# Chemistry Question about ACIDS And BASES

Discussion in 'MCAT Discussions' started by christian15213, May 1, 2007.

1. ### christian15213 Banned Banned

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Ok, I was wondering if anyone can explain this phenomenon.

It is true to say PH + POH = 14...

as it is to say Pka + Pkb = 14 ... correct me if I am wrong...

So why when some Pka's are like 20 or 15 or something that is obviously not going to have a pkb that is what? -4 or something?

just confuses me a bit...

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not so weird.

pKa/pKb can be really large or really small.

just like pH/pOH can be negative, so can pKa/b

It just means that one species is particularly strong and the other is particularly weak...

4. ### christian15213 Banned Banned

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ok, but can you work out a problem, where the pka is 20 and the base pkb would be -6... cause I am not thinking of this correctly... and with that pkb... does that mean super weak or super strong???? you know what I mean?

5. ### scottj72

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I think first you really have to understand what "p" really means and how it applys to all the numbers with acids and bases. "p" is always pX = - log X
X is usually a measurement of concentration.
Example
pH = - log [H3O+] pH 4 = - log [1.0 x 10 -4]
pH 5 = - log [1.0 x 10 -5]
pH 6 = - log [1.0 x 10 -6]
When you start changing values from the base ten the exponets can be decimal values as well and makes it little harder to see the comparison. But if you understand the concept and realize 2.5 X 10 -5 > 1.0 x 10 -5 you can probably understand how :
[FONT=Arial, Helvetica, sans-serif] pH 4.602= - log[2.5 X 10 -5]

So

[FONT=Arial, Helvetica, sans-serif]pH = -log[H3O+]

pOH = -log[OH1- ]
[FONT=Arial, Helvetica, sans-serif]pKa = -log Ka Ka = [H+][A-] / [HA].
pKb = -log Kb Kb = ( [BH+][OH-] / )
.pKw = -log([H+][OH-]) (is 14 at 25c)
.
The other thing to realize is what the equations represent. Pure water is neutral. So
H2O <--> H+ + OH-
Kw = [H+] [OH-]
[H+] = [OH-] = 1.0 x10-7
Kw = [1.0x10-7][ 1.0x10-7] = [1.0x10-14]
pKw = - log [1.0x10-14]
pKw = 14

So the equation pH + pOH = 14 applies to any single solution.
If you are given a pH you are also indirectly given pOH
Solution has a pH 3 then it also has a pOH of 11
Which really means it has: [H3O+] = 1.0x10-3
[OH1- ] = 1.0x10-11
So the equation pKa + pKb = 14 deals with acid/base pairs. So a strong acid will have a weak conjugate base and a strong base will have a weak conjugate acid.
pKa = 20 (means very weak acid)
pKb = -6 ( means a very strong base)
Maybe this will make some sense to you. I believe this is a tough concept for many to actually get a grasp.

6. ### sehnsucht

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the Pka and Pkb work just like the negative log system governing pH where a higher pH = weak acid/ strong bass = low H+ solution concentration and vice versa for low pH.

thus, a high Ka means that an acid is strong because it dissociates easily in solution. its corresponding pKa value decreases with acid strength. a high Kb means that a base is a strong base. its corresponding pKb decreases with base strength.

7. ### scottj72

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To clarify why pKa 20 is weak and pKb -6 is strong.
Remember that pka = - log [H+][A-] / [HA]
so basically pKa is equal to the log of some number written in scientific notation raised to a power of X. So pKa 20 means that the value is 1.0 x 10^-20. Very small number weak concentration.
With the pKb = -6 the opposite is true. The concentration is 1.0 x 10^6. This is a large number. I believe anything 1 or lower pKa or pKb is considered strong. So the relationship is consitant.
pH/pOH/pKa/pKb lower number means stronger because higher concentration of what you are measuring.
However, actual Ka and Kb values have a direct relationship because they are an expression of the concentrations and not log values.

8. ### scottj72

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To clarify why pKa 20 is weak and pKb -6 is strong.
Remember that pka = - log [H+][A-] / [HA]
so basically pKa is equal to the log of some number written in scientific notation raised to a power of X. So pKa 20 means that the value is 1.0 x 10^-20. Very small number weak concentration.
With the pKb = -6 the opposite is true. The concentration is 1.0 x 10^6. This is a large number. I believe anything 1 or lower pKa or pKb is considered strong. So the relationship is consitant.
pH/pOH/pKa/pKb lower number means stronger because higher concentration of what you are measuring.
However, actual Ka and Kb values have a direct relationship because they are an expression of the concentrations and not log values.

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