once and for all: What is the difference between pH and pKa. in words pleaseee!

[puffylover]

I never understood this. Every time I've asked someone, they've just replied with the henderson-hasselbach equation and said pH=pKa when the acid and conjugate base are in equal concentrations.

To make myself more clear, does pH describe a solution only? And pKa only describes an acid or base? What about salt?

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[RogueUnicorn]

the pKa is a PROPERTY of an ACID that describes how a solution of the acid would affect the pH. it is merely a restatement of the equilibrium of dissolution for an acid.

 

[puffylover]

woops. acid.

sounds good. for now... lol. i'll see if any questions mess me up again

 

[unique135]

Agree with bleargh and apologize if my explanation doesn't help you.


Ka = [H+][A-]/[HA] ...................Ka = Acid-Ionization Constant/Acid Dissociation Constant
Ka = [H+]. ([A-]/[HA])
-logKa = -log[H+] - log([A-]/[HA])...............using rule log(A.B) = log A + log B
pKa = pH - log([A-]/[HA])...............Henderson-Hasselbach equation.

The point here is that
1. pKa represent Ka
2. pH represent concentration of H+
3. Henderson-Hasselbach equation is derived from simple equlibrium equation.

Who describes Who?
Properties of an acid such as polarity, bond strength, and stability of conjugate base describe pKa. pKa describes pH. (For instance, adding HCl to water: pKa will be low and pH will be low.)


Similarly, there is Kb: Base dissociation constant.

pKa can help you find pKb that describes base by formula:
pKa + pKb = pKw
pKa + pKb = 14

Salts can have their own pKa or pKb. This depends on whether salt is acidic, basic or neutral in nature.

 

[homestar]

I believe that one of the problem arises because pH's cannot be directly calculated for weak acids and bases based on the molar concentrations of the acid or base--they do not completely dissociate.

Understanding what the p means is a bit of a key to the mess. The p in pKa, pKb, pH, and pOH simply means: "the negative log of something."

pKa's and pKb's are commonly stated for weak acids and bases whereas pH's are commonly stated for strong acids and bases. This can be tricky, because it ties in the concept of Keq's (equilibrium concentration expressions), Bronsted-Lowry conjugate pairs, acids/base chemistry and logarithms. If you don't understand Keq's, then I would look into this further--it is a key part of the MCAT, as far as I'm concerned.

In terms of numbers, a low pKa means that the concentration of protons that the weak acid contributed is relatively high. A high pKa means that the concentration of protons that the acid contributed is relatively low. To understand this, you need to understand what an equilibrium concentration expression means, and what a negative logarithm does.

pH is a similar monster, but it is usually used for strong acids and bases.

Best

 

[kentavr]

My 2c

Correct me if I am wrong, but that's how I understand it.
1. pKa(=-log(Ka)) is a constant for given acid and given temperature. pH(=-log[H]) is not.
2. The point that Ka is a constant means that whatever volume of acid you place in any volume of solvent does not affect Ka. The fundamental fact is that for Rx like HA <-> H+ + A- the ratio Ka=[H+]*[A-]/[HA] is always a constant.
However the [H+] depends on volume of solvent and acid. More solvent means less [H+] since it is moles divided on volume.
3. Let say we take 1 mole of HCl and place it in 1L of water. Then HCl disassociates completely(not exactly true but for simplicity). Then Ka is 1*1/0 = infinite. However [H+]=1. and pH=-log(1)=0. Let say we get 10 moles of HCl then the same math gives us that Ka still infinity, but pH = -log(10)=-1 etc.
4. Let's clarify this infinity Ka. As I said before, the HCl will not be completely disassociated. Small part still be binded. According to the standard table pKa for HCl is -8. Or Ka = 10^8. It means that it is very large number and we are not very wrong in saying that it's almost completely disassociates with Ka close to infinity. More realistic Ka exists for weak acids, but the math stays the same.

 

[ABCZXX]

LOL at the serendipitous timing. I just spent a week straight learning this stuff and reviewing it. You can PM me OP and I will go back and forth with you until you understand it completely. I would say that at this moment I am a pH and pKa guru, and would even challenge Bleargh on his expertise! (but I <3 Bleargh because I love star-trek and hate his new avatar!)

As an aside, one of the reasons that the concept is confusing is because water is a reactant, yet it is omitted from the equilibrium equation. Once you suspend this covenant, and work a few acid/base problems; and, actually use water in the equilibrium expression (55 moles-per-liter) then you will be able to learn acids/bases more easily. Once you learn acid/base equilibria, then you can return to omitting water as a reactant from the equilibrium expression, but at least you'll understand acids & bases. I am an acids/bases expert and would not say that in a "gunners forum" like this, unless I can defend my ego. 😉

 

[RogueUnicorn]

i humbly defer then, to ABCZXX.. clearly i am outmatched. i suggest OP that you take advantage of this willingness to teach on his/her part, since pKa and pH has felled many a worthy premeds..

 

[boaz]

pH and pKa are both "p functions." A "p function" just means converting "x" to "-log(x)". In pH, x=[H+]. In pKa, x=Ka (the acid dissociation constant).

That is the fundamental difference.

 

[ABCZXX]

i humbly defer then, to ABCZXX.. clearly i am outmatched. i suggest OP that you take advantage of this willingness to teach on his/her part, since pKa and pH has felled many a worthy premeds..

