Lesson 3: Acids - Names and Formulae

Part b: Naming Acids Containing Oxyanions

Part 3a: Naming Acids (no O Atoms)
Part 3b: Naming Acids Containing Oxyanions

 

The Big Idea

This lesson explains how to name an oxyacid by using the name of the corresponding polyatomic ion. Numerous examples are given of how to use the anion’s root, change ‘-ate’ to ‘-ic’ or ‘-ite’ to ‘-ous,’ and add ‘acid’, resulting in names like nitric acid (from nitrate) and sulfurous acid (from sulfite).

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What are Oxyanions?

The previous page of Lesson 3 discussed the rules for naming and formula writing for acids that do not contain an oxygen atom. Now we will turn our attention to the names and formulae for acids containing oxygen. Like all acids, the acids of focus on this page have the generic formula of HA where A represents the anion that combines with hydrogen (H). This page focuses on anions known as an oxyanions. An oxyanion is a negatively charged ion that contains at least one oxygen atom.
 
An inspection of our Polyatomic Ion List would reveal that most of the polyatomic ions contain oxygen. Here are the formulae and names of common ones. Observe that the names all have an -ate or -ite ending.

 
 

 

Rules for Writing the Names of Oxyacids

The rules for naming an acid are dependent upon whether the acid contains an oxyanion. Acids containing an oxyanion are referred to as oxyacids. The graphic below displays the names of several oxyacids. 

 
Some clear patterns emerge. There are two words. The second word is acid. The first word is the root name of the anion with either an -ic ending or an -ous ending. If the name of the anion ends -ate, then it is replaced with the -ic ending. And if the name of the anion ends -ite, then it is replaced with the -ous ending. The root name of an anion is typically the anion name with the ending removed. As discussed later, there are exceptions for sulfur and phosphorus containing oxyanions.
 

 

Procedure for Writing Names of Oxyacids

We can summarize this procedure as follows:

1. Identify the formula of the anion.
2. Use a Polyatomic Ion List to identify the full name of the anion.
3. Identify the root name of the polyatomic ion.
4. Add the -ic ending (for -ate ions) or the -ous ending (for -ite ions) to the root name.
5. Add acid as the second word.

 

Examples of Names of Oxyacids

The table below shows several examples of how the name of an oxyacid is derived from the formula of the acid.
   
Rows c and d and rows e and f may include some surprises. It might be expected that the root name for sulfate and sulfite would be sulf. And it might be expected that the root name for phosphate and phosphite would be phosph. This is not the case. Instead, sulfur and phosphor are the root names for these two oxyanions. The -ic and the -ous are added to these root names as seen in rows c and d and rows e and f.
     

How to Write the Formula of an Oxyacid

Identifying the formula that matches the name of an oxyacid involves working backwards. Begin by determining the anion formula (including the charge). Then determine the subscript that hydrogen will have. The subscript on hydrogen will be the charge of the oxyanion. Finally, write the formula for the oxyacid. Some examples are shown below.  
 
 
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Before You Leave - Practice and Reinforcement

Now that you've done the reading, take some time to strengthen your understanding and to put the ideas into practice. Here's some suggestions. 

• Download our Study Card on Acids - Names and Formulae. Save it to a safe location and use it as a review tool. The Study Card covers all of Lesson 3, not just this page. 
• The Check Your Understanding section below includes questions and problems with answers and explanations and solutions. It provides a great chance to self-assess your understanding.

 
 
 

Check Your Understanding of Naming Oxyacids

Use the following questions to practice your skill at naming oxyacids. Tap the Check Answer buttons when ready.
 
1. Based on the formula, which of the following are oxyacids? Select all that apply.

1. H₂S
2. HNO₃
3. NH₃
4. H₃AsO₄
5. HC₂H₃O₂
6. NH₄OH

 

 
2. Based on the formulae, which of the following are oxyacids? Select all that apply.

1. hydrosulfuric acid
2. sulfuric acid
3. phosphorous acid
4. dihydrogen monoxide
5. hydronitric acid
6. potassium hydrogen phthalate

 

 
3. Write the name of the following oxyacids.

1. HClO₃  
   
   Check Answer
   Answer: chloric acid
   
    
   
    
2. HClO 
   
   Check Answer
   Answer: hypochlorous acid
   
    
   
    
3. HMnO₄ 
   
   Check Answer
   Answer: permanganic acid
   
    
   
    
4. H₂Cr₂O₇ 
   
   Check Answer
   Answer: dichromic acid
   
    
   
    
5. HBrO₃ 
   
   Check Answer
   Answer: bromic acid
   
    
   
    
6. HBrO₂ 
   
   Check Answer
   Answer: bromous acid