## Acid and Base

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

### Acid and Base

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

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.

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

### Re: Acid and Base

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

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.