Acid and Base

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Dakota_Campbell_1C
Posts: 51
Joined: Fri Sep 28, 2018 12:15 am

Acid and Base

Postby Dakota_Campbell_1C » Thu Jan 17, 2019 9:30 pm

Why does the KA and KB value have to add up to 10x^14?

Parth Mungra
Posts: 72
Joined: Fri Sep 28, 2018 12:28 am

Re: Acid and Base

Postby Parth Mungra » Thu Jan 17, 2019 9:54 pm

pKa + pKb equals the pKw, which is usually 14 (Kw= 10^-14) at 25 degrees Celsius. This is true if you add more acidic reactant or basic reactant because of the inverse relationship between the two. Ka x Kb involves canceling out concentrations of molecules and ions, so you end up with [H30+] x [OH-], which should equal 10^-14.

MadisonB
Posts: 63
Joined: Fri Sep 28, 2018 12:19 am

Re: Acid and Base

Postby MadisonB » Fri Jan 18, 2019 2:57 am

If you think of the negative log of 1x10^-14, which is 14. The pH of the acid and the base always add up to 14, in a neutral solution both will be 7.

(the below table is found at http://chemed.chem.purdue.edu/genchem/t ... h17/ph.php)

Pairs of Equilibrium Concentrations of H3O+ and OH- Ions That Can Coexist in Water

Concentration (mol/L)
[H3O+] [OH-]
1 1 x 10-14 ---->
1 x 10-1 1 x 10-13
1 x 10-2 1 x 10-12
1 x 10-3 1 x 10-11 Acidic Solution
1 x 10-4 1 x 10-10
1 x 10-5 1 x 10-9
1 x 10-6 1 x 10-8
1 x 10-7 1 x 10-7 Neutral Solution
1 x 10-8 1 x 10-6 ---->
1 x 10-9 1 x 10-5
1 x 10-10 1 x 10-4
1 x 10-11 1 x 10-3 Basic Solution
1 x 10-12 1 x 10-2
1 x 10-13 1 x 10-1
1 x 10-14 1

Sean Reyes 1J
Posts: 67
Joined: Fri Sep 28, 2018 12:24 am

Re: Acid and Base

Postby Sean Reyes 1J » Sat Jan 19, 2019 10:42 am

If you were to write out the equilibrium constant for the concentrations of an acid, it would look like the following with the example using the acid ammonium:
Ka = [H3O+][NH3] / [NH4+]
Same goes for a base, using ammonia as the base:
Kb = [OH-][NH4+] / [NH3]
When multiplying these together, you would be left with only [H3O+][OH-], which you can then equate to Kw, 1.0 * 10^-14, as Kw simplifies down to the concentration of hydronium multiplied to the concentration of hydroxide.


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