A solution of 0.205 M aspartic acid, the charge neutral form of the amino acid,

Question

A solution of 0.205 M aspartic acid, the charge neutral form of the amino acid, is titrated with 0.0683 M NaOH. The pKa values for aspartic acid are 1.990, 3.900, and 10.002, corresponding to the -carboxylic acid group, the -carboxylic acid group, and the amino group, respectively. Calculate the pH at the first equivalence point of this titration.

pH = ___

hint:

Aspartic acid is the charge neutral, intermediate form of the triprotic amino acid. It reacts with a strong base to form the second intermediate species of aspartic acid, which has an overall charge of minus one, at the first equivalence point of this titration.

The second intermediate formed at the equivalence point, like the first intermediate, is amphiprotic, meaning it can act as either an acid or a base. Begin by determining the formal concentration of the second intermediate at the equivalence point. When determining the H concentration, make sure you are using the correct Ka values in your calculations.

Explanation / Answer

I general, the pH in the first equivalence point is: pH = pKa1 + pKa2 / 2

This is mainly because the substance between these two points is the H3A, H2A- and HA2- .

In this case, pH = (pK2+pK3) / 2

Calculate, pH of solution of 0.205 M aspartic acid, the charge neutral form of the amino acid, is titrated with 0.0683 M NaOH. The pKa values for aspartic acid are 1.990, 3.900, and 10.002, corresponding to the -carboxylic acid group, the -carboxylic acid group, and the amino group, respectively as follows:

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