Another way of looking at the acid strength is to look at the percent ionization of weak acids.
What we can look at is the H_3O+ concentration at equilibrium
or remembering that the same thing we've been calling the H+ concentration.
those mean exactly the same thing. Divided by the initial HA concentration
and multiplied by 100. The bigger the value of the percent ionization
the stronger the acid
and vice versa. The smaller the percent ionization
the weaker the acid. Let's look at an example of how we can use this
information to figure out the pH of the solution.
Here we have HA in equilibrium with H+
and A-. It is 0.350 molar weak acid that is 13.2 percent
ionized and we want to find the pH of the solution.
We have percent ionization
is equal to H+ at equilibrium over HA initial times a hundred.
I could plug in the values I know is equal to H+ at equilibrium over HA
initial times a hundred.
I could plug in the values I know
13.2 percent equals H+ it equilibrium
over 0.350 initially
times 100. Now I can rearrange
and solve for my H+ concentration at equilibrium
and find that is 0.462.
That gives me the concentration it does not yet give me
the pH value. I have to take the negative log of that number
to find the pH value which is 1.335.
We have three-digit in our decimal place and the mantissa of the log
because we have three significant figures in our concentration.
When I look at a mixture of strong and weak acid
I have a couple of things to consider. First of all the strong acid
When I look at a mixture of strong and
weak acid
I have a couple of things to consider. First of all the strong acid
completely dissociate or ionize.
Because I go from HCl to producing H+ and Cl- and all of my HCl will dissociate.
So I have a fairly large concentration of H+ present.
When I look at the HA, if I just had the weak acid by itself
I would see the formation of some H+ and some A-.
This number even by itself is going to be a small number.
So compared to this H+ that I get from the acid.
However because the week acid process is in equilibrium process
and we've got our equilibrium arrow there. It is an equilibrium process.
What we're going to see is that the present at the H+ from the strong
acid ionization
will actually drive the equilibrium towards the left
for the weak acid and as a result the small amount of H+ I actually had
present from dissociation the HA
will actually get smaller in this mixture with a strong acid.
So what I find is that the primary H+ source
will be the strong acid and the amount of H+ produced from that weak acid
is so small
that it becomes insignificant and I can ignore it. Now from looking at a mixture
of two or more weak acids.
I have multiple reactions going on and just like I did with the strong and weak acids
I'm gonna find the strongest have those acids. if there are guys are fairly
different from one another say we have K_a values of 10^-3
10 ^ -5 and 10 ^-9
those K_a values are far enough separated from one another
that I really only have to look at the one that has the K_a of 10^-3.
And I will do all my calculations as if that were the only one at present.
Now I will a look at an example which one of the following is associated with the acid?
Now note these all have K_a values so they are all weak acids.
So we're asking which one is the strongest week acid at these four.