# Relation between strength and proticity of an acid

The strength of an acid is related to the number of molecules which have dissociated into hydronium ions in aq solution of that acid while proticity is the number of hydronium ions furnished by 1 molecule of that acid in water. But what is the relation between the two. Does a strong acid have a high proticity or vice-versa?

• There's no real connection. Higher proticity can make a compound slightly more acidic but it depends on what you compare with what. Also that kind of usage of "basicity" is misleading. – Mithoron Jun 8 '16 at 22:11

I'm a chemist. I've never heard the term "proticity". If your definition of it is correct, then consider phosphoric acid, H3PO4. It has a proticity of 3. Is it a stronger acid than HCl or H2SO4? There's really NO useful relationship I can see between strength and the number of donate-able protons. The term "acid" has three of four different meanings. The three most common are Arrhenius, Brønsted, and Lewis acids, but note that there are other definitions (usually used in very specialized contexts). The difference between Ar. and Br. is that Arr. applies only to water as the solvent while Br. applies to any compound which will donate an H(+) ion (solvent unspecified and possibly without any solvent present), the most general definition, L., has to do with compounds (or ions) which accept electron density, so it is often used in descriptions of covalent (not (necessarily) ionic) reactions. You can look up the dissociation constants for each of the three HnPO4 species which can give up an H(+) ion. None of them are "strong" acids, on the other hand H2SO4 is a very strong acid while H2CO3 is a very weak acid. (If phosphoric acid were strong, those of us who drink carbonated soda wouldn't have any teeth left since they add it in order to increase the solubility of the carbonate ion (CO2 +H2O → HCO3(+) + OH(-)...you do the mass balance! (it's incomplete the way I've written it). Both sulfuric and hydrochloric acids can easily have pHs well below 0. (anhydrous HCl is a gas, and H2SO4 is often sold as 98% H2SO4 in water (what's the molarity of that?!)). You can also have NaOH solutions in water with a pH well above 14, but don't tell your teacher, s/he may argue with you (but I know better!).(although to be truthful, at some concentration NaOH(aq) is no longer usefully thought of as a "water solution",(its more a solution of water IN NaOH (or in H2SO4)).

• You should really consider breaking this up into paragraphs. It's hard to look at. – ringo Jun 9 '16 at 2:36

I tend to think of acids as proton donators and bases as proton acceptors. You are correct that an acid is anything that increases the concentration of $\ce{H+}$ in solution. A general acid will have the form $\ce{HA}$, which after losing an $\ce{H+}$ becomes $\ce{A-}$. While there are a couple of different ways to calculate the strength of an acid, one of the most common methods is to see how much the acid tends to dissociate. The $pK_a$ of an acid is equal to $-log(K_a)$, where $K_a$ is the acid dissociation constant for the particular acid. The lower an acid's $pK_a$, the larger its $K_a$, and thus the stronger the acid.

While the $pK_a$ is one way to think about the strength of an acid, another thing to think about is the strength of the conjugate base. A stronger acid is one that dissociates more readily, meaning that its conjugate base is more stable. A more stable conjugate base in turn means that after losing a proton, the acid is unlikely to pick up one from solution. This means that a strong acid will create a weak base while a weak acid will create a strong base after dissociating.

• OP's asking about multibasic acids... – Mithoron Jun 8 '16 at 22:38
• chemistry.stackexchange.com/questions/16804/… also @Pearu – Mithoron Jun 8 '16 at 22:41
• This is a general explanation about acid strength. At the time of writing, it was unclear what the question wanted. – Niels Kornerup Jun 9 '16 at 15:12

For Brønsted acids and bases, a strong acid (with high acidity) means it will easily give up a hydrogen ion. A strong base (with high basicity) will easily accept a hydrogen ion. For Lewis acids and bases, a strong acid will accept electrons more easily than a weak acid, and strong bases will donate electrons more easily than a weak base.

• I'm afraid you didn't get OP's point. – Mithoron Jun 8 '16 at 22:36
• Then answer instead of writing useless condescending comments. Basicity is related to the strength of the acid. And acidity is related to the basicity of the acid. In aqueous solution at 25 degrees Celsius there is an equilibrium constant, determined by the concentration of the acid and base products, which means that acidity and basicity are always related. Compounds with weak basicity will have strong acidity, and vice versa. – Pearu Jun 8 '16 at 22:53
• OP is asking about so called "basicity" with the meaning equal to proticity i.e. about mono and multibasic acids - dissociating one or more protons. I also at first thought he was asking about "normal" basicity. Sorry for being overly laconic. – Mithoron Jun 8 '16 at 23:05