Weak
Acid/Base Solver
a
freeware program by Daniel J. Bishop
A
Brief Chemistry Lesson
(Skip
this if you already know it.)
In many
chemistry courses, you will be expected to do calculations with acids in
bases. Usually, acids and bases are
defined by how they disassociate in water.
Disassociation
of an acid: HA + H20 => A- + H3O+
Disassociation
of a base: Z + H20 => HZ+ + OH-
A- is the conjugate base of HA and HZ+ is the conjugate acid of Z. For strong acids and bases, the
disassociation reactions go to completion.
For weak acids and bases, they do not.
The equilibrium concentrations can be found with the Ka or Kb constant.
Ka = [A-]final[H30+]final/[HA]final
Kb = [HZ+]final[OH-]final/[Z]final
To solve the
equations, let x be the amount of the weak acid or base that dissociates. Then make the following substitutions:
For weak
acids:
[A-]final = [A-]initial + X
[H30+]final = [H30+]initial + X
[HA]final = [HA]initial - X
For weak
bases:
[HZ+]final = [HZ+]initial + x
[OH-]final = [OH-]initial + x
[Z]final = [Z]initial - x
Then, do some
algebraic manipulation, and you can use the quadratic formula to solve for
x. Now, you’ll probably need at least
one of these formulas to finish your calculations:
pH = -log [H+]
pOH = -log
[OH-]
pH + pOH =
14.0 (at 25°C)
A
Better Way
Fortunately,
your calculator can do all these calculations for you. All you need to do is use TI-GraphLink to
copy the WEAKACID and WEAKBASE programs to your calculator.
An
Example
Two liters of
solution at 25°C contains .20 mol of acetic acid (Ka= 1.8x10-5) and .50 mol of sodium acetate. Find the pH.
Instructions
1. Type weakacid() at the home screen.
2. Enter 1.8E-5 for Ka.
3. Enter .10 for INITIAL MOLARITY OF WEAK ACID.
(.20 mol / 2
L = .10 M)
4. Enter 1E-7 for INITIAL [H+].
(Initially,
there are no hydrogen ions except those already in the water. At 25°C, the hydrogen ion concentration in
pure water is 1.0x10-7 M.)
5. Enter .25 for INITIAL MOLARITY OF CONJ.
BASE.
(In water,
sodium acetate completely breaks into sodium ions and acetate ions. Acetate is the conjugate base of acetic
acid. Since there is one mole of
acetate ions in each mole of sodium acetate, the molarity of acetate is .50 mol
/ 2 L = .25 M.)
6. The calculator now displays the pH,
5.14. Also note that [ACID]=.100 and
[BASE]=.250. [ACID] is the final
concentration of acetic acid and [BASE] is the final concentration of
acetate. They are indistinguishable
from the initial concentrations!
Contact
Me
If you have
any questions or comments, please e-mail me at dbishop@neo.tamu.edu.
Versions of
this program for other TI calculators have already been released:
TI-83/83+
version (released 2000-04-26)
http://www.ticalc.org/pub/83/basic/science/acidbase.zip
TI-82 version
(released 2000-06-05)
http://www.ticalc.org/pub/82/basic/science/acidbase.zip
TI-86 version
(released 2000-10-20)
http://www.ticalc.org/pub/86/basic/science/acidbase.zip