AP Chemistry 2026 Free Response Question 6 - SOLVED!

Ajouté : 2026-05-09

[00:00:00]Hi there, my name is Jeremy Krug and this is the place for all things AP Chemistry. As I record this video, the free response questions for the 2026 AP Chemistry exam have just been released.

[00:00:12]And this is my walkthrough for free response question number six, which is a short FRQ worth points. If you like what you see or you learned something from this video, remember to smash that like button and leave a comment down below.

[00:00:26]If my videos have been helpful to you this year, I'd be very honored if you'd recommend my videos to next year's AP Chem students. And just for full disclosure, this answer key is my best prediction for the point breakdown. Full scoring guides get released by College Board later in the summer. And as always, other answers that are chemically correct are also acceptable.

[00:00:49]Now, here's my walkthrough for question number six.

[00:00:53]Question six is a good laboratory question. We have spectrophotometry here and the question says, "A student performs a spectrophotometry experiment with venadium 2 plus solutions and produces the calibration curve shown in figure one by measuring the absorbance of several solutions of known concentration. So we have the different concentrations, we have the absorbances and we have the calibration curve. Part A says the student measures an absorbance of 32 for a venadium 2 plus solution. The concentration of V2 plus ions in the solution is represented by the particle diagram in the circle in figure two. So we see this up here in the circle in figure three. Draw the correct number of venadium 2 plus ions to represent the concentration of a solution with an absorbance of 08. Water molecules have been omitted. For clarity, assume that each particle diagram represents the same volume. So notice that in figure two, if we uh count the the circles here, we have 1 2 3 4 5 6 7 8. So there are eight particles drawn for an absorbance of 32.

[00:02:05]Well, in figure three, the absorbance is 0.08, which is exactly 1/4th what it was in the previous picture. So since the absorbance is 1/4 what it was in the previous picture, we should have 1/4 as many particles. So instead of having eight particles drawn up here, we should have well 1/4 of eight, which is only two particles drawn. So hopefully you drew two circles or two particles there to represent that concentration. So if you did that, give yourself one point for drawing in those two particles. As we move on to part B, we have one and two. On part B, it says the student measures the absorbance of a different venadium 2 plus solution with an unknown marity, but the absorbance is higher than.36, the upper limit of the graph. The student then transfers 3.00 milliliters of this venadium 2 plus solution to a 25ml volutric flask, dilutes to the mark on the flask with distilled water, and measures the absorbance of the diluted solution. Part one says the absorbance of the diluted solution is 0.2 22.

[00:03:12]Determine the marity of venadium 2 plus ions in the diluted solution. So all we have to do is read the graph. We just take a look at where 0.22 is. So it's right there, that first uh hash mark above 0.20. And if we slide that over to the calibration curve, it hits right around here. And if we drop down to the x-axis, we see that we have a concentration of just around 055 molar. So I would say 055 molar or something very close to that. If you got that, give yourself one point for reading the graph correctly. Now part two says determine the marity of venadium 2 plus ions in the original undiluted solution. Show the work that leads to your answer. So since we're talking about dilution, I would use the dilution equation which is m1 v1 equals m_sub_2 v2. the marity of the initial solution times the volume of the initial solution equals the marity of the final solution times the volume of the final solution. So the marity of the initial solution m1 is what we're solving for that undiluted value. So my m1 is going to be my unknown in this question. Now the v1 the volume of that undiluted solution it says was 3.00 milliliters. So I'll put 3.00 milliliters in for my v1. Now my final marity after it was diluted down well we see that that is 055 molar and that was what we got there in part B1. So that answer that you got from B1 basically had to be dragged down to M2. Now V2 the final volume it says that it was diluted down to 25 milliliters in your volutric flask. So 25 milliliters is your V2.

[00:05:03]Whenever we do the algebra here and solve for M1, we find that the initial marity is about 0.46 molar. So hopefully you got that or something very close to that as your answer. And that makes sense because a 46 molar solution would be basically off the chart here on the absorbent. So give yourself one point if you got that correct. So that is the third point on question uh on question six here. Let's take a look at the last part of this part C. It says the concentration of venadium 2 plus ions calculated in part B2 is lower than the actual concentration of the undiluted solution. The student claims that the calculated concentration is too low because during the dilution step the final level of the solution was higher than the mark on the volumetric flask.

[00:05:55]Do you agree or disagree and justify your answer? I would agree because whenever you use a volutric flask, whenever you add water to the line, that is, you know, the correct way to do it.

[00:06:06]If your water mark is higher than the mark on the flask, that means that you added too much water than you should have. And that means that your solution is going to have a lower concentration than it should have. And so that's basically what we see here. The concentration was lower than it should have been. So uh that tells us that yes we should agree because we added too much water. So if you said that give yourself a point and gave a reasonable justification for that. So we had some good laboratory experimental type questions here. If you did spectrophotometry in lab in your class then hopefully this helped you as you answered that question. So that's it. I hope this was useful for you as you reflect on your exam performance for question six or you get ready for a future exam. And if you are getting ready for a future AP Chem exam, remember that my ultimate review packet and ultimate exam slayer have dozens of practice free response questions and nearly a thousand practice multiple choice questions with full explanations to help you slay your AP exam. The link is in the description down below. Thank you so much for watching this video and I hope to see you soon.