Spreadsheet #14: Re-iterative Calculations or Successive Approximations for Weak Acids
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Calculate
Equation: [H+] = [ Ka * (Ca-[H+]) ] ^ 0.5
where Ka = ionization constant for the acid, and Ca = Initial Concentration of the acid. For the first calculation (“Iteration 0”), use [H+]=0. In the following iterations, use [H+] from the preceding iteration.
Calculate the [H+], the associated pH, and the %
ionization for acetic acid at four different initial acid concentrations (Ca): 1.8e-5,
5.0e-5, 1.0e-4, 1.0e-3 moles/Liter. Calculate
20 iterations for each concentration. Plot the values you obtain for [H+]
at each iteration and observe the convergence.
Show all four cases on the same graph.
Comment on the number of iterations to reach a solution for each
case. For acetic acid, Ka=1.75e-5 (Click to download the
data set: 14-Ka-pH.)
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Acetic Acid |
Ka: |
1.75E-005 |
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Ca: Initial concentrations of Acid (mole/L): |
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Iteration#: |
1.8E-005 |
5.00E-005 |
1.00E-004 |
1.00E-003 |
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0 |
1.77E-005 |
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... |
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1 |
2.10E-006 |
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2 |
... |
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3 |
... |
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20 |
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pH= |
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%
Ionization: |
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Chemistry Helps: |
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pH is the negative log the hydrogen concentration (-log[H+]). |
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% Ionization is the equilibrium concentration of [H+] divided by the initial concentration of acid (Ca). |