Question

How can I determine the formula of an acid?

Answer 1

First you need to determine if the acid is binary (only H and one other element) or ternary (H and more than one other element).

Explanation:

Binary acids will have names which begin with the prefix "hydro". The ending of the name for the acid will come from the second element found in the acid.

HCl = hydrochloric
HBr = hydrobromic

Notice how chlorine changes to chloric and bromine changes to bromic.

Ternary acids do not have the prefix hydro in their names unless the polyatomic ion has a name which has the "ide" suffix. The name comes from the element listed first in the polyatomic ion (oxyanion).

`H_3PO_4` = phosphoric acid
`H_3PO_3` = phosphorous acid

Because `H_3PO_4` contains the polyatomic ion phosphate, the ending for the name of the acid will be "ic".

Because `H_3PO_3` contains the polyatomic ion phosphite, the ending for the name of the acid will be "ous".

You should memorize
"ate" goes to "ic"
"ite" goes to "ous"

Here is a video to help further with this concept.

Video from: Noel Pauller

Hope this helps!