How Do You Calculate the pH at Equivalence Points in Malonic Acid Titration?

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SUMMARY

The discussion focuses on calculating the pH at the equivalence points during the titration of malonic acid, a dibasic acid with pKa1 of 2.85 and pKa2 of 5.70. The initial attempt calculated the pH at the first equivalence point incorrectly as 7.77, while the correct value is 3.50. The solution involves recognizing the presence of an amphiprotic salt at the first endpoint, which requires a different approach to determine the pH accurately.

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  • Ability to calculate pH and pOH using the formulas for weak acids and bases
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  • Study the calculation of pH for amphiprotic salts in titration scenarios
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Chemistry students, educators, and laboratory technicians involved in acid-base titration experiments and those seeking to deepen their understanding of pH calculations in complex titration scenarios.

nmr
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Malonic acid can ionise in two stages as it is a dibasic acid.
The values of pKa1 and pKa2 of malonic acid are 2.85 and 5.70 respectively. Calculate the pH of the first and second equivalence points of the titration of 10cm^3 of malonic acid of concentration 0.1 M with sodium hydroxide of concentration 0.1 M.

my attempt at a solution:
maximum buffering capacity before first endpoint: pH=pKa1 = 2.85

[OH-] = [(Kw/Ka1)*(0.05)]^(0.5)
therefore pOH = 6.23
pH = 14-6.23 = 7.77

im stuck here as the answer given for the pH of the first endpoint is 3.50 and I have no idea how to work it out
 
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First endpoint pH given as an answer is wrong, so don't worry if you can't reproduce it.

At first endpoint you have a solution of an amphiprotic salt.
 
so is my pH value correct ?
 
Nope.
 
how would i go about solving for the pH then?
 
I have pointed you in the right direction in my first answer.

If you plan to ignore hints please tell, so that I don't waste my time answering.
 
ah ok sorry
 

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