Relationship between pH & pOH ?

2 Answers
Feb 21, 2018

Well, they have an equation that connects them together...

Explanation:

Well first, let's look at the definitions of #pH# and #pOH#. The #pH# is the potential of hydrogen #(H^+)# ions in a solution, while the #pOH# is the potential of hydroxide #(OH^-)# ions in a solution. They are like opposites of each other.

We know that solutions can be acidic, neutral, or basic (alkaline).

The #pH# measures the acidity of a solution, while the #pOH# measures the alkalinity of a solution. Less #pH# means more acidic, while more #pH# means more alkaline. At #pH=7#, the solution is considered neutral, i.e. its concentration of #H^+# ions is equal to the concentration of its #OH^-# ions.

An important fact to know that, is that a water solution at #25^@C# will satisfy the following equation:

#pH+pOH=14#

Source:

https://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Calculating_pHandpOH.htm#RelationshippHpOH

Feb 21, 2018

These are related by the autoprotolysis reaction, the which water undergoes at a given temperature....

Explanation:

#underbrace(2H_2O(l)rightleftharpoons H_3O^+ + HO^-)_"autoprotolysis of water at 298 K"#

And at #298*K# the extent of this equilibrium has been very carefully measured...

#K_w=[H_3O^+][HO^-]=10^-14#..

Now back in the day, before the advent of cheap electronic calculators, chemists (and physicists, and accountants, and students), used logarithmic tables to multiply and divide small and large numbers, and the #pH# and #pOH# scales are a throwback to this time...we can use logarithms to the base 10....and so..

#log_10K_w=log_10{[H_3O^+][HO^-]}#

#log_10[H_3O^+]+log_10[HO^-]=log_10K_w#

But #log_10K_w=log_10(10^-14)=-14# by definition of the logarithmic function...

And so #-14=log_10[H_3O^+]+log_10[HO^-]#

OR.....

#+14=-log_10[H_3O^+]-log_10[HO^-]#..and to simplify we introduce a couple of definitions....

#+14=underbrace(-log_10[H_3O^+])_"pH"underbrace(-log_10[HO^-])_"pOH"#

The which gives our working relationship:

#+14=pH+pOH#...

And if you think that is hard, spare a thought for your parents and grandparents...when they did this stuff, they had to use logarithmic tables rather than electronic calculators, which calculate logarithms at the touch of a button.

And so, finally, if a chemist reports #pH=0# for a solution of hydrochloric acid, then what is #[HCl]#?