Awesome! I can't believe I'm acknowledged by Bleargh. 🙂 He got my "dream score" and I'll be taking the MCAT on May 1st. One of my favorite SDN'ers also had trouble with acids/bases and I plan to do a full write-up on this confusing topic with as good of quality as QofQuimica and Shrike as seen in the following topics-thread: http://forums.studentdoctor.net/showthread.php?t=516521

I also aspire to be a MCAT teacher some day since I loathe the pedagogy of conventional textbooks and MCAT course books alike. Look at the Wikipedia article of Resonance Hybrid and see how intuitive and natural the explanation is? Read the Vector analogy and tell me awesome and intuitive that explanation is! I have sent the OP and lengthy PM trying to teach him calculating Ka but using water as a reactant so that way he understands from the explanation "products over reactants" then I'm showing him to remove water from the equilibrium expression and that's how easy it is to calculate Ka.

To go from Ka to pKa is you take the negative log (base 10) of the Ka.

That sentence makes clear sense to me and bleargh, but is not much help to the OP. I am not kidding that I spent a week learning this stuff and it is all super-recent. I was initially motivated to help my favorite SDN'er so I have triple motivation to make a "Ka and pKa once and for all thread". Oh and I'm a nerd and a Wikipedian. If my writeup is good enough, I'll upload it to the SDN wiki! I would post the PM to the OP but it might be confusing. I'll modify the PM I sent and make sure it is very clear and methodically explained.

 

[RogueUnicorn]

Awesome! I can't believe I'm acknowledged by Bleargh. 🙂 He got my "dream score" and I'll be taking the MCAT on May 1st. One of my favorite SDN'ers also had trouble with acids/bases and I plan to do a full write-up on this confusing topic with as good of quality as QofQuimica and Shrike as seen in the following topics-thread: http://forums.studentdoctor.net/showthread.php?t=516521

I also aspire to be a MCAT teacher some day since I loathe the pedagogy of conventional textbooks and MCAT course books alike. Look at the Wikipedia article of Resonance Hybrid and see how intuitive and natural the explanation is? Read the Vector analogy and tell me awesome and intuitive that explanation is! I have sent the OP and lengthy PM trying to teach him calculating Ka but using water as a reactant so that way he understands from the explanation "products over reactants" then I'm showing him to remove water from the equilibrium expression and that's how easy it is to calculate Ka.

To go from Ka to pKa is you take the negative log (base 10) of the Ka.

That sentence makes clear sense to me and bleargh, but is not much help to the OP. I am not kidding that I spent a week learning this stuff and it is all super-recent. I was initially motivated to help my favorite SDN'er so I have triple motivation to make a "Ka and pKa once and for all thread". Oh and I'm a nerd and a Wikipedian. If my writeup is good enough, I'll upload it to the SDN wiki! I would post the PM to the OP but it might be confusing. I'll modify the PM I sent and make sure it is very clear and methodically explained.

yikes. i appreciate the sentiment, but i'm just a dude on a forum.... i applaud your enthusiasm to help people out though 👍 pKa is a bedevilling subject

 

[Econ2MD]

I came across a statement in my biochem text I thought is helpful.

pKa is the pH at which an acid half dissociates


(i.e. equal concentrations of acid and conjugate base)

It's a measure of how tightly the acid likes to hang onto its proton (H+)

Using HH equation, the log(A-/HA) drops out, equating pH and pKa

 

[ABCZXX]

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[Steelersfan2009]

you are not Sal from Khan academy are you?

 

[ABCZXX]

you are not Sal from Khan academy are you?

No, but I think so highly of him and also of MIT OpenCourseWare since I am 100% self taught. I have sent a thank you email to Sal and he wrote me back. 😀 There are great learning materials online nowadays. Wikipedia is very good also. Their explanation of Solvent-System definition of acids & bases is when a light bulb went off in my head. I hope that all matters resolving pKa and pH are once and for all settled! 😛

Feel free to shoot me a PM anytime between now and May 1st 2010 (my Mcat). Good luck everyone.

 

[fizzgig]

maybe the horse has been beaten to death, but to reiterate in words vs math (though i guess the math is needed on the mcat? that's farther away for me).

pka is, as others said, a constant that is associated with an acid (the pka, at standard temp and pressure i guess, is not going to change). when the pH of a solution is equal to that pka, half of the acid's H+ is associated and half is dissociated.

when the pH gets lower than that pka (by dumping in HCl for example, which adds H+ to the solution), some of the dissociated H+ reattaches to A- to form HA, effectively bringing the pH back up slightly, closer to the pka.

when the pH gets higher than the pka (dumping in NaOH, removing some H+ from the solution), some of the HA dissociates, adding H+ to the solution, acidifying it, and bringing the pH back down a bit, closer to the pka again.

the importance of all this is seen in the weak acid/base chemistry we study, and is of course most significant near physiological pH, where you can see how the above actions act as a buffer. at pka=pH, there's plenty of HA as well as H+/A-, and it's at its happy point, happy to do more dissociating or associating. if you disturb the balance and shift the pH away from the pka by adding strong acid or base, the acid solution re-establishes equilibrium by adding or removing H+ from the solution, bringing the solution back towards the weak acid's happy point.

the happy point itself is determined by how well H+ and A- stick together. if you've got 100 molecules of HA constantly breaking up and getting back together, then the pka is going to be determined by how likely HA is to stick together longer when a collision occurs between a free floating H+and A-.

anyway, lots of good math stuff involved but thought i'd throw a verbal explanation out there. corrections welcome if i'm wrong - been a long time since general chem.