# Question #1ed19

Jan 23, 2016

${\text{K"_3"PO}}_{4}$

#### Explanation:

You can actually approach this question from two perspectives. You can

• start from the name of the compound and try to understand why its chemical formula is written like that
• start from the chemical formula and try to understand why the compound is named like that

So, let's take the first approach. You know that the compound's name is potassium phosphate. As you known, ionic compounds are always named cation first, followed by the anion.

Cations are ions that carry a positive charge and anions are ions that carry a negative charge.

So right from the start, you know that the cation must be the potassium cation, ${\text{K}}^{+}$. This ion carries a $1 +$ charge because potassium is a group 1 element.

This means that the anion must be the phosphate anion. Here is where a little knowledge of polyatomic ions can prove to be very valuable.

The phosphate anion is actually a polyatomic ion made up of one phosphorus atom and four oxygen atoms. The chemical formula of the anion is ${\text{PO}}_{4}^{3 -}$.

Now, ionic compounds are always neutral. This means that you must balance the $3 -$ charge on the anion by using three potassium cations.

The chemical formula of the compound will thus be

$3 \left[{\text{K"]^(color(red)(+)) ["PO"_4]^(color(blue)(3-)) <=> "K"_color(blue)(3)("PO"_4)_color(red)(1) <=> color(green)("K"_3"PO}}_{4}\right)$

Now let's take the second approach. You can identify the cation and the anion by taking apart the chemical formula of the compound.

${\text{K"_3"PO}}_{4}$

As you know, ionic compounds follow the criss cross rule, which states that the charge on the cation becomes the subscript of the anion, and vice versa.

Once again, it is very important to remember that cations are always written first!

So, you have

${\text{K"_color(blue)(3)"PO"_4 <=> "K"_color(blue)(3)("PO"_4)_color(red)(1) <=> 3"K"^color(red)(+)"PO}}_{4}^{\textcolor{b l u e}{3 -}}$

So, the cation will carry a $1 +$ charge, since you have ${\text{K}}^{+}$. The anion will carry a $3 -$ charge, since you have ${\text{PO}}_{4}^{3 -}$.

Once again, your cation will be the potassium cation and the anion will be the phosphate anion, hence the name potassium phosphate.