Questions about pKa and pH

[Ultimeaciax]

Pka + pKb = 14

PH + pOH = 14

I know there's a relationship about pKa and pH, but looking at that equation just makes me think they are the same!

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[soby10]

Pka+PKb=pkw
Pka is the -log of ka
Pkb is the -log of kb
POH+PH=14
POH is -log of [OH-]M
PH is -log of [H+ ]M

The relation comes from Kw=[H+][OH-]
kw=10-14 This is from the autoionization of water
water reacting with itself at 25 degrees C which is equal to 10-14
The difference is that Ka/kb is a equlibrium value of H20 the acid/base dissociation constants. POH/PH are molar concentration units. So they are related but not identical.

 

[cheesier]

They're related through the Hendserson Hasselbach equation:

 

[dougkaye]

pH is not a measurement of concentration per se, but it is derived from concentration: pH = -log [H+]. I think this is what you meant, but wanted to clarify.

 

[BerkReviewTeach]

Pka + pKb = 14

PH + pOH = 14

I know there's a relationship about pKa and pH, but looking at that equation just makes me think they are the same!

pH describes the [H+] in a solution on a log scale that is easier to think about than the actual concentrations in molarity. The lower the pH, the more concentrated the H+ in solution.

pKa describes the acidity of a compound. It can be thought of as a physical property, describing the strength of an acid.

You question is analogous to asking what the difference is between temperature and boiling point. One describes the heat in the environment (and thus varies) while the other describes the compound and the state it's in at a given temperature. Raising the temperature does not change the boiling point, but can convert the compound into a gas.

Lowering the pH does not change the pKa for the acid, but it does change the amount that is in the protonated state.

Hope this helps.

 

[liam braoski]

pH describes the [H+] in a solution on a log scale that is easier to think about than the actual concentrations in molarity. The lower the pH, the more concentrated the H+ in solution.

pKa describes the acidity of a compound. It can be thought of as a physical property, describing the strength of an acid.

You question is analogous to asking what the difference is between temperature and boiling point. One describes the heat in the environment (and thus varies) while the other describes the compound and the state it's in at a given temperature. Raising the temperature does not change the boiling point, but can convert the compound into a gas.

Lowering the pH does not change the pKa for the acid, but it does change the amount that is in the protonated state.

Hope this helps.

You gave a vague definition of pH and didn't really give the definition of pKa. pH is -log[H+] and pKa is -log[Ka] both in base 10. Your analogy is poorly worded and your explanation awkward. Temperature does not describe the heat in the environment. What you should have said was that temperature (analogous to pH) describes the state of the system (analogous to solution) and boiling point (analogous to pKa) is a physical property of a substance (analogous to an acid).

 

[Zedor]

You gave a vague definition of pH and didn't really give the definition of pKa. pH is -log[H+] and pKa is -log[Ka] both in base 10. Your analogy is poorly worded and your explanation awkward. Temperature does not describe the heat in the environment. What you should have said was that temperature (analogous to pH) describes the state of the system (analogous to solution) and boiling point (analogous to pKa) is a physical property of a substance (analogous to an acid).

I understood what BerkleyReviewTeach was saying

 

[xianghui]

One thing to remember is that when [A]- = [HA] for weak acid, pH=pka. Other than that, use the formula. The lower the pka, the higher the acidity. For pkb, the lower the pkb, the higher the basicity.