## conjugate base and pH

Katie Bart 1I
Posts: 104
Joined: Sat Aug 24, 2019 12:16 am

### conjugate base and pH

Why doesn't the conjugate base of a strong acid affect pH?

Sebastian Lee 1L
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Joined: Fri Aug 09, 2019 12:15 am
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### Re: conjugate base and pH

The conjugate base of a strong acid will not affect the pH because it is weak and stable. For example, if you have a Cl- anion in a salt, that is the conjugate base of HCl, a strong acid. Remember that strong acids will completely dissociate into ions so for HCl: $HCl\rightarrow H^{+} + Cl^{-}$. Cl- does NOT turn back into HCl because it is stable as it is in solution and HCl is a strong acid that will dissociate anyways. So if you had a salt like NaCl, the Cl- anion doesn't affect pH because it doesn't react with the water molecules to form OH-. The reaction $Cl^{-} + H_{2}O \rightleftharpoons HCl + OH^{-}$ does not actually form so the conjugate base of a strong acid does not make the solution more basic.

On the contrary if you had a conjugate base of a weak acid (like acetic acid), that would increase the pH. For acetic acid, the CH3CO2- conjugate base is strong enough that it will take a proton from water and form OH- ions in solution. Remember that in acetic acid, the reaction goes both ways so it is definitely possible for the acetate ion to bond with a hydrogen to form acetic acid and OH- ions (making it more basic).

Posts: 58
Joined: Thu Jul 25, 2019 12:15 am

### Re: conjugate base and pH

Katie Bart 1H wrote:Why doesn't the conjugate base of a strong acid affect pH?

The conjugate base of a strong acid is a stable compound that is also a weak base. As it is a stable compound, it does not have much charge and will not prompt further electron or proton transfer, therefore not affecting the pH

brennayoung
Posts: 51
Joined: Fri Sep 20, 2019 12:17 am

### Re: conjugate base and pH

Katie Bart 1H wrote:Why doesn't the conjugate base of a strong acid affect pH?

The conjugate base of a strong acid is a stable compound that is also a weak base. As it is a stable compound, it does not have much charge and will not prompt further electron or proton transfer, therefore not affecting the pH

So if this was a strong base and unstable would it then affect the pH?

Sean Cheah 1E
Posts: 105
Joined: Wed Sep 18, 2019 12:20 am

### Re: conjugate base and pH

The seesaw relationship between the relative strengths of a conjugate acid/base pair states that the conjugate base of a strong acid will be a very weak base and the conjugate acid of a strong base will be a very weak acid. In these cases, the conjugates are so weak that they have basically no effect on the pH. On the other hand, with weak acids and bases, the resulting conjugates do affect the pH because the reaction will not go to completion and will instead settle into some kind of equilibrium.