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Tips for Conjugate Acid and Base Formulas - Concept

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Teacher/Instructor Jonathan Fong
Jonathan Fong

U.C.Berkeley
M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here's some tips and tricks for writing the chemical formula of a conjugate acid or a conjugate base from the original base or acid. Say for example, if I start off with an acid, remember acids basically match up with a conjugate base. What happens is, let's say for example if we have say HF; Hydrofluoric acid, and what we do is we put that in water. We'll draw our equilibrium, our reversibility yield sign because this is a weak acid. And so, what ends up happening is since HF is the acid, water is acting like a base. What happens is an acid is proton donor and so it will donate a H+ to the water. And so you end up with Hydronium or H3O+ plus and then you end up with F- Fluoride.

What happens is the HF basically became the F-, and so the F- is the conjugate base. The shortcut, because everyone likes shortcuts is this; If I go HF and I have the acid and I want to get to it's conjugate base, all I do is I subtract an H and a + from it, so I take away that. Basically I have HF I take away the H and then I basically subtract 1 from the charge; -1 from charge, and then so what I'm left with, is I'm left with F-.

Another one is say, say if I have HSO4- and we're told that's an acid. Then we want to get the conjugate base. All I do is I take away an H so I'm left with the SO4 and then I take away 1 from the -1 so I'm left with -2 or 2- so I end up with SO4-2. That's a short cut for going from an acid to it's conjugate base.

Now on the flip side, base to conjugate Acid. Say if I have a base let's say I have Ammonia or NH3 and that's on water so we use our double yield sign. Water is acting like an acid in this case. Water is going to donate a proton to the NH3. Now if I have my products I end with NH4+ plus and I get Hydroxide or OH-. The NH3 is paired up with NH4+ because basically that's what it changes into by adding the H+ and so this is the conjugate acid of that. Just a side note water is an acid in this particular equation and then in the previous equation we used it as a base. Something that can act like and acid or base is called Amphoteric because it can either donate or accept a proton.

Here's shortcut for going from a base to a conjugate acid, because everyone likes shortcuts again. So we go NH3 and then what we do is in this case we add an +H so that should be pretty easy. We have the base and then we'll add an H+ so we get NH4+. Then this is the conjugate acid.

Another example could be like say if I have CH3NH2, Methylamine. This is a base. So I add a +H. Now where do I add it to? Well the Carbon would already be bonded up with 3 Hydrogens and the Nitrogens so I can't put it there. Usually I put it on the end. So I'll put it here on the end with the Nitrogen. Keep in mind whenever you're adding the H+, you're going to add it usually to where the Nitrogen is. I end up with CH3NH3 (now instead of 2 I add one more Hydrogen so I have 3) and then I add the + charge and then this is the conjugate acid of this base.

Hopefully these shortcuts help you figure out what the formula of a conjugate base would be from an acid or the conjugate acid from a base.