Question 1

What is the oxidation state of Mn in $$\mathrm{$$KMnO_4$$}$$?

Accepted Answer

$$+7$$

Question 2

Given the cell notation $$\mathrm{Pt(s)|Fe$$^{2+}$$(aq.), Fe$$^{3+}$$(aq.) || Mn$$^{2+}$$(aq.), $$MnO_4$$^-(aq.), H^+(aq.)|Pt(s)}$$, which of the following is the correct reduction half-reaction at the cathode?

Accepted Answer

$$\mathrm{$$MnO_4$$^-} + 8\mathrm{H^+} + 5\mathrm{e^-} \rightarrow Mn$$^{2+}$$ + 4\mathrm{H_2O}$$

Question 3

Calculate the standard cell potential $$E_{cell}^0$$ for the cell: $$\mathrm{Pt(s)|Fe$$^{2+}$$(aq.), Fe$$^{3+}$$(aq.) || Mn$$^{2+}$$(aq.), $$MnO_4$$^-(aq.), H^+(aq.)|Pt(s)}$$ using the table provided.

Accepted Answer

$$1.51\ \mathrm{V} - 0.77\ \mathrm{V} = 0.74\ \mathrm{V}$$

Question 4

Which equation correctly relates the standard cell potential $$E_{cell}^0 to $$the standard Gibbs free energy change $$\Delta G^0$?

Accepted Answer

$$\Delta G^0$ = -nFE_{cell}^0$$

Question 5

A voltaic cell uses a saturated calomel electrode (SCE, $$E_{SCE} = 0.241\ \mathrm{V}$$\ vs. SHE) as the reference and a platinum electrode in $$\mathrm{Fe$$^{2+}$$/Fe^{3+}$$}$$ solution as the cathode. If $$[$$Fe^{2+}$$]/[$$Fe^{3+}$$] = 2.8 	imes $$10^{-3}$, what is the cell voltage?

Accepted Answer

$$E_{cell} = 0.771\ \mathrm{V} - 0.241\ \mathrm{V} + \frac{0.05916}{1} \log(2.8 	imes 10$$^{-3}$$)$$

Question 6

During calibration, a standard buffer with pH 6.881 gave a voltage reading of $$-18.3 mV$$, and a second buffer with pH 4.002 gave $$+146.3 mV. $$What is the observed slope (in mV/pH)?

Accepted Answer

$$\frac{146.3 - (-18.3)}{4.002 - 6.881} = -57.1\ \mathrm{mV/pH}$$

Question 7

What is the correction factor $$\beta if $$the observed slope is $$-57.1\ \mathrm{mV/pH}$$ and the theoretical slope is $$59.16\ \mathrm{mV/pH}$$?

Accepted Answer

$$\beta = \frac{-57.1}{59.16} = -0.97$$

Question 8

A sample gives a voltage reading of $$+100.0 mV on $$the same pH electrode calibration as above. What is the pH of the sample?

Accepted Answer

$$pH = 4.002 + \frac{100.0 - 146.3}{-57.1}$$

Question 9

Which of the following is the correct Nernst equation for the half-cell $$\mathrm{Fe$$^{3+}$$ + e^- \rightarrow Fe$$^{2+}$$} at$$ $$25^\circC$$?

Accepted Answer

$$E = E^0$ - \frac{0.05916}{1} \log\left(\frac{[Fe$$^{3+}$$]}{[Fe$$^{2+}$$]}ight)$$

Question 10

Which of the following best describes the function of the salt bridge in a voltaic cell?

Accepted Answer

It completes the electrical circuit by allowing ion flow between half-cells.

Question 11

In the cell $$\mathrm{Pt(s)|H_2$(g, 1.00\ bar)|HA (aq, 0.050\ M), A^- (aq, 0.005\ M) || KCl (aq, 0.10\ M)|AgCl(s)|Ag(s)}$$, what is the expression for the acid dissociation constant $$K_A of HA$$?

Accepted Answer

$$K_A = \frac{[H^+][A^-]}{[HA]}$$

Question 12

Which of the following is the correct balanced net spontaneous reaction for the cell: $$\mathrm{Fe$$^{2+}$$ + $$MnO_4$$^- + 8H^+ \rightarrow Fe$$^{3+}$$ + Mn$$^{2+}$$ + 4H_2O}$$?

Accepted Answer

$$5Fe^{2+}$$ + $$MnO_4^- + 8H$$^+ \rightarrow $$5Fe^{3+}$$ + $$Mn^{2+} + 4H_2O$

Question 13

If the standard reduction potential for $$\mathrm{AgCl(s) + e^- \rightarrow Ag(s) + Cl^-(aq)} is$$ $$0.220\ \mathrm{V}$$, what is the cell potential if the measured voltage is $$0.402\ \mathrm{V}$$ and the other half-cell is a hydrogen electrode?

Accepted Answer

$$E_{cell} = 0.402\ \mathrm{V}$$

Question 14

Which of the following is the correct formula for the equilibrium constant $$K in $$terms of $$E_{cell}^0$$?

Accepted Answer

$$\ln K = \frac{nFE_{cell}^0}{RT}$$

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