What does a parenthesis mean in a formula?

1 Answer
Nick
Sep 13, 2016

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_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+"#.

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