# How many moles of Cl^- ions are needed to completely combine with 0.25 moles of Mg^(2+) ions?

Aug 19, 2016

To form $M g C {l}_{2}$? Clearly we need $0.50 \cdot m o l$ of chloride ion.

#### Explanation:

In all these sorts of problems, reference to a balanced chemical equation is necessary as a first step:

$M {g}^{2 +} + 2 C {l}^{-} \rightarrow M g C {l}_{2}$

This equation is balanced with respect to (i) mass, and (ii) charge, and is therefore a reasonable representation of chemical reality, of physical reality if you like. The equation clearly states that each equiv magnesium ion requires 2 equiv of chloride ion to form the salt. Since we started with a given NUMBER of $M {g}^{2 +}$ ions ($0.25 \cdot m o l$) it follows that we need TWICE this number of chloride ions for reaction.

Why do I refer to the $\text{mole}$ as a number?