Here's why:
* Magnesium is the cation (positive ion) and is named as it is.
* Dihydrogen arsenate refers to the anion (negative ion) which is formed by arsenic acid (H₃AsO₄). When two hydrogen ions are removed from the acid, it leaves a dihydrogen arsenate ion (H₂AsO₄⁻).
Since the compound is neutral, the charges of the ions must balance. Magnesium has a +2 charge and dihydrogen arsenate has a -1 charge, so the formula for the compound is Mg(H₂AsO₄)₂.
However, the commonly used name for this compound is Magnesium hydrogen arsenate because it is simpler and avoids the use of the prefix "di."
