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What is the Dissociation Constant?

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn about the dissociation constant of acids in solution. We will learn how to calculate the dissociation constant and why it is important.

Dissociation Constant

When we're working in a laboratory, we need to be very careful around strong acids, as they could burn the skin. But how do we determine if an acid is weak or strong? One way to do this is by comparing the dissociation constants of the acids. The dissociation constant is the ratio of dissociated ions (products) to original acid (reactants). It is abbreviated as Ka.

Typically we determine the dissociation constant by seeing how much of it dissociates in water. Whenever an acid is added to water, some of the hydrogen atoms from the acid are added to water, leaving behind the conjugate base of the acid. When we first add the acid to the water, we have a bunch of the hydrogen atoms transferring to water, then some of the hydrogen atoms return to the conjugate base (reforming the acid), and then they keep going back and forth between the water and the acid. This continues until the products and reactants reach equilibrium. Equilibrium is when there are no changes in concentration of products and reactants over time.

When an acid (HA) is put into water, some of it dissociates into a conjugate base (A - ) and hydronium ion (H 3 O + )
Equilibrium

Once the reaction reaches equilibrium, we can determine the dissociation constant. Since it is the ratio of products to reactants, we can calculate it with a quotient:


Equation


The brackets around each group means the concentration of that molecule. Water is not included because it is technically a solvent not a reactant.

Example Calculation

Let's now measure the strength of acetic acid (vinegar). We'll put 1 mL of acetic acid into 1 mL of water. We allow the reaction to occur. Once it comes to equilibrium, we measure the concentration of each product and reactant:

Conjugate base: 0.004 mL

Hydronium: 0.004 mL

Acid: 0.996 mL

Now let's calculate the dissociation constant:


Ka equation


So the dissociation constant is really small!

Logarithmic Constant

The most common way to use the dissociation constant is by taking the negative of log base 10 of the dissociation constant; this is referred to as the pKa.


pKa equation


We use pKa because this gives us an easy comparison between acids. Ka gives us a range of several orders of magnitude, which makes it difficult to compare values. The pKa range is typically somewhere between -10 to about 50. If the pKa is less than -2, then we call it a strong acid. A pKa of -2 to 12 is a weak acid. A pKa above 12 is not typically considered acidic.

So let's now calculate the pKa for acetic acid. The dissociation constant was 0.0000161, and the log of that is -4.79. We take the negative of that and get 4.79. The pKa is between -2 and 12, so acetic acid is a weak acid.

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