Unit for is a mix of foundational skills, like stoichiometry, and an introduction to later topics.
Unit for is a mix of foundational skills, like stoichiometry, and an introduction to later topics (0:32). The questions in this episode are: 2018 - Question 3 d) through i) and 2014 - Question 1 a) - f).
These are released FRQs from previous exams and copyright of the College Board (1:32). 2018 starts with three questions from a different unit. Part d) asks for a redox reaction (2:05). In part e) you are asked to calculate the concentration of iron(ii)plus in the solution (2:36) and in f) to discuss lab equipment (3:58). G) and i) refer to a second experiment involving iron impurities (4:39) and the effect of incomplete oxidation (5:34). Question 1 from 2014 is a gravimetric analysis with the goal of determining the iodide content in a potassium iodide tablet (7:00). Therefore we write the net-ionic equation (7:22), reflect on our data (7:52) and ion concentrations (8:06) before we calculate the number of moles of precipitate (8:23) and percentage iodide (8:53). Part f) concludes with a question about a potential error (9:38).
Question: True or false: You MUST indicate the state of matter in parentheses in your balanced chemical equation, even if not asked for it.
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Unit 4 Free-response questions.
Let’s Zoom Out
The FRQ questions on the AP exam often combine content from two or three different units. In today’s episode, we are focusing on questions that require knowledge and skills from Unit 4: Chemical Reactions. This is an interesting unit, because it has a lot of overlap with other units. Unit 4 introduces types of chemical reactions, like acid base reactions and redox reactions. But it also has some of the most fundamental content: balancing equations, net-ionic equations, chemical and physical change and stoichiometry. Therefore, we find unit 4 rarely as separate questions, but interwoven with content from other units.
The questions we are using are online accessible. Our suggestion: Answer the questions yourself and then listen to this episode to hear the explanations, as well as do’s and don’ts for answering questions of Unit 4. The questions used are: 2018 - Question 3 d) through i) and 2014 - Question 1 a) - f).
These are released FRQs from previous exams and copyright of the College Board.
Lets Zoom in:
Question 3 of the 2018 exam is about iron and it’s ions, iron(II) plus and iron(III) plus. Parts a), b) and c) are referring to content from Unit 1 - Atomic Structure and Unit 3 - Intermolecular forces, so we will skip these for now and jump into the question with part d), where we are asked to write the balanced equation for the half reaction for the oxidation of iron(ii) plus to iron(iii) plus. Since oxidation indicates that an electron is lost - loooosing electrons is oxidation, which is also reflected in the change in charges from 2+ to 3+, the balanced equation is: Iron(ii) plus yields iron(iii) plus plus one electron.
In part e) you are asked to calculate the concentration of iron(ii)plus in the solution using data from a redox titration. Whenever we have calculations, keep our mantra in mind: Units are friends! Make sure you use units to help organize your work and have a way to “check” your setup. Also keep in mind: In our titration reaction, we are titrating the sample containing iron(ii) plus with permanganate. At the equivalence point, all of the iron(ii) has been converted by the permanganate. And so in a first step, we can calculate the number of moles of permanganate - 0.000612 mol. In a second step, we use the mole ratio of the balanced chemical equation between iron(ii) plus and permanganate. The coefficients tell us that each mole of permanganate reacts with 5 moles of iron(ii) plus. That means, we have 0.003059 moles of iron(ii) plus. Since we are looking for a concentration, we have to take into account that our iron(ii) sample is 0.01L, which means the sample had an iron(ii) concentration of 0.306 M. Part f) of the question is related to lab skills. A 25 mL volumetric flask would be a poor choice to deliver 10.0 mL of the sample solution, because the volumetric flask has only one marking - at 25.00 mL.
The question continues with a second experiment, in which there is a powdered iron sample with impurities. The sample is being oxidized to iron(III) oxide with the goal of determining the percent mass of iron in the original sample. The data given shows the mass of the powdered iron with the inert impurity as well as the mass of iron(III) oxide that is being produced. In question part g) you are prompted to calculate the number of moles in the iron(III) oxide. This is a two step problem: in the first step, we convert the mass of iron(III) oxide to moles, using the molar mass. We have 0.04716 moles of iron(III) oxide. Using the mole ratio of 2 moles of iron for each 1 mole of iron(III) oxide, we can calculate to have 0.09431 moles of iron. In part h) of the question, we can now use the moles of iron and calculate the percent by mass of iron in the sample. We convert the moles of iron to 5.267 g of iron. We know that our sample with the impurity weighs 6.724 g. So, the percent iron by mass in the sample is 78.33%. Finally, in part i) of question 3, we are asked to discuss the effects of an incomplete oxidation of iron on the calculated mass percentage. If the oxidation is incomplete, you would produce iron(II) oxide, which will have a higher mass percentage of iron - 77.7% vs. 69.9%. This would lead to having a lower number of moles of iron and therefore also a lower mass percentage.
The second question we are discussing is from the 2014 exam, question 1. In this question, we have a gravimetric analysis with the goal of determining the iodide content in a potassium iodide tablet. The tablet is dissolved in water and lead(ii) nitrate is added to form a precipitate of lead(ii) iodide. In part a) you are asked to write the net-ionic equation and explain why this is the best way to represent the reaction. The net-ionic equation is the best representation, because it shows the formation of the precipitate from the ions and eliminates all spectator ions: Pb(II) plus, plus 2 I minus yields lead(II) iodide. Net-ionic equations are a very important topic and do need some practice! Part b) is a question related to the presented gravimetric analysis. The filter paper is weighed and dried three times until the mass is constant to ensure that all the water has been driven off.
Part c) asks about the relative concentration of potassium cations and nitrate anions. The concentration of the nitrate anions will be greater than the concentration of the potassium cations, because the lead(II) nitrate was added in excess. Now, in part d) we are getting to the stoichiometry part, calculating the number of moles of precipitate. Using the experimental data, we can subtract the mass of the dried filter paper from the “mass of filter paper and precipitate after third drying” and calculate to have 0.236g of lead(II) iodide. Using the molar mass of lead(II) iodide, we calculate 5.12 x 10-4 moles of lead(II) iodide. Our original goal was to determine the mass percentage of iodide in the tablet. Therefore, in part e) we use the moles of lead(II) iodide and the mole ratio of 2 moles of iodide per mole of lead(II) iodide to calculate 1.02 x 10-4 moles of iodide. Using the molar mass of iodine, we determine that we have 0.130g of iodide in one tablet. As a last step to calculate the mass percentage, we divide the 0.130g of iodide by the total mass of the tablet, 0.425g, which gives us 30.6% of iodide in the potassium iodide tablet. In part f) we need to predict how dissolving the tablet in more water will change the mass percent of iodide and justify the answer. Since all the water will evaporate during the drying, it actually doesn't change the mass percentage - it will just take a tad longer to dry! So, no change!
To recap…
Unit 4 is an interesting and important mix between topics that will be discussed in later units, but also includes foundational knowledge and skills like balancing equations and stoichiometry. Stoichiometry usually involves mole calculation: Keep our mantra in mind: Units are friends! Use them to organize, structure and guide your work! Both discussed questions have a lab-related subquestion. When you are working through a lab activity, question yourself about possible experimental or human errors and how it would affect your results.
Coming up next on the APsolute RecAP Chemistry Edition: Concentration changes
Today’s Question of the day is about chemical equations.
True or false: You MUST indicate the state of matter in parentheses in your balanced chemical equation, even if not asked for it.