Define Lewis acid and base. Explain what a buffer solution is and provide one of its uses. Calculate the pH of a buffer solution composed of 0.01 M butanoic acid and 0.1 M potassium butanoate.

The solution is a buffer made from a weak acid (butanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$) and its conjugate base (butanoate ion from potassium butanoate, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOK}$).
$[\text{Acid}] = 0.01 \text{ M}$
$[\text{Base}] = 0.1 \text{ M}$
$\text{K}_\text{a} = 1.5 \times 10^{-5}$
Use the Henderson-Hasselbalch equation: $\text{pH} = \text{pK}_\text{a} + \log\left(\frac{[\text{Base}]}{[\text{Acid}]}\right)$.

d) i) When the cell is connected:
A. The species that is oxidized is $\boxed{\text{Sn(s)}}$. (Since $\text{Sn}^{2+}/\text{Sn}$ has a more negative standard reduction potential, $\text{Sn}$ will lose electrons and be oxidized).
B. The species that is reduced is $\boxed{\text{MnO}_4^-\text{(aq)}}$. (Since $\text{MnO}_4^-/\text{Mn}^{2+}$ has a more positive standard reduction potential, $\text{MnO}_4^-$ will gain electrons and be reduced).

d) ii) A suitable instrument that can be used to determine the emf of the cell is a voltmeter (or potentiometer).

d) iii) The composition and function of the part labeled S:

• Composition: A salt bridge, typically containing an inert electrolyte such as potassium nitrate ($\text{KNO}_3$) or potassium chloride ($\text{KCl}$) in a gel.
• Function: It completes the electrical circuit by allowing the migration of ions between the two half-cells, thereby maintaining electrical neutrality and preventing charge buildup in the solutions.

d) iv) Calculate the emf of the cell.

The reduction half-reaction is $\text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \to \text{Mn}^{2+} + 4\text{H}_2\text{O}$, so $\text{E}^\circ_{\text{cathode}} = +1.5 \text{ V}$.
The oxidation half-reaction is $\text{Sn} \to \text{Sn}^{2+} + 2\text{e}^-$, so $\text{E}^\circ_{\text{anode}} = -0.14 \text{ V}$.

d) v) Platinum is used in the RHS of the cell because it is an inert electrode. It provides a surface for the electron transfer to occur in the $\text{MnO}_4^-/\text{Mn}^{2+}$ redox couple without reacting itself, as there is no solid reactant or product to serve as an electrode.