Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made.
Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students.
In chemistry, pH is a measure of the hydrogen ion (H+) concentration in a solution.
The pH of a buffer can be calculated from the concentrations of the various components of the reaction.
The balanced equation for a buffer is:
$HA \rightleftharpoons H^+ + A^-$
The strength of a weak acid is usually represented as an equilibrium constant.
The acid-dissociation equilibrium constant (Ka), which measures the propensity of an acid to dissociate, for the reaction is:
The greater [H+] x [A-] is than [HA], the greater the value of Ka, the more the formation of H+ is favored, and the lower the pH of the solution.
ICE Tables: A Useful Tool For Solving Equilibrium Problems
ICE (Initial, Change, Equilibrium) tables are very helpful tools for understanding equilibrium and for calculating the pH of a buffer solution.
They consist of using the initial concentrations of reactants and products, the change they undergo during the reaction, and their equilibrium concentrations.
Consider, for example, the following problem:
Calculate the pH of a buffer solution that initially consists of 0.0500 M NH3 and 0.0350 M NH4+.
(Note: Ka for NH4+ is 5.6 x 10-10).
The equation for the reaction is as follows:
$NH_4^+ \rightleftharpoons H^+ + NH_3$
We know that initially there is 0.0350 M NH4+ and 0.0500 M NH3.
Before the reaction occurs, no H+ is present so it starts at 0.
During the reaction, the NH4+ will dissociate into H+ and NH3.
Because the reaction has a 1:1 stoichiometry, the amount that NH4+ loses is equal to the amounts that H+ and NH3 will gain.
This change is represented by the letter x in the following table.
Therefore the equilibrium concentrations will look like this:
Apply the equilibrium values to the expression for Ka.