Acids will release hydrogen ions, and bases will accept them. However, if you remove a hydrogen ion from a molecule, and then add a hydrogen ion, you get the same molecule. For instance:
$$HNO_3 - H^{+} + H^{+} \rightarrow HNO_3$$
However, immediately after removing the hydrogen ion, you have a nitrate ion, \(NO_3^{-}\). A conjugate acid-base pair is a pair of chemicals whose molecular formulas differ by only one hydrogen ion. Therefore, by adding and removing hydrogen ions, you can toggle between the two chemicals in a conjugate acid-base pair.
As stated before, the two chemicals that are part of an acid-base conjugate pair will have chemical formulas that differ by only one hydrogen atom, and the charge difference between the two chemicals will be 1.
Example 1: Do the chemicals \(HClO_4\) and \(ClO_4^{-}\) constitute an acid-base conjugate pair?
Solution: The chemical formulas differ by one hydrogen atom, and the charges differ by \(1\), so this is an acid-base conjugate pair.
Example 2: Do the chemicals \(HNO_3\) and \(HNO_2^{2+}\) constitute an acid-base conjugate pair?
Solution: These chemicals are not an acid-base conjugate pair, because one chemical has more oxygen atoms than the other. Also, the difference in charge is \(2\) instead of \(1\), and both chemicals have one hydrogen atom, with no difference in hydrogen atoms, so a hydrogen atom cannot be transferred to or from one of the chemicals to get the other.
This is a slightly different and more difficult example.
Example 3: Use each of the following chemicals once to create two acid-base conjugate pairs:
$$H_3PO_4$$
$$H_2PO_4^{-}$$
$$HPO_4^{2-}$$
$$PO_4^{3-}$$
Solution: These chemicals are ordered in a special way. As you go down the list, the number of hydrogen atoms decrease, and the charge gets more negative, each in increments of 1 for each chemical you pass on the list. Therefore we have a chain of chemicals, and there are many acid-base conjugate pairs. However, we can only use each chemical one time, so pick the two chemicals with the most hydrogen atoms for one pair:
$$H_3PO_4$$
$$H_2PO_4^{-}$$
That leaves the other two chemicals, which are also a conjugate acid-base pair:
$$HPO_4^{2-}$$
$$PO_4^{3-}$$
In this example, notice that \(H_2PO_4^{-}\) and \(HPO_4^{2-}\) are also a conjugate acid-base pair, but the other two chemicals are not, so this separation of the chemicals does not fully solve the problem.
A conjugate acid is a chemical that results from adding a hydrogen ion to a base. A conjugate base is a chemical that results from removing a hydrogen ion to an acid. A conjugate pair can be either a conjugate acid and a base, or an acid and a conjugate base; really there are no special properties that distinguish these two groups from each other aside from the terminology.
Example 4: \(HClO_2\) is an acid. If you remove a hydrogen ion from it, you get its conjugate base, the chlorite ion, which has the chemical formula \(ClO_2^{-}\).
Example 5: \(CO_3^{2-}\) is a base. If you add a hydrogen ion to it, you get its conjugate acid, the bicarbonate ion. Its chemical formula is \(HCO_3^{-}\).
Acid-base conjugate pairs are important in acid-base chemistry because the two chemicals differ by only one hydrogen ion. The hydrogen ion is a centerpiece of acid-base chemistry for this very reason-many reactions involve two or more conjugate pairs, which means that the transfer of hydrogen ions will occur.