Untitled - Kelly Walsh High School

224 CHEMISTRY FOR THE UTTERLY CONFUSED
Quick Tip
Don’t Forget!
Quick Tip
The HA is not necessarily neutral, and the A does not necessarily have a negative
charge.
We can write an equilibrium constant expression for this system:
K c [H 3 O ][A ]
[HA]
The [H 2O] is assumed to be a constant and is incorporated into the K a value. It
does not appear in the equilibrium constant expression.
Since this is the equilibrium constant associated with a weak acid dissociation, this
particular K c is the weak acid dissociation constant, K a. The K a expression is:
K a
[H 3 O ][A ]
[HA]
Many times, we will show the weak acid dissociation reaction in a shortened
notation omitting the water:
HA(aq) K H (aq) A (aq) with the K a
[H ][A ]
[HA]
The greater the amount of dissociation, the larger the value of the K a. Refer to
your textbook for a table of K a values.
For every H formed there is an A formed.