Weak acids and as

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Weak acids and as
Read Chapter 6-7 to 6-9.

• Last semester you learned how to determine the pH
  of weak acids.
• Today we will learn a different way to solve the
  same problem using a more powerful method.
• This method has 3 advantages
• it is exact for all pH values,
• it can be readily extended to more complex
  problems,
• it is easy to implement with mathcad or excel.

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Consider the following reaction
Ka
Kw
How many equations and unknowns are there above?
Mass balance
Charge balance
We have 4 equations and 4 unknowns
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Solution of equations
To solve these equations it is useful to
introduce the concept of the fractional amounts
of A containing species.
a0 fraction of HA a1 fraction of A
Lets simplify these equations.
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Solution of equations
Sketch the as as a function of pH.
Excel or Mathcad can be used to make these plots.
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Use CB eqn and ai to calculate H
Recall the CB equation
insert as shown to obtain
We have 1 eqn and 1 unknown, so we can solve for
xH.
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If the first term can be neglected, and it
usually can be, then the equation simplifies to
We can rearrange this to put it into the form
with which you are familiar.
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Solution of Equations
How can we solve the below equation without
approximations
We cant solve for H as a function of CHA
so we do the reverse and Solve for CHA as a
function of H.
See Numerical Results
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pH of Weak Acid
I II III
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Analysis of Region 1
Region 1 is where CHA is very small.
pH 7
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Analysis of Region 3
Region 3 is where CHA is large compared x.
Compare with numerical result.
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Weak Acids Overview

• Today we learned a powerful method for solving
  for the concentrations of weak acids.
• This method is
• exact for all pH values,
• easy to implement with mathcad or excel,
• can be readily extended to more complex
  problems.

There are analytical solutions in 3 regions.
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Suggestion

• You have been introduced to 3 concepts that will
  be used repeatedly throughout this course. They
  are mass balance, charge balance, and as. Become
  familiar with these concepts.
• Go home, get out a blank sheet of paper and try
  to rederive the expressions for the as.