PH after titration of Weak Acid with a Strong Base

In summary, the conversation is about titrating a solution of .1006M CH3COOH with .2012M NaOH, with a given pKa and Ka value. The question is what the pH would be at the equivalence point of the titration. The solution involves setting up an ICE table and using the Ka value to find [H+] at the equivalence point. However, the calculated pH may be in the 4-5 range instead of above 7, so the discussion turns to considering the effect of the sodium salt alone in the water. The final answer is that the pH at equivalence will be above 7, with a pOH value between 5-6 that can be used to calculate the pH using
  • #1
FrogsWithSock
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Homework Statement


Titrating .1006M CH3COOH with .2012M NaOH. pKa=4.74, Ka=1.8*10^-5
What is the pH at the equivalence point of the titration?


Homework Equations


pKa = -log(Ka) pH = -log[H+]


The Attempt at a Solution



Alright, after setting up an ICE table (in order to find [H+] at the equivalence point) I realized that when you work it out Ka = [H+], however when I plugged that in I got a pH in th 4-5 range when it should be above 7.
Here is my Ka reasoning
Ka = [A-][H+]/[HA]
Ka = [Co+x][x]/[Co-x] Co = Initial Concentration
x is very small (Weak acid, no/little disassociation)
Ka = x
Ka = [H+]


Am I missing something or is my logic just wrong? Anything that could point me in the correct direction would be greatly appreciated.
 
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  • #2
The pH at equivalence is the same as if you dissolved the sodium salt in water, without any acid or base. Now, you calculation should be based on just the effect of the sodium salt alone in the water.
 
  • #3
Late answer but i came across this question when i was looking for the same answer.

Don't forget the answer your getting is pOH!
Therefore pH = 14 - pOH (where your pOH will be between 5-6 for this type of question)
pH = > 7
 

1. What is the purpose of titrating a weak acid with a strong base?

Titrating a weak acid with a strong base allows for the determination of the acid's dissociation constant (Ka) and the pH at the equivalence point. It also helps to understand the acid-base equilibrium and the relationship between the concentration of the acid and its dissociation.

2. How does the pH change during titration of a weak acid with a strong base?

The initial pH of the weak acid solution will be low due to the presence of undissociated acid molecules. As the strong base is added, it will neutralize the acid and form a salt, increasing the pH. However, there will be a gradual decrease in the rate of pH change as the equivalence point is approached. At the equivalence point, the pH will be determined by the dissociation constant of the weak acid.

3. What is the significance of the equivalence point in titration?

The equivalence point is the point at which the moles of acid and base are equal, indicating that all of the acid has been neutralized. It is an important point in titration as it allows for the calculation of the concentration of the acid and the determination of its Ka value. It also marks the point of maximum buffering capacity.

4. How does the strength of a weak acid affect the pH after titration with a strong base?

The strength of a weak acid is directly related to its Ka value. A stronger acid will have a higher Ka value, indicating a higher degree of dissociation. Therefore, a strong acid will require a larger volume of strong base to reach the equivalence point, resulting in a higher pH at the equivalence point compared to a weaker acid.

5. Can the pH after titration of a weak acid with a strong base be predicted?

The pH after titration can be predicted by using the Henderson-Hasselbalch equation, which takes into account the initial concentration of the weak acid and the volume of strong base added. However, this prediction may not be accurate if there are other factors affecting the titration, such as the presence of impurities or the formation of a buffer solution.

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