A 100.0-mL buffer solution is 0.175 M in HClO and 0.150 M in NaClO. a. What is the initial pH of

A 100.0-mL buffer solution is 0.175 M in HClO and 0.150 M in...

Key Concepts

Acid–Base Buffers

Acid–base buffers consist of a weak acid and its conjugate base (or a weak base and its conjugate acid) and are used to maintain a relatively constant pH when small amounts of a strong acid or base are added. They work by neutralizing added acids or bases, thus preventing drastic changes in the hydrogen ion concentration of the solution.

Henderson–Hasselbalch Equation

The Henderson–Hasselbalch equation is a mathematical expression used to estimate the pH of a buffer solution. It relates the pH to the pKa of the weak acid and the ratio of the concentrations of the conjugate base to the acid, allowing for quick calculation of the pH under equilibrium conditions.

Reaction Stoichiometry in Buffers

Reaction stoichiometry in buffers involves understanding how the strong acid or base added to the buffer reacts with its components. The added acid will react with the conjugate base, whereas the added base will react with the weak acid. Correct stoichiometric calculations are critical for determining the resultant concentrations of the buffer components and the subsequent shift in pH.

Conversion of Mass to Moles

Conversion of mass to moles is a fundamental concept necessary to quantify the number of particles, which is particularly important when dealing with additions of reactants (like strong acids or bases) to a buffer. This conversion uses the molar mass of the substance to translate a given mass into the number of moles, which can then be employed in stoichiometric calculations to assess the effect on the system's pH.

Transcript

00:01 Okay, so we have a question that is asking us to calculate the ph of a buffer solution, specifically 100 mils of 0 .175 hclo.

00:21 Pca of hclo is 7 .53.

00:27 All right.

00:30 So, when you have a mixture of a weak acid and its conjugate base or vice versa, all you need to use is the hand.

00:38 Anderson hustleback equation to solve it.

00:40 So that's going to be p .k .a.

00:47 Log base time.

00:51 Now, if you get confused as to why this equation is like this or why it's the conjugate base on top instead of the acid, this equation is just derived from a standard acid base or acid equilibrium.

01:12 So you can imagine if you take the log of this, you'll get pca, you'll get ph, and then you'll be able to have with the log of this portion here.

01:20 Okay, so to start, part a of this question says, what's the ph? and to do that, the h is, e .k .a.

01:32 7 .53 plus log, and that equals 7 .53 plus, and that equals 7 .46.

02:03 Now, if you want to do a sanity check on these questions, so the p .k .a.

02:09 The significance of it is that is going to be the ph of a solution with equal parts, conjugate acid, and conjugate base.

02:17 So given that these concentrations are quite close together, we would expect the ph to be near the pca, but a little bit below it because we have slightly more of the weak acid.

02:27 And we see that this number we got does that, so we can be confident that's correct.

02:33 Now, question b says, what is the ph when? 85 milligrams or sorry 150 milligrams of hbr is added.

02:50 Now we're not expecting that the hbr would change the volume of the solution so we can still use 100 mils our volume and what we're going to want to do is figure out what hbr does.

03:01 So hbr is a strong acid which means that it will fully react with our weak base.

03:11 So hbr plus sodium hypochlorite is going to go all the way, it's going to go all the way to, basically the amount of each, for every mole of hbr we add, we're converting one mole of our conjugate base back into our acid.

03:38 So if we look back at the henderson -oxelac equation, naclo over h -c -l -o.

03:50 So the concentration of this is going to go up and the concentration of this, sorry, the concentration of this is going to go down.

03:59 Concentration of this is going to go up.

04:02 Basically, they're going to be related like this.

04:04 So the new, so let's say, call this f, is going to be the initial concentration.

04:16 Call it i, subtract the concentration of hbr we add.

04:23 Okay? and same thing with hclo.

04:28 The final concentration is going to be the initial concentration, plus concentration of hbr that was added.

04:38 Okay, because remember the hbr completely converts at one equivalent of sodium hypochlorite into hypochloric acid.

04:48 All right, so during the page here, so what we have is ph plus log of concentration of naclo, subtracts hbr, h -s -l -o, zo plus hb .r.

05:20 Now we need to know what the hbr concentration is.

05:25 So hbr is just going to be number moles over the volume.

05:32 And we got the mass of it, so it's going to be the mass over the molar mass times the volume, which is going to be 80, 150 milligrams, so 0 .150 grams over molar mass, 80 .90 grams over frames per mole and times of volume 0 .10...

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