# pH, pKa, Ka.. Differences?

Discussion in 'MCAT Discussions' started by DocInProgress, Jun 14, 2007.

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1. ### DocInProgress 7+ Year Member

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Jan 1, 2007
Can someone please clarify what exactly is the main difference between these three?

Also, what is the pI and what does that measure?

One other thing.. the half equivalence point is when half of the acid is neutralized by the base whereas the equivalence point is when all of the acid is neutralized by the base. Am I correct?

3. ### jochi1543President, Gunner Central 2+ Year Member

5,456
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Apr 4, 2007
MDApps:
Ka = [H+][A-] divided by [HA] --> for an acid HA.

pKa = - log Ka

pH = - log([H+])

4. ### jade1013 2+ Year Member

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May 8, 2007
UConn
pI is the isoelectric point and it is the pH at which a protein carries no net electrical charge.

5. ### DocInProgress 7+ Year Member

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Jan 1, 2007
Like, what exactly do they measure? Can you describe them?

6. ### kstreet 7+ Year Member

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Jan 29, 2007
Ka is the only true "measurement". It measures the strength of an acid. A stronger acid will have a greater [H+] concentration and hence a greater Ka.

Any time you see a "p" in terms of acid-base chemistry, it automatically signifies -log (whatever is after p).

So, pKa is the -log of (Ka). A larger Ka means a smaller pKa.

For pH, we are taking the -log of the [H+] concentration. A greater concentration of protons means a smaller pH.

7. ### DocInProgress 7+ Year Member

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Jan 1, 2007
Bingo, thanks!

8. ### koopa_troopaJunior Member 7+ Year Member

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Mar 28, 2005
Also, Ka is just an equilibrium constant (K=[products]/[reactants]) that you see in any reversible reaction. Ka is just a term used specifically for acids.

9. ### mrmilad 2+ Year Member

693
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Jun 7, 2007
MDApps:
Heres a little side note that I found useful, for amino acids when the pH of the surrounding is lower than the pI of the amino acid, then the substance is acidic (holds + charge), if the pH is higher than the PI then the substance is basic. Isoelectric point is pI= Sum of (pKa's of solution /(# of acidic species contributing to those pKa's) of all the different acids in the solution.

10. ### sehnsucht 2+ Year Member

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Feb 21, 2007
Ka = dissociation value for a compound in water; relationship with pKa is: high Ka value indicates that something dissociates in H20 well, inversely, a low pKa value indicates that the compound in question dissociates in H20 well.

pH = measures the [H+] ion concentration in an aqueous solution. because of the inverse log relationship to [H+], a higher [H+] indicates a lower pH value, ie. more [H+] = acidic solution. the same applies for pOH except you are measuring [OH-] ions in solution.

pI is the isoelectric point of a molecule, usually a protein or amino acid. the pI of a molecule indicates that the compount carries no net charge. for something to have a defined pI, the compound must have negative and positive functional groups (amphoteric).

hope this helps.