Writing Acid Formulas

Writing the formula for an acid from its nam requires us to recognize the basic structure of all acids. Acids are similar to ionic compounds, in that they are made of postive and negative pieces. The difference is that the positive piece is always hydrogen ions (H+1).

Because of that, the name of the acid is based on the negative ion alone. The names of negative ions always end in either "ate," "ite," or "ide." The name of the acid is related to the ending on that negative ion according to the system below. You will need to know your polyatomic ions for this.

"ic acids" (like nitric acid, acetic acid, oxalic acid) are formed from "ate" ions (nitrate, acetate, and oxalate)

"ous acids" (like nitrous acid and chlorous acid) are formed from "ite" ions (nitrite and chlorite)

"hydro___ic acids" (hydrocyanic acid, hydrochloric acid, hydroselenic acid) are formed from "ide" ions (cyanide, chloride, selenide)

To write the formula of an acid, then, requires that you add enough hydrogens to the beginning of the ion to balance the negative charge.

Some examples

Nitric acid

nitric acid (according to the system above) must be made from nitrate. Nitrate is NO3-1. Since it has a -1 charge, it requires one hydrogen. The formula is therefore HNO3.

 

 

 

Another example: oxalic acid

 

 

 

An example of an "ous" acid: chlorous acid

 

 

 

An example of a "hydro___ic" acid: hydrochloric acid

 

 

 

Another example: hydroselenic acid

 

 

 

 

And a last example with a polyatomic ion: Hydrocyanic acid

 

 

 

A Note about Acid Names

Some acids (listed below) don't quite follow the naming system and can be confusing. For example Sulfuric acid should be made from the ions sulfurate, but there is no such thing. Sulfuric acid is made from sulfate. The table below lists the acid names that misbehave this way and the ions from which they are made.

acid name ion
sulfuric acid sulfate
sulfurous acid sulfite
hydrosulfuric acid sulfide
phosphoric acid phosphate
phosphorous acid phosphite
hydrophosphoric acid phosphide

 

Your Turn

Try these on your own. The answers are here.

  1. hydrobromic acid
  2. iodous acid
  3. hydrosulfuric acid
  4. acetic acid
  5. sulfuric acid
  6. hypochlorous acid
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