Weak Titrate-Strong Titrant Curves - Video Tutorials & Practice Problems
Get help from an AI Tutor
Ask a question to get started.
1
concept
Acid-Base Titration Curve
Video duration:
2m
Play a video:
In this video, we now take a look at a weak tit rate being titrated by a strong tight trend. We're gonna say under this type of titration between a weak, tight trait and a strong tight trend, a buffer region can exist. So if we take a look, we have a weak titrate, strong titrate curve. And here there are a few key features that we need to keep in mind. All right. So our pure tran is just again, the beginning of the titration before any tran has been acted. So we're starting off here, the buffer region is just a region where the ph is most resistant to change. So what we need to realize here is that we're starting out with pure titrate and our peach is gonna increase pretty quickly initially, but then it's gonna start leveling off and gradually increasing. So in this region here that I've highlighted, that's why we have the slow, gradual grind up in ph. That's because at that point, our buffer has been created. Now, we'll see mathematically how this works out. But for right now, just realize that we start out with pure titrate in the beginning. But near the beginning, there's a sharp increase of PH and then there's a, there's a moment where a buffer is created and because a buffer is created, there's a slow down in the gradual increase of the ph. Now, of course, eventually you add too much tran and the buffer is destroyed and then you start increasing your ph drastically again. So here we have our buffer region, here we're gonna say our half equivalence point. This is just a midpoint within the buffer region where the amount of weak acid is equal to the amount of conjugate base. This is where we have an ideal buffer. So midway here. So around here, around here. So now the equivalence point, this is just the middle region of the curve itself that has the steepest incline. So here, this would be our equivalence point. So this is our equivalent point and this is our half equivalence point, they're related to each other. So it takes about 50 ML to get to the equivalence point. So it'll take about half of that 25 ML to get to the half equivalence point after the E Colman's point, which is this region here, this is where there is an excess of tiran still being added. So when it comes to weak tight trait and strong, tight tra, these are the key features you need to keep in mind when looking at a typical curve
2
example
Weak Titrate-Strong Titrant Curves Example
Video duration:
3m
Play a video:
Consider the titration of 55 mls of 0.120 molar hydro cyanic acid with 0.160 molar lithium hydroxide, calculated the volume needed to reach the half equivalence point. All right. So let's look at the following steps that we can utilize to help us find this answer. So step one, we determine the volume of the tran to reach the equivalence point. All right. So who's our title? We're not given the V of lithium hydroxide. That's because it's behaving as the tran here. We'd say that M acid times V acid equals M base times V base. When it comes to acid base titration, there are formulas that you should keep in mind. So our equivalent volume can be calculated using this equation, acid times V acid equals M base times V base. Now, before the equivalent point, we have a buffer region that can exist. And because of that, we utilize the Henderson Hasselback equation here. I'm just showing the variant where we have PK A involved Ph equals PK A plus log of conga base over weak acid. And then our half equivalence point is where the amount of weak acid and conjugate base are equal to one another, meaning that this would be equal to one. And so we drop out because log of one is equal to zero. The Henderson Hasselback equation would simplify to just PH equals PK A. And if we were to isolate K A, it would be restructured as K A equals 10 to the negative Ph. Right now, let's go back, we're using the equivalent volume formula so that we could figure out the volume of the tran to reach the equivalence point. So here we're gonna say we have 0.120 molar of our acid, 55 MLS of acid, our basis 0.160 molar and we don't know the volume of our base divide both sides by 0.160 molar. So cancels cancels volume of our base to get to the equivalent point would be 41.25 mL. Step two, utilize the correct formula based on the region of the titration curve. Utilize this step only if necessary. All right. So we just determined the volume to get to the equivalence point. Well, the half equivalence point would just mean we need half of that volume. So all we do here is we divide this by two and that will give you my half equivalent volume. What y'all just put as the sub half. So dividing 41.25 MLS by two, gives me 20.625 mL. So this would represent the volume to get to the half equivalent point for this particular example question.
