2026 AP Chemistry FRQ Question 6

Hinzugefügt: 2026-05-10

[00:00:01]Hey guys, welcome back. This is question number six of the 2026 AP Chemistry FRQ.

[00:00:07]A student performs a spectra spectrophotometry experiment with a uh V2 plus solution and produces calibration curve as shown in figure one by measuring the absorbance of several solutions of known concentrations. So this is where we put on our beers at law, right?

[00:00:24]Beer's law. It's actually or in grad school called beers Lambert law, right?

[00:00:28]But it's called Beer's law. Poor Lambert. All right. So, here we go. Part A. The student measures absorbance of 324 V2 plus solution. Concentration of V2 plus ions in the solution is represented by the particle diagram in the circle in figure two. In the figure, we draw the correct number of Oh, this is stupid, right? Look. So it measures absorbance 0.32 that is uh 1 2 3 4 5. So it's going to 32 is right here right that's here and that's at uh 0.08 08 molar right 0.08 molar the concentration of V2 of the solution is represented by the di so that is representing this figure two in figure three draw the correct number of ions to represent concentration of solution with an absorbance of 0.08 0.08 08. What is that? That's going to be uh each one of these is 0.02, right?

[00:01:37]2 4 5 uh 2 4 6 8 10, right? So, it's 0.08 is going to be right here somewhere. So, that lands you they they even put a dot. I mean, how silly.

[00:01:52]That's right here. So, that's a concentration of 0.02 molar. So we went from figure 2 is 0.08 molar to figure three you want 0.02.

[00:02:08]So what is that? That's um 25% right or 1/4 or a quarter. All right. So we want to show quarter. How many we get here? 1 2 3 4 5 6 7 8. So we have eight here. So 8 / 4 is what? two two dots. It's that easy. All right, so let's do it. Figure three. Going to make two beautiful dots and we're done. Right, so part A, 5 A, 5 A, C, figure uh three, right? So that's part A. Part A done. Let's check it off.

[00:02:52]All right, let's go to B. The student measures the absorbance of different V plus 2 plus solution with an unknown marity. But the absorbance is higher than 0.36 which is upper limit of the graph. So this is too concentrated.

[00:03:07]Solution then transfers 3 ml of this solution to a 25ml volutric flask.

[00:03:12]Dilutes the mark on the flask with the distilled water and measures the absorbance of the uh diluted solution.

[00:03:19]All right. The absorbance of the diluted solution is 0.22. Determine the marity um of uh V2 plus in the diluted solution. All right, let's go. Um this is B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B R Roman one. All right, so um did we just do this absorbance of this? All right, so part B.

[00:03:46]Wait, what? This is so stupid.

[00:03:49]Absorbance is 22.

[00:03:53]Wait, you're just reading it, right?

[00:03:54]This like this elementary school kids could do this. 22 is right here, right?

[00:04:01]They even put a dot like what? That means it's this, right? Wait, could it be this? Yeah, it's this easy. That was really stupid.

[00:04:12]All right. So 5B I right um the marity of the V2+ at what? Um is 0. What did we say? It was 0.55.

[00:04:34]Is that is that what we said?

[00:04:39]No. 0.05.

[00:04:41]Right. zero. Absorbance was 0.2, right?

[00:04:45]Or was it 0 22? A 22. It's a little bit higher. So absorbance is like a hair higher, right? So it lands at like right there, which is like right over here. So it's not 0.05. It's actually just a shmear higher than that. Right.

[00:05:04]2 E.

[00:05:07]No. Oh, no. Actually, see, it's it's even easier. Look at this.

[00:05:12]Each each y is 0.2 right? 2 point 2 2 point.22.24.26.28 point. So 22 lens you actually right here which is right in between 0.05 and 0.06. Therefore it's 0.55 0.055 right 0.055.

[00:05:36]So therefore uh 0.05 0 55 molar is your answer. Right? All right. Part two.

[00:05:46]Determine the marity of the original undiluted solution. Show the work. All right. Well, um this is very it's just you know dilution equation, right?

[00:06:05]Right. What do we got going here? Well, let's go. Um, what is our um new dilution right over here? Our our new concentration is 0.055 molar, right? And then let's go back to and then how much of that do we have?

[00:06:23]This this 0.055 molar was the fully um diluted, right? So that's the that's the um that's this one, right?

[00:06:37]Right. So that was 0.25.

[00:06:44]Can't believe I put M2. It's V2, right?

[00:06:48]Milliliters, right? We good? Um I'm sorry. 25.0 25.0 milliliters, right? And then what what else we know? We took originally 3.00 0 0 milliliters and we don't know what the concentration is. That's it, right? So what is this? Um it's M1 V1 equals M2 V2. So therefore V1 is equal to M2 V2 over M1. And then plug in everything in U. No, what am I doing? We're solving for V1.

[00:07:26]We're solving for V1, which is M2 V2 over M1, right? No, wait.

[00:07:34]That's sorry.

[00:07:38]Uh, it's a long day. All right. So, it's m1 is equal to v1 like that, right? So, m1 v1 equals m2 v2. So, move the v1 to the other side. All right. So, we got m2 0.055 molar time v2, which is 25.0 ms all over 3.00 ms. And what do we got? Um that gives me two sigfigs 0.46 right so what will we say we say the marity of um what is that determine the marity of the V2+ is equal to 0.46 46 molar. Circle that. Circle that. That's 52. And part C. The concentration uh is calculated is lower than the actual concentration of the undiluted solution.

[00:08:36]Student claims that the calculus concentration is too low because during the dilution step, the final level of the solution was higher than the mark on the volumetric flask. Do you agree or disagree? Just right answer. This is so easy. 5C. Right. What do we know? Um yes, I agree. Why?

[00:08:54]Um the student overshot the mark, right?

[00:08:57]Overshooting, right? Um in the volutric flask means he how do you know it's a he? I don't know. Because he did something stupid.

[00:09:18]It's always a he, right? Means he added too much water.

[00:09:28]Water water. Thus, um it was over diluted and concentration value would be lower, right?

[00:09:56]um is the student claims that the calculator answer is too low. Yep. And that's the reason why. And that's it. We're done.

[00:10:08]Is that it? Get out. This was an easy question, man. I hope you guys all got this one right. All right. I'll see you on the next one.