En qué difiere la titulación de un ácido monoprótico fuerte...

En qué difiere la titulación de un ácido monoprótico fuerte de la titulación de un ácido monoprótico débil con una base fuerte en los aspectos siguientes? (a) cantidad de base necesaria para alcanzar el punto de equivalencia; (b) $\mathrm{pH}$ al comenzar la titulación; (c) $\mathrm{pH}$ en el punto de equivalencia; (d) $\mathrm{pH}$ después de agregar un pequeño exceso de base; (e) selección del indicador para establecer el punto de equivalencia

Key Concepts

Equivalence Point pH

During titration with a strong base, the equivalence point for a strong acid is characterized by the formation of a neutral salt and water, generally yielding a pH near 7 (though slight deviations may occur due to ionic strength). In contrast, when a weak acid is titrated, the salt formed hydrolyzes in water, generating a basic solution and resulting in an equivalence point pH that is higher than 7.

Acid-Base Titration Stoichiometry

In titrations of both strong and weak monoprotic acids with a strong base, the stoichiometry remains 1:1. This means that, regardless of the acid’s strength, the number of moles of base needed to react completely with the acid (to reach the equivalence point) is the same, as long as the acid is monoprotic.

Acid Strength and Initial pH

A strong acid completely dissociates in solution, resulting in a much lower initial pH compared to a weak acid, which only partially dissociates. Therefore, even for solutions of equivalent nominal concentration, the starting pH of a strong acid will be significantly lower than that of a weak acid.

pH Changes After the Equivalence Point

Immediately after adding a small excess of strong base, the pH change becomes more pronounced. The steepness of the pH jump is influenced by the acid’s dissociation characteristics and buffering capacity. A titration involving a weak acid typically exhibits a different post-equivalence pH response compared to one involving a strong acid due to the different nature of the species present in solution.

Indicator Selection Based on pH Transition Range

The choice of indicator in a titration is dependent on the pH range over which the titration’s equivalence point occurs. In titrations of a strong acid with a strong base, the rapid pH change around pH 7 calls for an indicator that changes color near neutral pH. Conversely, for titrations of a weak acid, where the equivalence point is at a higher pH, an indicator with a color change range in the basic region is more appropriate.

Transcript

00:01 We are asked to answer some questions regarding a strong acid titrated with a strong base and a weak acid titrated with a strong base.

00:10 Our first question says how does this compare, how to each of these compare regarding quantity of base required to reach the equivalence point.

00:46 Okay, and this is going to be dependent on moles.

00:53 If the moles are the same, i'm going to make sure i specify that.

01:14 And it's the same base.

01:15 These two are the same base.

01:22 Quantity of base is the same for each.

01:38 Our second question, let me get back to my question here, says the ph at the beginning of the titration.

01:59 And again, we're asked how these compare for the strong acid and the weak acid.

02:06 The strong acid will have a lower initial ph.

02:29 Question c asks us to compare the ph at the equivalence point.

02:45 And it'll be the ph will be 7 at, i'm just going to abbreviate it, eqpt for strong acid and a strong base.

03:07 And the ph will be greater than 7 for the weak acid with the strong base.

03:22 And if you've looked at anything else.

03:27 At any other problems, you might have seen that.

03:32 Part d, let's just switch colors anyway here.

03:39 The question for d is, how is the ph going to compare after the addition of excess base? once we're past the equivalence points, that says base.

04:06 And they'll be about the same with excess base.

04:22 And last but not least, question e...

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