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Tips for Identifying Acid and Base Strength - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

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 are some tips and tricks for identifying if an acid or base is strong or weak, and if it's an acid or a base. Here we have a list and so I have strong acids.

Strong acids are easier to remember and there's only six of them. The first 3 that I'm going to write down are HCL; Hydrochloric acid, HBr; Hydrobromic acid, and HI; Hydroiodic acid. Now if you notice the n ions here are Cl, Br and I, so they're all Halogens from group 7a. If I have a Hydrogen infront and a Halogen and it's aqueous, then we have a strong acid. Remember, strong means it dissociates completely, so it dissociates into H+ ions and then the n ions, so in this case; Cl-, Br- and I-.

All the Halogens are included except for HF, so not HF. HF is actually a weak acid and so just Cl, Br and I and it's because they have a lot of electrons. They are not as electronegative as the Fluorine. Fluorine is a special case. Anyway the other 3 are; HNO3 Nitric acid, H2SO4 Sulphuric acid, only the first proton dissociates completely, and then HClO4 Perchloric acid. So HNO3, H2SO4, HClO4, and so those 3 are also strong acids.

Sometimes Periodic acid HIO4 is considered strong but not commonly. Then just one more to throw in, Hydronium sometimes is considered a strong acid. These are the your 7 that you would know and the easiest way for identifying them, is to memorize them. If you memorize the strong acids, then that will make things pretty easy.

Now, moving over to the strong base. All the strong bases, if you notice LiOH. We have NaOH Sodium Hydroxide, we have KOH Potassium Hydroxide, RbOH Rubidium Hydroxide, and then CsOH. Note which group that the cation comes from, these are all group 1A cations. Then they all have Hydroxide in there because strong bases have the Hydroxide. All the group 1A cations, the metal cations plus Hydroxide those are all strong bases. Then, moving over to group 2A, if we go down the list Calcium Hydroxide, Strontium hydroxide and Barium Hydroxide. This group 2A cations with Hydroxide are considered as strong bases they have limited solubility.

Now Beryllium and Magnesium, which are the first two elements in group 2A, they are not strong bases so they don't count. Just these 8 here, so once again you memorize these then your sitting pretty right there. These are the strong bases, the pattern group 1A cations plus a Hydroxide or group 2A cations plus a Hydroxide, but only these three that are listed here.

Now moving down to the weak acids. When you have a weak acid, you have basically any acid with a formula, and commonly you usually see them with an H in the front. So starting with a H in the formula, so Hydrogen, because that's where the Hydrogen gets donated, the H+. You might see it like in HF as an example, or in HC2H302, acetic acid. You have a H in the front and it's not one of those strong acids. Let's see, you generally have the H plus you have the n ion. Here we have Hydrogen and we have Fluoride, here you have Hydrogen and you have Acetate ion.

Or, the other option would be if it ends in COOH. Like CH3COOH and that's also Acetic acid and notice how this is the proton or the H+ that gets donated, when the acids donate their proton. So it would be like that. Weak acids generally have a Hydrogen in front or an COOH and that last Hydrogen, is the one that gets donated.

Then we also have weak bases, which is the last flavor that we are going to take a look at. Weak bases are basically any base with an Amino group that you have there. You may have seen the word Amino like an Amino acid, like in your Biology class. An Amino group is something like NH2 or has some variation. A variation can be like N or NH, hence we have those there. Examples of weak bases are like N2H4 or CH3NH2 Methylamine. Here there's Hydrogens here, but if you notice, there's some variation of an Amino group with the Nitrogen there.

Weak bases you have those, and then generally, remember they accept the protons so they can take an H+. Just to summarize strong acids and strong bases; the strong acids are only these 6 or 7 here that are listed. The strong bases are only these 8 that are listed from group 1A and group 2A cations plus a Hydroxide. If you memorize those, then weak acids and weak bases, they are pretty easy to identify. Weak acids have an H in the front or ending COOH and then weak bases have an Amino group, something with a Nitrogen usually.

Keep in mind, not everything is an acid or a base. Some things are just plain old ionic compounds and so make sure that if you take a look at these, that they follow these rules here. Hopefully these tricks help you identify whether an acid or base is strong or weak, and it will help you with acid base chemistry a little more. Have a good one.