Give an example of a divalent anion and a trivalent cation. Write the formula of the compound formed by their combination.

Aluminium is trivalent cation with a valency of 3 → Al³⁺

Sulphate is divalent anion with a valency of 2 → SO₄^2-

Formula:

$\text{Al}^{3+} \space \text{SO}$4^{2-} \\[1em] \overset{3}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}4} \Rightarrow \underset{2}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \underset{3}{\text{SO}4} \\[1em] \text{Al}2(\text{SO}4)3

Hence, the formula is : Al₂(SO₄)₃