What is difference between pH and pKa , pOH and pKb?

In summary, pH and pKa refer to the same concept, but pOH and pKb also. pH and pKa were originally intended to measure "acidity", but as the definition of acid morphed, the measurement preference changed from pH to pKa.
  • #1
donaldparida
146
10
You have to show your attempts at answering the question, this is a forum policy. This is just a case of checking definitions.
Do pH and pKa refer to the same concept? The same question for pOH and pKb also.
What does kWh actually mean. In have read that it is the ionic product and is the product of [H+] and [OH-] but what does it indicate. Does it have any full form?
 
Chemistry news on Phys.org
  • #2
This is not a complete answer, but pH and pOH are more or less inverse. The lower case p referred to "partial pressure of", and H vs OH was for Hydrogen and Hydroxide (acid vs base). pKa has come along much later, and is generally considered to be a better measure of acidity, but I can't define it better than that. Both pH and pKa were originally intended to measure "acidity", but as the definition of acid morphed, the measurement preference changed from pH to pKa.

-Jeff
 
  • #3
pH and pKa measure different things.
pH is a property of a specific solution. pKa is a fundamental property of a substance.
The definition is that pKa is the pH at which half of the acid is dissociated.
 
  • #4
Not very surprisingly pKa has the same relation to pH as Ka has to H - or really to [H+]...

I have only seen the constant that you wrote written as Kw. It is defined as the product of [H+] and [OH-], which is constant for aqueous solutions (whatever the pH).

But this is not the place to find out about these things - they are in any elementary physical chemistry textbook and loads of online sources with the same coverage. Read a textbook and if you have any difficulties, If something does not make sense to you or you can't do the problems (having tried) then come here. We can only supplement textbooks we cannot write them, we cannot really be as good as they are.
 
  • Like
Likes Bystander
  • #5
Since pH is such a dominate reference for acidity and alkalinity in many areas of science it may be instructive to review the pH values and ranges of different chemical systems in nature. Here is a reference sheet I've passed out to my classes over the years and find it is a good basis for many interesting discussions. It is not by any means complete, but represents some common systems the student may have heard about.pH values and ranges for natural systems.*

Classification of soil pH ranges*

The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows: [17]
upload_2016-10-28_14-6-31.png


Living systems*
upload_2016-10-28_14-6-31.png


Seawater
See also: Ocean acidification

The pH of seawater is typically limited to a range between 7.5 and 8.4.[18] It plays an important role in the ocean's carbon cycle, and there is evidence of ongoing ocean acidification caused by carbon dioxide emissions.[19] However, pH measurement is complicated by the chemical properties of seawater, and several distinct pH scales exist in chemical oceanography.[20]

*From => https://en.wikipedia.org/wiki/PH
 
  • #6
  • #7
Why would negative pH complicate anything?
 
  • Like
Likes Bystander
  • #8
It would if you were taught, and 'believe', that the pH range goes from 0 to 14
 
  • #9
CapnGranite said:
'believe', that the pH range goes from 0 to 14
"pH = 0" corresponds to what concentration?
 
  • #10
Are you asking me because you would like to know?
 
  • #11
CapnGranite said:
Are you asking me because you would like to know?
What's the pH of a 10 M HCl solution?
 
  • #12
I think that's a bit condescending and patronizing. I don't do pissing contests. I'm sorry I entered your domain of homework helper and science advisor. I suggest you google the answer, or better yet, try to work it out for yourself.
 
  • #13
@Bystander
FWIW - negative pH is a real value and has use in Soil Science for example.

pubs.acs.org/doi/pdf/10.1021/ed083p1465
 
  • #14
CapnGranite said:
It would if you were taught, and 'believe', that the pH range goes from 0 to 14

Sure, but that's just a common misconception, that probably comes from the fact people don't care about how the logarithm works. Sadly, in popular thinking math is considered as not needed for chemistry, add to that fact that most people ignore the obvious connections, and you have a disaster waiting to happen. I still don't consider negative pH a complication - it is a way people ignore what they already know that produces problems.

And Bystander was just trying to force you to find out by yourself that pH values outside of 0-14 are obvious when you combine the scale definition with real life examples, nothing condescending nor patronizing about it. It is called a Socratic method and that's a way we work here.
 
  • Like
Likes donaldparida and Bystander

What is the difference between pH and pKa?

pH is a measure of the acidity or basicity of a solution, while pKa is a measure of the strength of an acid. pH is a unitless value on a logarithmic scale, while pKa is a constant specific to each acid.

What is the difference between pOH and pKb?

pOH is a measure of the hydroxide ion concentration in a solution, while pKb is a measure of the strength of a base. Like pH, pOH is also a unitless value on a logarithmic scale, while pKb is a constant specific to each base.

How are pH and pKa related?

pH and pKa are related through the Henderson-Hasselbalch equation, which describes the relationship between the pH, pKa, and the concentrations of acid and conjugate base in a solution. This equation is pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid.

Which is more important, pH or pKa?

Both pH and pKa are important in understanding the properties of an acid or base. pH is important for determining the acidity or basicity of a solution, while pKa is important for determining the strength of an acid. Both values are necessary in understanding the behavior of acids and bases in different environments.

Can pOH and pKb be used interchangeably?

No, pOH and pKb cannot be used interchangeably. While they both measure the strength of a base, they do so in different ways. pOH measures the hydroxide ion concentration, while pKb measures the equilibrium constant for the reaction of a base with water. However, they are related through the equation pOH = pKb + log([B]/[BOH]), where [B] is the concentration of the base and [BOH] is the concentration of the conjugate acid.

Similar threads

Replies
4
Views
964
  • Biology and Chemistry Homework Help
Replies
1
Views
2K
Replies
8
Views
3K
Replies
14
Views
2K
  • Chemistry
Replies
2
Views
810
Replies
2
Views
1K
Replies
8
Views
797
  • Chemistry
Replies
2
Views
3K
Replies
5
Views
3K
  • Chemistry
Replies
1
Views
633
Back
Top