 

2

5

 

3

5

 

4

10

 

5

10

 

6

10

 

Total

50

 

 

 

 

 

 

By signing below I acknowledge that I will abide by the SDSU Academic Code of Honor as it pertains to academic integrity. For this exam I will not make use of any unauthorized materials, nor will I engage in any practices that otherwise fall within the broader definition of “cheating”.

_____________________________________________________________________________________

Signature Date

If this pledge is not signed the instructor reserves the right to not grade the exam and award a score of 0 for this exam in the calculation of the final grade.


  1. (10 points) What is the cell potential for the galvanic cell described by the line notation below?

Mg(s) | Mg(OH)+ (45.92 mM), pH = 6.05 || IrCl62– (15.47 mM), Cl– (9.02 mM) | Ir(s)

word image 904

  1. (5 points) Write out the Beer’s law equation and identify all the variables.


 

 

 

 

𝞮 = molar absorptivity (1/M•cm)

 

        1. = path length (cm)
        2. = molar concentration (M)

 

 

 

  1. (5 points) For the following questions consider the three principle types of spectrometers that are used for analyses: flame, graphite furnace, and inductively coupled plasma (ICP). For each question select the best system:

        1. Which instrument is capable of analyzing solids?


 


 Graphite furnace

 

        1. Which instrument reaches the highest temperatures?


 

ICP

 

        1. Which instrument has the longest path length?


 

Flame spectrometer

 

        1. Which is best for atomic emission spectroscopy?


 

 

ICP

        1. Which can execute a programed temperature cycle?


Graphite furnace

  1. (10 points) A tin (Sn) metal indicator electrode is being used to determine the concentration of sulfide (S2–) in an unknown solution. The measurement is done by mixing a known tin nitrate (Sn(NO3)2) solution with some of the sulfide solution (details below) and measuring the potential of the solution with a tin electrode, and a SCE reference electrode (0.241 V).


 

The mixture was made by combining 20.00 mL of 30.12 mM Sn(NO3)2 and 50.00 mL of the unknown sulfide solution. The Ksp for tin sulfide is 1.3×10–26.


 

The cell potential was measured to be –1.080 V. What is the concentration of S2– in the unknown solution?


word image 905

  1. (10 points) You are measuring the amount of iron via a colorimetric reaction with 1,10phenanthroline. You can quantify the amount of iron by the absorbance that you can measure form the metal-dye complex.


 

You have added an excess of the 1,10-phenanthroline to a solution containing an unknown amount of iron. When the absorbance is measured you obtain a value of

0.488 AU.


 

You then mix 15.00 mL of that unknown solution with 30.00 mL of an iron standard solution that contains 73.23 µM iron and an excess of the 1,10-phenanthroline. When you measure the absorbance of this solution you get an absorbance value of 0.682 AU.


 

What is the concentration of iron in the unknown solution?


 

word image 906

  1. (10 points) A liquid chromatographic (LC) instrument is being used to quantify the calcium content of a soy milk. To test the detector for the LC a standard solution containing 515 ppm Ca2+ and 145 ppm Cd2+ is measured, and the peak areas for the two metals are found to be 1531 and 583 respectively.


 

A 20.00 mL sample of the soy milk is mixed with 5.00 mL of the cadmium internal standard (225 ppm). When the peak areas are obtained for calcium and cadmium they are found to be 1450 and 181 respectively.


 

What is the ppm concentration of calcium in the soy milk?


Note: keep all concentrations in ppm throughout this problem for simplicity.

word image 907

E =K+ 0.05916 log([A]+KA,I [I]zzAI ) KA,I = [Az]zAI

zA

word image 2620 [I]

SA Sspike

=

CA %VA (+Cspike %’Vspike (*

CA ‘ * &VT ) & VT )

SA Sspike

=

%VO ( C %V ( %V (

CA ‘ * A O *+Cspike spike *

&VF ) &VF )

SA kACA K×CIS

= =

SIS kISCIS CA

b± b2 −4ac

x=

2a

& VF )

word image 908 word image 909

CIS SA

K = ×

CA SIS

Selected Standard Reduction Potentials

Reaction

E° (Volts)

Mg(OH)+ + H+ + 2e– ⇌ Mg(s) + H2O

–2.022

Mg(OH)2(s) + 2e– ⇌ Mg(s) + 2OH–

–2.690

IrCl62– + e ⇌ IrCl63–

1.026

IrCl62– + 4e ⇌ Ir(s) + 6Cl

0.835

Sn4+ + 2e ⇌ Sn2+

0.139

Sn2+ + 2e ⇌ Sn(s)

–0.141

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