Tabla de Contenidos
pOH = – log [OH – ]
pOH can also be related to pH. For example, a solution with a concentration of pH = 4.0; therefore, the concentration of protons would be:
[H+] = 1.0 × 10 -4 M
Knowing that the autoprotolosis constant of water is constant and has a value of 10 -14 , the concentrations of hydroxide ions according to protons can be calculated as follows:
Kw =[H + ] [OH – ]
10 -14 = (1.0 × 10 -4 ) · [OH – ]
[OH-] = (10 -14 ) / (1.0 × 10 -4 ) = 1.10 -10 M
There is also another way to know the pOH by knowing the pH, given the following relationship:
pH + pOH = 14
Knowing that pH = 4 we can clear:
4 + pOH = 14
pOH = 14 – 4 = 10
Calculation of the pOH of different solutions
Example 1. Calculate the pOH of a 0.5 M sodium hydroxide NaOH solution.
NaOH is a strong base, so the concentration of the base is equal to the concentration of the hydroxide ions.
[OH-] = 0.5M
pOH = – log [OH-] = – log [0.5] = 0.3
Example 2. Calculate the pOH of a solution of nitric acid HNO3 with a concentration of 3.8.10-4 M
Nitric acid is a strong acid, which dissociates having the same concentrations of protons as acid, therefore:
[H+] = 3.8.10 -4M
pH = – log [H + ] = – log [3.8.10 -4 ] = 3.42
pH + pOH = 14
3.42 + pOH = 14
pOH = 10.58
Fountain
The pOH Concept | Chemistry for Non-Majors. (2021). Retrieved 20 June 2021, from https://courses.lumenlearning.com/cheminter/chapter/the-poh-concept/
