# pH, pKa, Ka, pKb, Kb

Ranking Acid Base Strength Using Ka pKa Values Leah4sci

Tip: This isn't the place to ask a question because the teacher can't reply.

## Key Questions

• The pH scale provides a way of measuring how acidic or basic solutions are. The scale ranges from 0-14. A pH of 0 is the most acidic, 7 is neutral and 14 is the most basic.

Here is a video of a lab which looks at a number of different solutions and measures their pH levels using a pH meter and an indicator.

video from: Noel Pauller

• $\text{p"K_"a}$ and $\text{p"K_"b}$ are measures of the strengths of acids and bases, respectively

Acids

When you dissolve an acid in water, it undergoes an equilibrium reaction with the water in an.

HA + H₂O ⇌ H₃O⁺ + A⁻

The value of the equilibrium constant is given by

K_"a" = (["H"_3"O"^+]["A"^-]]/["HA"]

The greater the value of ${K}_{\text{a}}$, the stronger the acid.

For most weak acids, ${K}_{\text{a}}$ ranges from ${10}^{-} 2$ to ${10}^{-} 14$.

We convert these exponential numbers into a normal range by taking their negative logarithm.

The operator $\text{p}$ means "take the negative logarithm of".

So $\text{p"K_"a" = -logK_"a}$.

For most weak acids, $\text{p"K_"a}$ ranges from 2 to 14.

Thus, the smaller the value of $\text{p"K_"a}$ , the stronger the acid.

Bases

When you dissolve a base in water, it reacts with the water in an equilibrium reaction.

B + H₂O ⇌ BH⁺ + OH⁻

The value of the equilibrium constant is given by

K_"b" = (["BH"^+]["OH"^-]]/["B"]

The greater the value of ${K}_{\text{b}}$, the stronger the base.

For most weak acids, ${K}_{\text{b}}$ ranges from ${10}^{-} 2$ to ${10}^{-} 13$.

$\text{p"K_"b" = -logK_"b}$.

For most weak acids, $\text{p"K_"a}$ ranges from 2 to 13.

The smaller the value of $\text{p"K_"b}$ , the stronger the base.

Here's a video on $\text{p"K_"a}$ and $\text{p"K_"b}$.

Ka= dissociation constant of acid; Kb= dissociation constant of base

#### Explanation:

this concept of Ka and Kb is valid only for weak acids and weak bases.
let HA be a weak acid, whose dissociation reaction is:

$H {A}_{a q} \rightarrow H {+}_{a q} + A - \left(a q\right)$
This is a reversible reaction

then Ka = [H+][A-] / [HA]
here A- is conjugate base, writing above reaction in reverse order

$A {-}_{a q} \rightarrow H A + - O H$

Kb for this reaction is:
$K b = \frac{\left[H A\right] \left[- O H\right]}{A -}$

Note: lower is the pKa value stronger is the acid
higher is the pKb value stronger is the base.

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