## What exactly are conjugate acids and base? [ENDORSED]

Jennifer1E
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### What exactly are conjugate acids and base?

I understand how to find the conjugate acids and bases but I am having a difficult time understanding what they are conceptually. What are conjugate acids and base?

Sarah_Wilen
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### Re: What exactly are conjugate acids and base?  [ENDORSED]

A conjugate pair of molecules refers to two molecules that have identical molecular formulas except that one of them has an additional H+. Here are some examples: HCl and Cl-, H2O and OH-, H2PO4- and HPO42-, Na+ and NaOH. The member of a conjugate pair with an extra H+ is called the conjugate acid and the one with fewer is the conjugate base.

In other words, in an acid-base reaction, the original acid gives up its proton to become a conjugate base. After losing its proton, the remaining ion is capable of gaining a proton, hence qualifying as a base. The original base accepts a proton, so it now is classified as a conjugate acid since it can release his newly acquired proton and thus behave like an acid.

main-qimg-95dd29dc9674e0c9c148b1a4fb325682.png (8.07 KiB) Viewed 1004 times

Here's how to calculate the equilibrium constants (K) for weak conjugate acid/base pairs. The sum of the pKa and pKb of a conjugate pair of a weak acid and weak base must always be equal to 14 at 298 K. Here is how I know that:

I am going to give an example of a conjugate pair of a weak acid and weak base: NH4+ and NH3. I will write out the balanced dissociation reactions for each in water.

Also, an important tip: just because a molecule is called conjugate acid or base doesn't mean it's actually acidic or basic in solution. For example, we take OH- and O2-. We see that by definition, hydroxide is the conjugate acid of the pair, but we know, hydroxide is actually a base in solution.

yay

805070989
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### Re: What exactly are conjugate acids and base?

When solving for a problem dealing with acids and bases will it always state if it is dealing with either a strong or weak acid or base?

Sarah_Wilen
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### Re: What exactly are conjugate acids and base?

I don't think the problem will ever explicitly say that you are dealing with a strong or weak acid or base, but if you know which acids and bases are strong, then through the process of elimination, you will be able to tell if the acid or base you are given is strong or weak.

Here are the strong acids we look at most often:
HBr (hydrobromic acid)
HCl (hydrochloric acid)
HI (hydriodic acid)
HNO3 (nitric acid)
HClO4 (perchloric acid)
HClO3 (chloric acid)
H2SO4 (sulfuric acid)

Here are the strong bases we look at most often:
Group 1 hydroxides
Group 2 (alkaline earth metals) hydroxides
Group 1 and 2 oxides

The weak acids and bases are the ones that aren't on that list.

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### Re: What exactly are conjugate acids and base?

Does having a strong acid mean having a weak base? and vice versa? Thank you in advance!

Sarah_Wilen
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### Re: What exactly are conjugate acids and base?

The word strong in front of acids and bases means that the compounds dissociate COMPLETELY into their ions. On the other hand, weak acids and bases do not completely dissociate, so the dissociation reaction is reversible. I think you are asking about conjugate acids and bases. If there is a strong acid, it will have a conjugate base, same for weak acids. If there is a strong base, it will have a conjugate acid, same for weak bases.

Weak acids have a greater pH than strong acids because their dissociation constant is lower than the strong acid's.
Weak bases have a greater pOH than strong bases because their dissociation constant is lower than the strong bases'.

Let me know if I can clarify anything for you! :)

Sarah_Wilen
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### Re: What exactly are conjugate acids and base?

Oops, mistake. You are right. I didn't realize what you were asking. Yes, relatively speaking, strong acids have weak conjugate bases. Strong bases have weak conjugate acids.

For example, HCl is a strong acid. Its conjugate pair is Cl-, which is a weak base.
HCl(g) + H2O(l) <-----> H3O+(aq) + Cl-(aq)

On the other hand, strong bases have a weak conjugate acid.

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### Re: What exactly are conjugate acids and base?

Thank you for the clarification Sarah :)

Sarah_Wilen
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### Re: What exactly are conjugate acids and base?

A way to quantitatively show the relationship between the strengths of conjugate acid-base pairs is like this...
We know that $[H_{3}O^{+}][OH^{-}=K_{w}]$
So it is only right that if the concentration of the acid is very high (strong acid), then the concentration of the base must be lower to make up for that. And vice versa too.