Question

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

Answer 1

To form `MgCl_2`? Clearly we need `0.50*mol` of chloride ion.

Explanation:

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

`Mg^(2+) + 2Cl^(-) rarr MgCl_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 `Mg^(2+)` ions (`0.25*mol`) it follows that we need TWICE this number of chloride ions for reaction.

Why do I refer to the `"mole"` as a number?