Consider the titration of 50.0 mL of a 0.100 M solution of the protonated form of the amino acid alanine (H₂A⁺: K_a1 = 4.6 × 10^–3, K_a2 = 2.0 × 10^–10) with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of base. (e) 100.0 mL

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Determine the initial moles of H2A+ in the solution using the formula: \( \text{moles} = \text{volume} \times \text{concentration} \).

Calculate the moles of NaOH added using the same formula: \( \text{moles} = \text{volume} \times \text{concentration} \).

Since 100.0 mL of NaOH is added, which is twice the initial volume of H2A+, determine the limiting reactant and the remaining species after the reaction.

Recognize that at this point, all H2A+ has been converted to A- (the fully deprotonated form), and the solution contains excess NaOH.

Calculate the pH of the solution by determining the concentration of OH- from the excess NaOH and using the relation \( \text{pH} = 14 - \text{pOH} \).

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant (in this case, NaOH) to a solution containing the analyte (the protonated form of alanine) until the reaction reaches its equivalence point. The pH changes during this process, and understanding how to calculate the pH at various points is crucial for interpreting titration results.

Acid-Base Equilibria

Acid-base equilibria involve the dissociation of acids and bases in solution, characterized by their acid dissociation constants (Ka). For alanine, the two dissociation constants (Ka1 and Ka2) indicate the strength of the acid in its protonated forms. Understanding these constants helps predict the pH of the solution at different stages of titration, particularly when calculating the pH after adding a specific volume of base.

Buffer Solutions

Buffer solutions resist changes in pH upon the addition of small amounts of acid or base. In the context of the titration of alanine, the solution can act as a buffer before reaching the equivalence point, where both the protonated and deprotonated forms of the amino acid are present. Recognizing the buffer region and its capacity is essential for accurately calculating the pH after the addition of NaOH.

Consider the titration of 50.0 mL of a 0.100 M solution of the protonated form of the amino acid alanine (H2A+: Ka1 = 4.6 × 10–3, Ka2 = 2.0 × 10–10) with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of base. (e) 100.0 mL