pH and pOH: Crash Course Chemistry #30

We can calculate the hydroxyl exponent, which is the negative logarithm of the hydroxide concentration. This is easy because the dissociation constant of water never changes. Although the concentrations of hydrogen and hydroxide are the same in pure water or only in completely neutral solutions, the product of the two concentrations is always equal to 1.0 times 10 to the power of negative 14 in any aqueous solution. Orange juice, for example, is actually a water solution of sugar, citric acid, and a few other things. Let us consider that the concentration of hydrogen ions in the juice is equal to 3.2 times 10, that is, 4 moles per liter.

Have a little fun and calculate the pH at that point, which turns out to be 3.5. But we can also use the dissociation constant of water and the concentration of the hydronium ion to do a very simple division and find the hydroxide concentration. The result is 3.1 times 10 to the negative 11 moles per liter. Once we know the concentration, we can go one step further, we can find the hydroxyl exponent of the solution, which is similar to pH, it is simply the negative logarithm of the hydroxyl concentration. The hydroxyl exponent in this case is 10.5. Now here is a wonderful and surprising tip.

The sum of the pH and the hydroxyl pH is always 14. In our example, the pH is 5.4 and the hydroxyl pH is 8.6. And that's right, you add them up and you get 14! a surprise! Okay, maybe that's cool for me, but that's never deterred me, I love this stuff. And next week I hope to really surprise you by showing you how to stabilize the pH of a solution even if you put a strong acid or a strong base in it. In the meantime, thanks for watching this episode. If you've been paying attention, you've learned how pure water ionizes to form hydronium and hydroxide ions in reversible reactions.

You learned about the equilibrium constant of that reaction and it has a special name: water dissociation constant. You learned some examples of acids, bases, and neutrals, plus why some acids and bases are called strong and other acids and bases are called weak. You learned about logarithms and how you can use them to calculate the pH of a substance, and you learned a little about the hydroxyl exponent, which can also be calculated with logarithms and subtraction. Finally, you learned some interesting mathematical connections between pH and hydroxyl pH.