Determining the voltage of a cell
[...] I am not sure which is reduced at the cathode. Would it be the sulfate ions?


orthocresol has already pointed out in his comment that there's more in $\ce{Fe2(SO4)3}$ than just the sulfate.

Take a look at the cation:

$$\begin{align}
\ce{Fe^3+&amp; + e- &amp;&lt;=&gt;&amp;Fe^2+\quad &amp;+0.77V} \\
\ce{Fe^3+&amp; + 3e- &amp;&lt;=&gt;&amp;Fe(s)\quad &amp;-0.04V}
\end{align}$$

(Standard electrode potentials are cited from wikipedia.)