Question

What does a parenthesis mean in a formula?

Answer 1

It means that the atoms in a particular structure are repeated in the molecule.

Explanation:

An example, `Ca_3(PO_4)_2`

The atomic weights of the components:

• Ca - calcium - 40.078
• P - phosphorus - 30.973762
• O - oxygen - 15.9994

`Ca_3` means there are three Ca; `PO_4` means there is one P and 4 oxygen, the `()_2` means there two `PO_4`. So, substituting in the values:

`Ca_3(PO_4)_2`
(3 x 40.078) + ((30.973762 + (4 x 15.9994)) x 2)
120.234 + ((30.973762 + 63.9976) x 2)
120.234 + (94.971362 x 2)
120.234 + 189.942724
310.177

So the answer is 310.177 g/mol

The `PO_4` carries a charge of -3, with the P and the 4 O linked by covalent bonds. One oxygen has a double bond with the P, and the other three oxygens have one covalent bond and each carry a negative charge, giving the overall charge of -3. The two `PO_4` have a total charge of -6, and form ionic bonds with the three `Ca^"2+"`.