## pKa v. pKa1 and pKa2

Acidity $K_{a}$
Basicity $K_{b}$
The Conjugate Seesaw $K_{a}\times K_{b}=K_{w}$

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Maricruz Diagut 3J
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Joined: Fri Sep 25, 2015 3:00 am

### pKa v. pKa1 and pKa2

Can someone please explain what the difference between pKa v. pKa1 and pKa2 is?

Heerali Patel 3A
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Joined: Fri Sep 25, 2015 3:00 am

### Re: pKa v. pKa1 and pKa2

pKa1 and pKa2 are the negative logs of the acidity constants for the first and second stage in which a polyprotic acid loses a proton. pKa(overall) is the negative log of the overall acidity constant for the overall ionization reaction of the polyprotic acid. For example, using H2CO3 as the polyprotic acid:

H2CO3(aq) + H2O(l) <=> HCO3-(aq) + H3O+(aq) Ka1=[H3O+][HCO3-]/[H2CO3] pKa1=-log(Ka1)
HCO3-(aq) + H2O(l) <=> CO3^2-(aq) + H3O+(aq) Ka2=[H3O+][CO3^2-]/[HCO3-] pKa2=-log(Ka2)
__________________________________________
H2CO3(aq) +2H2O(l) <=> CO3^2-(aq) +H3O+(aq) Ka(overall)=Ka1 Ka2=[H3O+]^2[CO3^2-]/[H2CO3] pKa(overall)=-logKa(overall)
Last edited by Heerali Patel 3A on Thu Dec 03, 2015 1:53 pm, edited 2 times in total.

Chem_Mod
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### Re: pKa v. pKa1 and pKa2

Ka refers to the equilibrium if an acid only has 1 proton to give. Ka1 and Ka2 are for polyprotic acids and refer to the first deprotonation and second deprotonation reactions.

Alondra Loera 1A
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### Re: pKa v. pKa1 and pKa2

Just like the pH, the pKa tells you of the acid or basic properties of a substance.
pKa <2 means strong acid
pKa >2 but <7 means weak acid
pKa >7 but <10 means weak base
pKa >10 means strong base

pKa1 is when an acid can undergo more than 1 ionization the pka1 is the disassociation constant for the first ionization
pKa2 is the disassociation constant for the second ionization

Kai_Chiu 1F
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### Re: pKa v. pKa1 and pKa2

So, pKa1 and pKa2 only really matter when the problem is asking for second and first ionization? E.g. in problem 12.35, it simply asks for Ka value and gives a pKa1. I could just take 10^-pKa1 and get the answer? I do not have to worry about the 1 subscript?

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