## Conjugate Acids and Bases (Uses)

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### Conjugate Acids and Bases (Uses)

What are the applications/uses of knowing conjugate bases and acids? How do you use this information?

Sebastian Lee 1L
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### Re: Conjugate Acids and Bases (Uses)

I think we should know conjugate acids/bases because they are related to the strength of acids/bases and how salts can or cannot affect pH of a solution. The conjugate base of an acid is basically the product once the acid donates the proton (assuming Bronsted acid). For example, the conjugate base of HCl would be Cl- because this is what is left once the H+ is given to a water molecule in solution. It's important to know that since HCl is a strong acid, this means that it completely dissociates into Cl- and hydronium which are more stable. $HCl\rightarrow H^{+} + Cl^{-}$. Therefore, Cl- is not likely to accept a H+ to become HCl, making it a weak base. On the other hand, if you have acetic acid (CH3COOH), the conjugate base is the acetate ion (CH3CO2-). $CH_{3}COOH \rightleftharpoons CH_{3}CO_{2}^{-} + H^{+}$. This is a strong base because it can easily go the other way and accept a proton from water to form acetic acid and hydronium ions.

An example of a conjugate acid is the Na+ cation. This forms from the dissociation of NaOH: $NaOH\rightarrow Na^{+} + OH^{-}$. Since NaOH is a strong base, the Na+ cation is a weak acid because it is stable as is and won't accept a lone pair.

Conjugate bases of strong acids are weak bases. Conjugate bases of weak acids are strong bases. Likewise, conjugate acids of strong bases are weak and conjugate acids of weak bases are strong.

When determining if salts will make solutions acidic or basic, we need to know if the ions are conjugate acids/bases of weak or strong acids. For example, in NaCl, the Na+ is a weak conjugate acid of NaOH and the Cl- is a weak conjugate base of HCl. Therefore, they won't affect the pH of the solution since they aren't acidic or basic in nature. However, something like NH4Cl will be acidic because NH4 is the conjugate acid of NH3 which is a weak base. The NH4 is able to donate a proton in solution and make it acidic. Conversely, the acetate ion (described above) in a salt will raise the pH because it will act as a base, taking a H+ from water to form acetic acid and hydronium.