A student titrates 25 mL of a solution of an unknown acid, HX, with a 0.10 M solution of sodium hydroxide, NaOH(aq), to the equivalence point. Which of the following pieces of experimental evidence would best help the student confirm that HX is a weak acid?
Answer:
Titration curve shows gradual pH rise, equivalence point pH >7, buffer region, and indicator choice (e.g., phenolphthalein) confirm weak acid.
Let’s delve into each piece of evidence in detail:
1. Titration Curve:
A titration curve is a graph showing how the pH of a solution changes as titrant is added. For a weak acid-strong base titration like this one, the curve starts at a relatively low pH (determined by the initial concentration of the weak acid) and gradually increases as the strong base (NaOH) is added. Near the equivalence point, where the moles of acid and base are stoichiometrically equivalent, there’s a steep rise in pH. This occurs because the added base neutralizes most of the weak acid, resulting in a solution that’s mostly the conjugate base of the weak acid. After the equivalence point, the pH continues to rise slowly because excess base is present. This gradual increase in pH beyond the equivalence point is a hallmark of weak acid titrations.
2. Equivalence Point pH:
In a titration of a weak acid with a strong base, the equivalence point occurs when the moles of acid are equal to the moles of base added. At this point, the resulting solution contains only the conjugate base of the weak acid and the salt formed from the neutralization reaction. Because the conjugate base of a weak acid is a weak base, the solution will be slightly basic. The pH at the equivalence point for a weak acid titration typically falls between 7 and 14, depending on the strength of the acid and base involved.
3. Buffer Region:
Before the equivalence point, there’s a region in the titration curve where the pH changes relatively slowly upon addition of small amounts of titrant. This region is known as the buffer region. In a weak acid-strong base titration, this buffer region arises because the solution contains a mixture of the weak acid and its conjugate base. As a result, when small amounts of strong base are added, the weak acid partially neutralizes the base, maintaining the pH relatively constant. The presence of this buffer region is indicative of a weak acid being titrated.
4. Indicator Choice:
Indicators are substances that change color in response to changes in pH. For a titration involving a weak acid, it’s crucial to choose an indicator whose pH range overlaps with the pH range around the equivalence point. For instance, phenolphthalein is commonly used in weak acid-strong base titrations because it undergoes a color change in the pH range of about 8 to 10, which corresponds well to the region near the equivalence point for many weak acid-strong base titrations.
By considering these pieces of evidence collectively, the student can confidently confirm that HX is a weak acid based on the behavior observed during the titration with NaOH.
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