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pOH concept

In the same way that it happens with the concentration of protons to calculate the pH, for the pOH it could be found from the concentration of hydroxide ions. Specifically, the logarithm of the hydronium ion concentration, as follows:

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

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The pOH Concept | Chemistry for Non-Majors. (2021). Retrieved 20 June 2021, from https://courses.lumenlearning.com/cheminter/chapter/the-poh-concept/

https://en.khanacademy.org/science/ap-chemistry/acids-and-bases-ap/acids-bases-and-ph-ap/a/ph-poh-and-the-ph-scale

https://www.quimicas.net/2015/05/definicion-de-poh.html