Consider the titration of butyric acid (HBut) with sodium...

Consider the titration of butyric acid (HBut) with sodium hydroxide. In an experiment, $50.00 \mathrm{~mL}$ of $0.350 \mathrm{M}$ butyric acid is titrated with $0.225 \mathrm{M}$ $\mathrm{NaOH} . K_{\mathrm{a}}$ HBut $=1.5 \times 10^{-5}$ (a) Write a balanced net ionic equation for the reaction that takes place during titration. (b) What are the species present at the equivalence point? (c) What volume of sodium hydroxide is required to reach the equivalence point? (d) What is the $\mathrm{pH}$ of the solution before any $\mathrm{NaOH}$ is added? (e) What is the pH of the solution halfway to the equivalence point? (f) What is the $\mathrm{pH}$ of the solution at the equivalence point?

Consider the titration of butyric acid (HBut) with sodium hydroxide. In an experiment, $50.00 \mathrm{~mL}$ of $0.350 \mathrm{M}$ butyric acid is titrated with $0.225 \mathrm{M}$ $\mathrm{NaOH} . K_{\mathrm{a}}$ HBut $=1.5 \times 10^{-5}$
(a) Write a balanced net ionic equation for the reaction that takes place during titration.
(b) What are the species present at the equivalence point?
(c) What volume of sodium hydroxide is required to reach the equivalence point?
(d) What is the $\mathrm{pH}$ of the solution before any $\mathrm{NaOH}$ is added?
(e) What is the pH of the solution halfway to the equivalence point?
(f) What is the $\mathrm{pH}$ of the solution at the equivalence point?

Key Concepts

Acid-Base Titration

Acid-base titration is an analytical procedure where a solution of known concentration (titrant) is used to determine the concentration of an analyte in solution. It involves the gradual addition of titrant to the analyte until the reaction reaches the equivalence point, where the amount of titrant added exactly reacts with the substance in the sample.

Net Ionic Equation

A net ionic equation represents the chemical species that actually participate in the reaction, omitting the spectator ions. In acid-base reactions, it typically focuses on the transfer of a proton from the acid to the hydroxide ion from the base, which simplifies the reaction representation.

Equivalence Point

The equivalence point in a titration is reached when the number of moles of titrant added equals the number of moles of the analyte originally present. For a weak acid being titrated with a strong base, the equivalence point is characterized by the complete neutralization of the acid, leading to the formation of its conjugate base.

Stoichiometric Calculations

Stoichiometric calculations in a titration context involve using molarity and volume to determine the number of moles of reactants. This allows one to calculate the volume of titrant needed to reach the equivalence point based on the balanced chemical reaction between the acid and the base.

Initial pH Calculation

The initial pH of a weak acid solution is computed using its acid dissociation constant (Ka) and concentration. By setting up an equilibrium expression for the weak acid dissociation and solving for the hydrogen ion concentration, one can determine the pH before the titration begins.

Half-Equivalence Point and Buffer Solutions

At the half-equivalence point of a titration, half of the acid has been neutralized, resulting in a buffer solution composed of the acid and its conjugate base in equal amounts. Here, the pH can be conveniently calculated using the Henderson-Hasselbalch equation, where the pH is equal to the pKa of the acid.

Conjugate Base Hydrolysis

At the equivalence point in the titration of a weak acid with a strong base, the resulting solution contains the conjugate base of the acid. This conjugate base undergoes hydrolysis with water to generate hydroxide ions, which causes the solution to be basic. The pH at this stage is determined by setting up and solving the equilibrium expression for the base hydrolysis reaction.

Transcript

00:01 Okay, so we're going to do a titration between butyric acid, which we'll write in that shorthand with a strong base, so we'll just write oh minus, and they'll make h2o and the conjugate wheat base.

00:20 So that's a net ionic equation.

00:23 At the equivalence point, what we'll have present, okay, is that conjugate base and water, and we'll also have some sodium ion, and that came from the sodium hydroxide, which is the strong base that we added.

00:41 So we're going to figure how much strong base we needed to add.

00:45 So let's start by finding how many moles of our weak acid we started with.

00:49 So we've got our molarity, like 35 molar.

00:54 We'll multiply that by the liters.

01:00 And we'll see that we started with 0 .1 .75 moles of our weak acid.

01:07 So we must have added that same number of moles of our strong base.

01:16 And since we know the molarity of our strong base, we'll go ahead and divide by that.

01:22 Right, 0 .225 molar.

01:27 And that will give us the liters, 0778 liters, or 77 .8 milliliters.

01:37 So that's the amount of sodium hydroxide we would have had to add.

01:44 And then the next step, we're going to try to figure out the ph before the titration.

01:49 So all we're doing is finding the ph of our weak acid.