what is this "salt of..." thing? You keep saying it. What do you mean by the "salt of" something?
Most of you have learned (in high school or in your review of chemical reactions) that if you combine and acid and a base you will get a salt and water. If you never learned that, then you need to catch up. Here, let me write it out in words for you...
acid + base → salt + water
And here is a real example:
HCl + NaOH → NaCl + H2O
This is a classic and fundamental reaction. It is an acid/base neutralization. Perfectly matched amounts (1:1 stoichiometry) of strong acid (HCl) and strong base (NaOH) react 100% and form the salt (NaCl) and water. You HAVE to know this before you will ever understand the term "the salt of". Learn the chemistry and then say the words:
"Sodium chloride is the salt of hydrochloric acid and sodium hydroxide."
Get it? The two parts of a salt, the cation, and the anion, each come from a part of and acid and a base. So what makes the salt? In general, the cation will come from the base, and the anion will come from the acid.
Strong Acid + Strong Base = neutral salt Why? Because the conjugates of any strong acid or strong base has zero strength as a conjugate. This is the case of the two truly opposite ends of the ionization spectrum... 100% ionized (strong) and its opposite, 0% ionized. So mix and match all those 7 strong acids and 8 strong bases that you memorized. You can write out 56 formulas for 56 neutral salts.
Weak Acid + Strong Base = weakly basic salt Why? Because the cation of the strong base has zero strength as an acid, while the anion of the acid (the conjugate base) actually has a small amount of basicity - it's weakly basic. The much over used example of this in textbooks is sodium acetate (NaCH3COO). How about a more obscure example... OK, lets pair cesium hydroxide (CsOH, strong base) with nitrous acid (HNO2, weak acid) and we get cesium nitrite, CsNO2. Once again, let me spell it out for you in a reaction:
HNO2 + CsOH → CsNO2 + H2O
So back to our original subject... We SAY... cesium nitrite is the salt of a weak acid which is a generic term for hundreds of salts of this type. The very specific way to say it is... cesium nitrite is the salt of nitrous acid. So is it acidic or basic? Well, it certainly is NOT nitrous acid anymore. ALL the protons were stripped from the acid to make the salt. So there is NO WAY this salt can be acidic. So yes, it is basic - although WEAKLY basic. How do I know the relative strength of this weakly basic salt?
Well... that is the topic for yet another discussion on "Opposites".
Really? You noticed I left off the other combination. I just assumed you would see that the other version was a foregone obvious conclusion. Ok, ok... I'll spell it out for you again...
Weak Base + Strong Acid = weakly acidic salt Why? Well, now the cation of the weak base (the conjugate acid) is ever so slightly acidic (they are "opposites" - see other discussion) while the anion of the strong acid has zero strength as a base. How about another over used example? Well we have it and it is ammonium chloride, NH4Cl. Mix hydrochloric acid (HCl, strong acid) and ammonia (NH3, weak base) and you'll get your salt, ammonium chloride. HEY! Notice in this case you do NOT get the by-product of water because there is really no hydroxide to accept it. This time, the weak base ammonia is accepting the proton and making the conjugate acid ammonium, NH4+.
Ok, once again, how about a more obscure example of this... Soooo, lets pair perchloric acid (HClO4, strong acid) with dimethylamine ((CH3)2NH, weak base) and we get dimethylammonium perchlorate, (CH3)2NH2ClO4. And yes, I'll spell it out for you in a reaction:
HClO4 + (CH3)2NH → (CH3)2NH2ClO4
So let's say it... dimethylammonium perchlorate is the salt of a weak base which is a generic term for hundreds of salts of this type. The very specific way to say it is... dimethylammonium perchlorate is the salt of dimethylamine. So is it acidic or basic? We just protonated the base so there is nothing basic left. Now, being FULLY protonated, the newly formed conjugate acid (the dimethylammonium ion) is acidic. It will be weakly acidic. How do I know the relative strength of this weakly acidic salt?
Like I said before... that is the topic for yet another discussion on "Opposites".