Monoprotic and Polyprotic Acids

Deduce the difference between monoprotic and polyprotic acids, then learn about how acids break apart and what that means for acid strength. Then, assess your new knowledge with a quiz.

Definition and Examples

According to both the Arrhenius and Bronsted-Lowry definitions, acids are compounds that contain hydrogen ions. When these compounds are added to a solution, the hydrogen ions break free or dissociate. The easier the hydrogens pop off, the stronger the acid is considered to be.

In this table are common monoprotic and polyprotic acids.

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Monoprotic Acid Examples Polyprotic Acid Examples
HCl H2 SO4
HBr H3 PO4

Monoprotic and polyprotic acids are types of acids that contain one or more hydrogen ions, which are often referred to as protons. Can you figure out the differences between monoprotic and polyprotic acids? If you’ve been studying chemistry for a while, you’ve probably encountered the prefixes mono- and poly- a few times. You might even recognize that mono- means one and poly- means many. Does this give you a clue?

Monoprotic acids, like HCl and HBr, contain just one hydrogen ion. Polyprotic acids, like H2 SO4 and H3 PO4, contain two or three hydrogen ions.

It’s tempting to think that polyprotic acid are stronger than monoprotic acids because they contain multiple hydrogen ions, but that’s actually not true. Sometimes a polyprotic acid only lets go of one hydrogen and keeps its other hydrogen ions attached. Keep going to find out why.

Monoprotic Acids in Action

In water, monoprotic acids lose their hydrogen to the surrounding water molecules, making a hydronium ion (H3 O+). What’s left of the acid after the hydrogen is lost is called the conjugate base. You can see how HBr (aka hydrobromic acid) reacts with water.

Hydrobromic acid (HBr) dissociation.
HBr Dissociation

The hydrogen in HBr joins with water making H3 O+ and Br- is left behind. HBr is a monoprotic acid that’s considered to be a strong acid because it readily loses its hydrogen ion in solution. Acids that do not readily lose their hydrogens in solution are considered to be weak acids.

In general, the extent that a monoprotic acid loses its hydrogen depends on how attracted the hydrogen is to the rest of the compound. A monoprotic acid like HBr loses its hydrogen quite easily because the bromide ion (Br-) and the hydrogen ion are not that attracted to each other. Meanwhile, a monoprotic acid like HF (hydrofluoric acid) doesn’t like to break apart at all. The hydrogen and the fluoride ion (F-) are very attracted to one another. Thus, HF is considered to be a weak acid.

We measure the likelihood of hydrogens breaking off using something called the acid dissociation constant (Ka). This value is both unique and constant for every acid. In general, the higher the Ka value, the higher the likelihood of hydrogen ions breaking off in solution. Since more hydrogen ions in solution means a stronger acid, higher Ka values mean stronger acids. Lower Ka values mean weaker acids.

The dissociation constant for a strong acid like HBr is so high it’s often just referred to as ‘large.’ This means nearly 100% of the acid breaks apart in solution. The dissociation constant for a weak acid like HF is 7.2 x 10-4. That means hardly any of the acid is breaking apart in solution.

Polyprotic Acids in Action

Just like monoprotic acids, polyprotic acids will also lose the hydrogen ions to water when in a solution. Initially, a polyprotic acid will only lose one hydrogen ion and keep the others. See how sulfuric acid dissociation how sulfuric acid (H2 SO4) loses one hydrogen and holds onto the other when put in a solution with water? This reaction forms a hydronium ion and the HSO4– ion.

Sulfuric acid (H2 SO4) dissociation.
Dissociation of sulfuric acid in water.

As previously noted, the Ka value for H2 SO4 is ‘large,’ so nearly 100% of it breaks up in water to form hydronium ions and the HSO4– ion.

HSO4– will seldom lose it’s other hydrogen to form another hydronium ion and the sulfate (SO4 2-) ion. The Ka for this is dissociation is 1.2 x 10 -2, so few of the HSO4– ions will break up.

HSO4– dissociation.
Dissociation of HSO4- ion in water.

Polyprotic acids will have multiple Ka values, one for each hydrogen removed. Generally, the Ka value decreases significantly as hydrogens are lost. The table shows the three Ka values for the polyprotic acid, H3 PO4. Notice how the Ka values decrease as the acid loses hydrogen ions.

Acid Form Ka Value
H3 PO4 7.5 x 10 -3
H2 PO4 6.2 x 10-8
HPO4 2- 4.8 x 10-13

Lesson Summary

The main difference between monoprotic and polyprotic acids is how many hydrogen ions are contained within the acid. Monoprotic acids contain one hydrogen ion; polyprotic acids contain multiple hydrogen ions. Any easy way to remember this key difference is understanding that mono- is the prefix for one, poly- is the prefix for many and the suffix -protic refers to protons, which are the same thing as hydrogen ions.

Acids break apart or dissociate in water to form a conjugate base and a hydronium ion (H3 O+).

The more easily an acid loses its protons the stronger the acid is considered to be. The less likely an acid is to lose protons the weaker the acid is considered to be. The strength of an acid is related to its acid dissociation constant (Ka). The higher the Ka value, the more likely an acid is to lose its hydrogen ion. Therefore, higher Ka values correspond to stronger acids.


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