Acids and bases are related by the transfer of hydrogen ions. However, some acids and bases accept/release one hydrogen ion, and some accept/release more than one. The former group of acids and bases are called monoprotic, and the latter group of acids and bases is called polyprotic. Here we will focus on the properties of monoprotic acids and bases, namely how hydrogen ions are transferred, and how concentrations of these ions in acidic and basic aqueous solutions are affected.
A monoprotic acid will only transfer one hydrogen ion to a base. The charge of a monoprotic acid is always one higher before it gives away the hydrogen ion. The following key statement is common sense:
Any acid with only one hydrogen atom is monoprotic.
At the same time, some acids with more than one hydrogen atom are monoprotic. Some examples of each scenario will be given.
Example 1: \(HCl\) is an acid. Therefore it is a monoprotic acid because it only has one hydrogen atom:
$$HCl \rightarrow H^{+} + Cl^{-}$$
Example 2: \(CH_5^{+}\) may have five hydrogen atoms, but it is a monoprotic acid. Consider what happens when we remove one hydrogen ion:
$$CH_5^{+} \rightarrow H^{+} + CH_4$$
We get methane, a very stable compound. \(CH_5^{+}\) is not polyprotic because \(CH_4\) will generally not give away hydrogen atoms; it is stable as is, because carbon atoms generally have four bonds.
Also, many positively-charged ions that have hydrogen atoms are acids, because the excess positive charge can be removed. Example 2 shows one of these ions.
Example 3: The ion \(CH_3^{-}\) may appear to be an acid, but to see why it is not, consider the reaction for removing a hydrogen ion:
$$CH_3^{-} \rightarrow H^{+} + CH_2^{2-}$$
This transformation makes the charge of the molecule further from zero, making it unlikely to occur. The resulting ion only has two bonds with carbon, rather than the usual four. We will revisit this ion in another section of the article.
A monoprotic base will accept a hydrogen ion from an acid, but only one hydrogen ion. If it commonly accepts multiple hydrogen ions, it is a polyprotic base.
Example 4: The molecule \(CH_3^{-}\) is a monoprotic base. It will accept a hydrogen ion to give the molecule a neutral charge and a stable configuration: This configuration is stable because Carbon atoms tend to form four bonds, and this configuration allows it to do just that.
$$CH_3^{-} + H^{+} \rightarrow CH_4$$
However adding another hydrogen ion gives a non-neutral charge and an unstable configuration.
Example 5: The hydroxide ion is an exception to the usual principles of monoprotic and polyprotic bases. It will combine with a hydrogen ion to get
$$OH^{-} + H^{+} \rightarrow H_2O$$
However, even though oxygen atoms generally do not form three bonds, water will often accept another hydrogen ion to form the hydronium ion, which is very important in acid-base chemistry.
$$H_2O + H^{+} \rightarrow H_3O^{+}$$
The pH of a chemical is defined mathematically as
$$pH = -\log_{10}[H^{+}]$$
The brackets around the ion indicate a concentration, in molars, i.e. moles per liter. This type of concentration is called a molarity. If you need to find the pH of a base, use this formula:
$$pH = 14 + \log_{10}[OH^{-}]$$
Now, depending on the type of acid or base in the aqueous solution you are working with, the molarity and the hydrogen/hydroxide ion concentration will have a different relationship.
A monoprotic acid or base will only release/accept one hydrogen ion per molecule of the acid or base, so for a monoprotic acidic solution,
$$Molarity = [H^{+}]$$
And similarly, for a monoprotic basic solution:
$$Molarity = [OH^{-}]$$
This makes pH calculations for monoprotic acids and bases easier than pH calculations for polyprotic acids and bases.
Example 6: Find the pH of a .433-molar solution of hydrobromic acid.
Solution: The chemical formula of hydrobromic acid is \(HBr\). There is only one hydrogen atom in this molecule, so this is a monoprotic acid, and
$$Molarity = [H^{+}]$$
We can go right to finding the pH:
$$pH = -\log_{10}(.433 \; M) = .36$$
Example 7: What is the pH of a \(.0919\) molar solution of lithium hydroxide?
Solution: The chemical formula of lithium hydroxide is \(LiOH\). Since there is only one hydroxide ion per molecule, this is a monoprotic base, and
$$[OH^{-}] = .0919 \; M$$
Now we can find the pH with one of the formulas provided:
$$pH = 14 + \log_{10}[OH^{-}] = 14 + \log_{10}(.0919 \; M) = 12.96$$
Example 8: Explain why a \(.0599\) molar solution of barium hydroxide does not have a hydroxide ion concentration of \(.0599\) molars.
Solution: Barium hydroxide's chemical formula is \(Ba(OH)_2\). For every molecule, there are two hydroxide ions, so the concentration of the hydroxide ions is twice as high as the concentration of barium hydroxide.
The transfer of hydrogen ions is very important in acid-base chemistry, as is how many hydrogen ions can be transferred in a particular scenario. Monoprotic acids and bases, as a pair, will only support the transfer of one hydrogen ion for a pair of the molecules. This has an interesting effect on many facets of acid-base chemistry.