What is formed when you leave iron(II) sulfate in plain air?
Usually, the original iron(II) sulfate is present as green $\ce{FeSO4.7H2O}$.

Oxygen can oxidize $\ce{Fe(II)}$ salts to $\ce{Fe(III)}$ salts; for example, at $\mathrm{pH}=0$:

$$\begin{alignat}{2}
\ce{[Fe(H2O)6]^3+ + e- \;&amp;&lt;=&gt; [Fe(H2O)6]^2+}\quad &amp;&amp;E^\circ = +0.771\ \mathrm{V}\\
\ce{O2 + 4H+ + 4e- \;&amp;&lt;=&gt; 2H2O}\quad &amp;&amp;E^\circ = +1.229\ \mathrm{V}
\end{alignat}$$

$\ce{Fe(II)}$ is even easier oxidized under alkaline conditions; for example, at $\mathrm{pH}=14$:

$$\begin{alignat}{2}
\ce{FeO(OH) + H2O + e- \;&amp;&lt;=&gt; Fe(OH)2 + OH-}\quad &amp;&amp;E^\circ = -0.69\ \mathrm{V}\\
\ce{O2 + 2H2O + 4e- \;&amp;&lt;=&gt; 4OH-}\quad &amp;&amp;E^\circ = +0.401\ \mathrm{V}
\end{alignat}$$

However, you cannot simply oxidize iron(II) sulfate $\left(\ce{FeSO4}\right)$ to iron(III) sulfate $\left(\ce{Fe2(SO4)3}\right)$ in dry air since you would need additional sulfate to balance the equation:

$$\ce{2FeSO4 + SO4^2- -&gt; Fe2(SO4)3 + 2e-}$$

Nevertheless, $\ce{Fe(II)}$ can be oxidized to $\ce{Fe(III)}$.

The resulting $\ce{Fe(III)}$ is subject to hydrolysis. By way of comparison, the ion $\ce{[Fe(H2O)6]^3+}$ is only stable under strong acidic conditions. Already at $\mathrm{pH}=0{-}2$, it turns into yellow $\ce{[Fe(OH)(H2O)5]^2+}$ and begins to form $\ce{[Fe(OH)2(H2O)4]+}$:

$$\begin{align}
\ce{[Fe(H2O)6]^3+ \;&amp;&lt;=&gt; [Fe(OH)(H2O)5]^2+ + H+}\\
\ce{[Fe(OH)(H2O)5]^2+ \;&amp;&lt;=&gt; [Fe(OH)2(H2O)5]+ + H+}\\
\end{align}$$

Further addition of base causes precipitation of amorphous iron(III) hydroxide.

Accordingly, the likely product when iron(II) sulfate is oxidized is basic iron(III) sulfate, i.e. approximately:

$$\ce{4FeSO4 + O2 + 2H2O -&gt; 4Fe(OH)SO4}$$

However, the real weathering and aging of iron(II) sulfate in dry air actually yields a mixture of various compounds, including iron(III) sulfate and iron(III) oxide-hydroxide.

You may observe this reaction in some iron fertilizers for lawns. The fresh product typically contains green iron(II) sulfate, which gradually becomes yellow.