How can I determine the formula of an acid?
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).
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).
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!