# A beaker was determined to have 5.89 x 10^25 calcium ions by a student using an ion sensing probe. How many moles of calcium chloride were dissolved to reach this number of ions?

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Calcium Chloride has the formula

`CaCl_2`

Each formula unit, when dissolved in water, produces one calcium ion and two chloride ions. This give us the unit factor 1 CaCl2/2 Cl-.

One mole of particles of any kind is equal to 6.02x10^23. This gives us the unit factor 1 mole/6.03x10^23 ions.

Multiplying the number of chloride ions by these two unit factors will give moles of CaCl2 formula units:

(5.89x10^25 Cl- ions) x (1 mol/(6.02x10^23) x (1 Cl-/2 CaCl2)

=** 48.9 moles of CaCl2**

In order to solve problems like this you need to write the formula of the compound so you know the subscript on the ion in question. Since the calcium ion has a charge of +2 and the chloride ion has a charge of -2 (Cl alone is -1), the formula CaCl2 results in a neutral compound.

The term "formula unit" is used in place of "molecule" because ionic compounds don't exist as discrete molecules. The formula is the simplest ratio of ions.