3
Problem
Problem
Consider the titration of 30.0 mL of 0.100 M HC2H3O2 with 0.100 M NaNH2. Which volume of NaNH2 would take place within the buffer region?
a) 30.0 mL b) 50.0 mL c) 10.0 mL d) 100.0 mL e) 1.5 L
A
30.0 mL
B
50.0 mL
C
10.0 mL
D
100.0 mL
E
1.5 L
4
concept
Weak Titrate-Strong Titrant Curve
Video duration:
2m
Play a video:
In these two types of titration curves, the PH changes gradually before the equivalent point because we're gonna have a buffer region that can be formed. So here we're gonna take a look at the left side and the following video, we'll take a look at the other side. But let's focus on this left side. First, here we have a weak acid, strong base titration curve. We're starting off with a Ph less than seven. So because the PH is less than seven here, it would have to be the weak acid because if it was a strong base, it'll be above side here. We're slowly adding our strong base to it. So our strong base is the tight trin because we're starting out with an acid, we see that the PH is less than seven and it increases gradually with added base. That's because initially there's a jump in Ph, but we get to a point where a buffer has been created. And so there's a slow grind up in terms of our Ph. This highlighted part is where our buffer region has been created now here because we have something weak versus something strong. The strong species will dominate. Therefore, the ph of the equivalence point would be greater than seven. So here we can see if the PH is around eight or so. Now, after the equivalence point, so we're talking about after this red dot we're gonna say the weak acid is neutralized and excess strong base remains, we just keep adding more and more strong base even after all that weak acid has been neutralized. So this is why our ph continues to grow and and increase over time. But eventually it's got a plateau because it can't go up forever, right? So when it comes to this weak acid, strong base titration curve, these are the key features to keep in mind when looking at a typical curve, when we have a weak acid versus a strong base, whatever a strong will dominate, since it's a base, that's the strong species. It wants the PH to be above seven at the equivalence point, right. So that's why we see our equivalent Ph greater than seven. It's around eight or so here, right? So now that we've taken a look at this titration curve, click on the next video and let's look at the opposite trend.
5
concept
Buffer Region
Video duration:
1m
Play a video:
Now, let's take a look at the opposite trait here. Our weak base will be our tight trait. We're starting at a PH above seven and we're adding strong acid to it. Strong acid will be our tight print. Now because we're starting out with a base. We'd expect our Ph to be greater than seven initially and it decreases gradually with added acid. That's because initially, there's gonna be a, a quick drop in Ph, but then it's gonna kind of level off because in this highlighted region, a buffer region has been created. So this is gonna resist additional drop, steady, clear drop in Ph. It's gonna gradually slow down. It's still gonna be decreasing, just not at a same fast rate. Now here, what's the Ph at the equivalence point? Remember we have a weak species and a strong species, the strong species will dictate where the Ph will be at the equivalence point because if a strong acid, it wants an acidic Ph, so we'd want the Ph to be less than seven at the equivalence point here, we can see that the equivalence point is around a ph of five. Now, after the equivalence point So after the red dot, we can see that the weak base is neutralized, but we still keep adding strong acid. So there's gonna be some excess, strong acid remaining, eventually, it's gonna level off in plateau. And when again, when it comes to weak and strong, whatever is strong will dictate, what kind of ph we will have at the equivalence point. In the first one, since we had a strong base, the ph was above seven at the equivalence point. And in this one, the acid is what's strong. So we're gonna have a peach less than seven at the equivalence point. So keep this in mind when looking at these two types of titration curves.
6
example
Weak Titrate-Strong Titrant Curves Example
Video duration:
2m
Play a video:
At the half equivalence point, the concentration of weak acid is equal to the concentration of conjugate base. Consider the titration of 100 mls of 0.200 molar of a weak monno protic acid, meaning it only has one acidic H plus with 50 mls of 0.200 molar sodium hydroxide determine our K value of the weak acid if the PH is 4.18. Now, here they're giving us amounts of our weak mono protic acid and our strong base title. But that's not the key to the question. I tell us that we're talking about the half equivalence point. So we would know that these amounts would somehow get us to this half equivalence point. Here, we're going to say if we're at the half equivalence point P equals PK A and remember I talked about earlier how we can rearrange this formula to help us isolate K A. How do we do this? Well, we're gonna say that PK A really needs negative log of K A. So if we take, if we divide both sides by negative one, by both sides by negative one, that's gonna be negative Ph equals log of K A and then take the inverse log of both sides. So when we take the inverse log of both side, it's gonna become 10 to the negative Ph equals when I take the inverse of log of the right side, the log just cancels out or disappears. So it becomes this formula K A equals 10 to the negative. Ph. Again, this happens when we're at the half equivalence point. That's because weak acid and conjugate base are equal to each other in amount. So what we do here is we do 10 to the negative 4.18. So when we do four ne 10 to the negative 4.18 that's gonna give me 6.61 times 10 to the negative five as the K A for this weak Monno protic acid. So this would be our final answer.
7
Problem
Problem
In titration of NH3 and HCl, what is the predominant species beyond the equivalence point?
a) NH3 b) HCl c) CH3NH2 d) Cl–
A
NH3
B
HCl
C
CH3NH2
D
Cl–
Do you want more practice?
We have more practice problems on Weak Titrate-Strong Titrant Curves