Обчислення pH для слабких розчинів кислот/основ

What does it mean for an acid or base to be weak?

Weak acids and bases do not fully dissociate in aqueous solutions. As we have seen in conjugate acid-base pairs a strong acid once dissociated, becomes a weak base and donated proton. In accordance, a weak base will become a strong acid once protonated.

On the pH scale, a weak acid is going to have a pH closer to 14. Similarly, on the pOH scale, a weak base is going to have a pOH closer to 14.

But how do we calculate if an acid or base is strong or weak? Based off their ability to be ionized! The acid dissociation constant ($K_{a}$) and base dissociation constant ($K_{b}$) are both mathematical ways to express the concentration of dissociated hydrogens, from an acid, or hydroxyl groups, from a base. The smaller the $K_{a}$ or $K_{b}$ the weaker the acid or base, respectively.

As we have seen in the pH of strong acids and strong bases, the concentration of the ions in solution is integral in determining the strength of the solution.

But if I’m given a pOH how do I determine the pH? As mentioned in our text explanation of the pOH scale, the pOH scale and the pH scale have an inverse relationship, and when summed together always equal 14. The stronger an acid is, the weaker its conjugate base. In accordance, the weaker an acid is, the stronger its conjugate base. The same inverse relationship is valid for basicity.

If pH+pOH=14, there are a few other equations that are also worthwhile to mention. These equations are:

$$K_{a}\times K_{b}=K_{w}$$ $$and$$ $$pK_{a}\times pK_{b}=pK_{w}$$

$K_{w}$ is equal to 14 and $pK_{w}$ is equal to $1*10^{-14}$. These are two ways of expressing water's autoionization product, and is the result of water molecules reacting with one another:

$$K_{w}=[H_{3}O^{+}][OH^{-}]$$

An example is shown below of how to find pH from a weakly basic solution.

If we know one of these variables, we can find out the other. I know it can look complicated, but remember $K_{a}/K_{b}$ and $pK_{a}/pK_{b}$ are two different ways of expressing the same thing. The lower-case “p” only denotes that we are now on a different scale, using the logarithmic equation: $pK_{(a\,or\,b)}=-log(K_{(a\,or\,b)}$).

References: 1, 2

(Video) 16.5 pH Calculations for Weak Acids and Bases | General Chemistry

by Chad's Prep

In this video, Chad walks you through calculating the pH of weak acids and bases. He'll show you how to use acid and base dissociation constants with ICE tables. He'll also show you how to work backward from pH to the constant. After watching this video, you'll know how to tackle any weak acid or base calculation.