What is the oxidation state of copper (Cu) in Cu_2O?

Which of the following compounds contains nitrogen with an oxidation number of +5?

In which of the following reactions does the oxidation number of oxygen increase?

What is the oxidation number of Fe in Fe_2O_3?

What is the oxidation number of aluminum in Al2O3?

In which reaction does the oxidation number of oxygen increase (i.e., becomes less negative or more positive)?

Determine the oxidation state of each element in $CO{H}_2$ (assume typical oxidation-state rules for covalent compounds).

In the compound $N_2{H}_4{O}_2$ (often written as $N_2{H}_4{O}_2$), nitrogen occurs in two different ions: $N_1^{+}$ in $N_1{H}_4$ and $N_1^{-}$ in $N_1{O}_2$. What are the oxidation numbers of nitrogen in these two ions (in the order $N_1{H}_4$ then $N_1{O}_2$)?

What is the average oxidation number of sulfur in sodium thiosulfate, $N_a{2}_{}{S}_2{O}_3$?