What is the volume of base added at second equivalence point?

In summary, the initial concentrations of H3PO4 and H2PO4- can be found using information about pH and the first equivalence point, and the second equivalence point can be reached by neutralizing all H3PO4 and having a solution with initial H2PO4- and H2PO4-. The total volume of base added to reach the second equivalence point is >30mL.
  • #1
PinguNinjitsu
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Homework Statement



You have a solution that is buffered at pH =2.0 using H3PO4 and H2PO−4 (pKa1 = 2.12;
pKa2 = 7.21; pKa3 = 12.68). You decide to titrate this buffer with a strong base. 15.0 mL are needed to reach the first equivalence point. What is the total volume of base that will have been added when the second equivalence point is reached?

I know that the answer is >30ml but I don't know how to get to that point.

Homework Equations



PH=(PKa1+PKa2)/2 First equivalence point
PH=(PKa2+PKa3)/2 Second equivalence point

The Attempt at a Solution



I honest don't know what to do, I did try finding the concentrations of OH at the equivalence points using the pH formulas above, but that didn't seem to get me anywhere since those values were tiny and I didn't know how to get mL from it..

Any help would be much appreciated! Thanks!
 
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  • #2
Use information about pH to find initial concentrations of H3PO4 and H2PO4-. First end point means you have neutralized all H3PO4. Now you have solution containing initial H2PO4- and H2PO4- from H3PO4 neutralization.

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What is a Second Equivalence Point?

The second equivalence point is a point in a titration where the amount of titrant added is equal to the amount of analyte present in the solution. It signifies the completion of the reaction between the titrant and the analyte.

How is the Second Equivalence Point determined?

The second equivalence point is determined by calculating the volume of titrant added to reach the first equivalence point and then adding half of that volume to the initial volume. This is because the second equivalence point occurs when the concentration of the titrant is equal to half of its initial concentration.

What is the significance of the Second Equivalence Point?

The second equivalence point indicates the presence of a second acid or base in the solution. It can also be used to determine the concentration of the second acid or base, as well as the pKa or pKb values of the acids or bases involved in the titration.

How does the Second Equivalence Point differ from the First Equivalence Point?

The first equivalence point occurs when the amount of titrant added is equal to the amount of analyte present in the solution. The second equivalence point occurs when the concentration of the titrant is equal to half of its initial concentration. Additionally, the first equivalence point signifies the completion of the main reaction, while the second equivalence point indicates the presence of a second acid or base in the solution.

How can the Second Equivalence Point be used in titrations?

The second equivalence point can be used to determine the presence and concentration of a second acid or base in a solution. It can also be used to determine the stoichiometry of the reaction, as well as the pKa or pKb values of the acids or bases involved. Additionally, the volume of titrant added at the second equivalence point can be used to calculate the molar mass of the analyte.

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