K+ it has the parent basis KOH. Cl- has the parent acid at HCl.
Because both the parent acid and base our strong
we will see no reaction happening with water and as a result we will get no
production of H+ ions
or hydroxide ions. Therefore if a substance has a strong parent acid
and a strong parent base, we will end up with a neutral salt.
And in fact this is what we see. If we took the pH
a KCl solution we would find that the pH
is neutral. Now
look at an good example of KF. Here we have the same
parent base, our KOH because we have a K+ ion.
It will not react with water so we don't see the production of any H+
however we see F- whose parent acid is HF
a week parent acid, F- will react with water to produce
HF and OH-. Because there's some residual
OH- present in the mixture in the solution
HF and OH-. Because there's some
residual
OH- present in the mixture in the solution
we will see that we have a basic salt.
Let's look at example a NH_4Cl. We have NH_4+ which has a parents base of NH_3, a weak base.
So we will get the production of some H_3O+ in solution.
Cl-, as we've seen before, has a parent acid of HCl
and therefore it will not react with water and not produce and OH- in solution.
Therefore when we combine NH_4+ and Cl- in a compound
we get that we have an acidic salt.
Now we only have one more scenario to go where we have a week parent acid
and a weak parent base. So we'll look at the example
of NH_4F. So NH_4+
will produce some H_3O+ in solution because it will
donate a proton to form some NH_3, a weak base.
F- has a parent acid of HF, so it's a weak acid.
and we will get a OH- to form. However because we have these competing factors
of some H_3O+ plus forming
and some hydroxide forming we don't know whether the salt is going to be acidic or basic.
What we will have to look at are the individual K_a and K_b values
for the two substances. What we will have to look at now
is compare the K_a and K_b values of the parent acid and base.
I'll compare the K_a value of HF to the
K_b value of NH_3. The one that is larger will dominate and will determine
I'll compare the K_a value of HF to the
K_b value of NH_3. The one that is larger will dominate and will determine
the pH of the salt.
So if NH_3 has a K_b value that is greater
than the K_a of HF I will find that this is a basic salt.
Conversely if I find that the K_a value of HF
is greater than the K_b of value of NH_3 I find that I have an acidic salt.
So until I know those values I cannot determine the acidity or basicity
of a NH_4F.
Now let's look at an example. Which have these will behave as an acidic salt?