The accompanying graph shows the titration curves for two monoprotic acids. (d) Estimate the pKa of the weak acid.

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Identify the titration curve for the weak acid. In this case, it is curve B.

Locate the midpoint of the titration curve for the weak acid. This is the point where half of the acid has been neutralized.

At the midpoint, the pH of the solution is equal to the pKa of the weak acid.

Estimate the pH value at the midpoint of curve B by finding the volume of base added at the equivalence point and then halving it.

Read the pH value at this midpoint volume from the graph, which will give you the estimated pKa of the weak acid.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Titration Curves

Titration curves graphically represent the change in pH of a solution as a titrant is added. For monoprotic acids, the curve typically shows a gradual increase in pH followed by a steep rise at the equivalence point, where the acid is neutralized. The shape of the curve provides insights into the acid's strength and the pKa value, which is crucial for estimating the acid's dissociation constant.

pKa and Acid Strength

The pKa is the negative logarithm of the acid dissociation constant (Ka) and indicates the strength of an acid. A lower pKa value signifies a stronger acid, as it dissociates more readily in solution. In titration curves, the pKa can be estimated from the midpoint of the steepest section of the curve, where the concentrations of the acid and its conjugate base are equal.

Monoprotic Acids

Monoprotic acids are acids that can donate only one proton (H+) per molecule during dissociation. This characteristic simplifies their titration analysis, as the pH change can be directly correlated to the amount of base added. Understanding the behavior of monoprotic acids in titrations is essential for accurately determining their pKa values and assessing their strength.

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You have to prepare a pH = 5.00 buffer, and you have the following 0.10 M solutions available: HCOOH, HCOONa, CH3COOH, CH3COONa, HCN, and NaCN. How many milliliters of each solution would you use to make approximately 1 L of the buffer?

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