Equivalence point weak base-strong acid

Figure 9.8b shows a titration curve for a mixture consisting of two weak acids HA and HB. Again, there are two equivalence points. In this case, however, the equivalence points do not require the same volume of titrant because the concentration of HA is greater than that for HB. Since HA is the stronger of the two weak acids, it reacts first thus, the pH before the first equivalence point is controlled by the HA/A buffer. Between the two equivalence points the pH reflects the titration of HB and is determined by the HB/B buffer. Finally, after the second equivalence point, the excess strong base titrant is responsible for the pH. 

A weak acid-strong base titration. The curve represents the titration of 50.00 mL of 1.000 M acetic acid, HC2H3O2. with 1.000 /W NaOH. The solution at the equivalence point is basic (pH = 9.22). Phenolphthalein is a suitable indicator. Methyl red would change color much too early, when only about 33 mL of NaOH had been added. Bromthymol blue would change color slightly too quickly. 

FIGURE 16.8 Various weak acid-strong base pH titration curves. The curves shown are for titration of 40.0 mL of 0.100 M solutions of various weak acids with 0.100 M NaOH. In each case, the equivalence point comes after addition of 40.0 mL of 0.100 M NaOH, but the increase in pH at the equivalence point gets smaller and the equivalence point gets more difficult to detect as the Ka value of the weak acid decreases. 

Beyond the equivalence point, the curves for the weak acid-strong base and strong acid-strong base titrations are identical because the pH in both cases is determined by the concentration of OH- from the excess NaOH. 

With a weak acid, strong base titration like this one, the situation is more critical for two reasons. First, the pH changes a bit more gradually than with strong acid titrations. As a result, an indicator like methyl red will change color well before the equivalence point is reached. 

Note The choice of indicator is critical in weak acid-strong base and weak base-strong acid titrations because the slope of the transition is not so steep and the pH of the equivalence point can be very different from 7. pK (indicator) pH at the equivalence point.)... 

FIGURE 16.4 pH profile of a weak acid-strong base titration. A 0.10 M NaOH solution is added from a buret to 25 mL of a 0.10 M CH3COOH solution in an Erlenmeyer flask. Due to the hydrolysis of the salt formed, the pH at the equivalence point is greater than 7. 

TLie pH at tlie equivaLence point of an acid-base titration depends on liydroiysis of the salt formed in the neutraiization reaction. For strong acid-strong base titrations, the pH at the equivaLence point is 7 for weak acid-strong base titrations, the pH at the equivalence point is greater than 7 for strong acid-weak base titrations, the pH at the equivalence point is less than 7. 

Therefore, as mentioned previously, in a weak acid-strong base titration, the solution at the equivalence point is slightly basic, pH > 7.00. We calculate [H3O ] as in Section 18.5 we first find K, of Pr from of HPr, set up a reaction table (assume [Pr ] >> [Pr ]reactina)> and solve for [OH ]. We need a single concentration, [Pr ], to solve for [OH ], so we do need the total volume. Then, we convert to [HsO" ]. These two steps are... 

Understand how an acid-base indicator works, how the equivalence point and end point in an acid-base titration differ, and how strong acid-strong base, weak acid-strong base, and strong acid-weak base titration curves differ explain the significance of the... 

Explain how strong acid-strong base, weak acid-strong base, and weak / axe-strong acid titrations using the same concentrations differ in terms of (a) the initial pH and (b) the pH at the equivalence point. (The component in italics is in the flask.)... 

What species are in the buffer region of a weak acid-strong base titration How are they different from the species at the equivalence point How are they different from the species in the buffer region of a weak base-strong acid titration ... 

At the various points in your titration curve, list the major species present after the strong base (NaOH, for example) reacts to completion with the weak acid, HA. What equilibrium problem would you solve at the various points in your titration curve to calculate the pH Why is pH > 7.0 at the equivalence point of a weak acid-strong base titration Does the pH at the halfway point to equivalence have to be less than 7.0 What does the pH at the halfway point equal Compare and contrast the titration curves for a strong acid-strong base titration and a weak acid-strong base titration. 

The curve of a weak acid-strong base titration is different fiom that of the plot we learned for the strong acid-strong base titration. Unlike the previous curve, this plot will have a region which has buffering properties. Besides that, the equivalence point will be above the pH value 7. The general shape of the curve is shown in Figure 9-2. 

Figure 17.9 The volume of base needed to reach the equivalence point would not change because this quantity does not depend on the strength of the acid. However, the pH at the equivalence point, which is greater than 7 for a weak acid-strong base titration, would decrease to 7 because hydrochloric acid is a strong acid.

Why is pH at the equivalence point larger than 7 when you titrate a weak acid with a strong base (a) There is excess strong base at the equivalence point, (b) There is excess weak acid at the equivalence point, (c) The conjugate base that is formed at the equivalence point is a strong base, (d) The conjugate base that is formed at the equivalence point reacts with water. 

The pH at the equivalence point is above 7.00 for the weak acid-strong base titration. 

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