Calculate the pH of a solution prepared by mixing 100.0 mL of 1.20 M ethanolamine, C2 H5 ONH2,

step 1 So we're going to combine two solutions here. Let's go ahead and find moles of each reactant so

Calculate the pH of a solution prepared by mixing 100.0 mL...

Calculate the $\mathrm{pH}$ of a solution prepared by mixing $100.0 \mathrm{~mL}$ of $1.20 \mathrm{M}$ ethanolamine, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONH}_{2}$, with $50.0 \mathrm{~mL}$ of $1.0 \mathrm{M} \mathrm{HCl} . \mathrm{K}_{\mathrm{a}}$ for $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONH}_{3}+$ is $3.6 \times 10^{-10}$.

Key Concepts

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is used to estimate the pH of a buffer solution. It relates the pH to the pKa of the acid and the ratio of the concentrations of the conjugate base to the acid. This equation simplifies the calculation of pH in systems where both a weak acid (or conjugate acid) and its conjugate base are present.

Conjugate Acid-Base Pairs

In acid-base chemistry, every weak acid has a conjugate base and every weak base has a conjugate acid. Understanding these pairs is essential, as the strength of an acid is inversely related to the strength of its conjugate base. In reactions between a weak base and a strong acid, the formation of the conjugate acid and the remaining unreacted base create the conditions necessary for buffer action.

Acid-Base Reaction Stoichiometry

This concept involves calculating the moles of acids and bases present before and after a reaction. In titrations or mixed solutions, it is crucial to determine how much of the base reacts with the acid and what remains afterward. The mole ratios between reactants help identify the limiting reagent and calculate the concentration of the unreacted species as well as the formation of conjugate acids or bases.

Buffer Solution Formation

Buffer solutions are created when a weak acid and its conjugate base (or a weak base and its conjugate acid) are present in solution in appreciable amounts. This composition helps resist changes in pH upon addition of small amounts of acids or bases. The concept is key when the reaction of a weak base with a strong acid produces a mixture that contains both the unreacted weak base and its conjugate acid, forming a buffer.

Transcript

00:01 So we're going to combine two solutions here.

00:03 Let's go ahead and find moles of each reactant so that we can see what's happening.

00:08 Okay, so to start with, i'm going to go ahead and find the moles of our amine.

00:13 I'm going to take its molarity and its volume and liters.

00:17 We'll multiply those to see that i have 0 .1 -20 moles, or of our ethanol amine.

00:29 And then our other solution, i have 1 molar and 50 milliliters, so 0 .05.

00:36 Liters so i can see that i have 0 .050 moles of hcl and so that'll be the same as the moles of h plus.

00:48 So i have a strong acid and a weak base.

00:52 So those two things are going to react with each other.

00:54 The h plus will react with the ethanol amine and they'll just come together to form its conjugate acid.

01:15 So let's make a table where we can keep track of our initial moles, well, the change, and then our final moles of each thing.

01:23 So to start with the h -plus, i had 0 .05 moles.

01:28 The ethanol -mean, i had 0 .1 -2 -0.

01:31 I didn't make any of this yet.

01:34 So you can see that our h -plus is our limiting reactant, so all of that will react.

01:39 And then the same amount of the ethanol -mean, and then i'll make the same amount of our weak base because it's all one -to -one ratio...

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