8 | FRQ (Long) | Practice Sessions | AP Chemistry

Advanced Placement

24 Apr 202311:55

EducationalLearning

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TLDRIn this AP Daily Practice session, Kristen Cacciatore guides students through the process of determining the iodide (I-) content in tablets containing potassium iodide (KI) and a water-soluble sugar. The video covers the chemical reaction between KI and lead nitrate (Pb(NO3)2), resulting in the formation of a yellow precipitate, lead iodide (PbI2). The session explains the importance of using net ionic equations to represent the reaction, the significance of drying and weighing the precipitate to ensure all water is removed, and how to calculate the mass percent of iodide in the tablet. Additionally, it addresses the impact of varying water volumes on the experiment and the potential use of silver nitrate (AgNO3) as an alternative to lead nitrate for precipitating iodide. The video concludes with a discussion on the limitations of measuring the mass of AgI to three significant figures with a balance that measures to the nearest 0.01 grams. Throughout, Kristen provides tips for further study and resources for AP exam review.

Takeaways

🔍 **Session Focus**: The session is dedicated to practicing long free response questions for the AP exam.
📚 **Resource Availability**: A PDF with practice questions is available for download via a link in the video or description.
🧪 **Chemical Reaction**: The net ionic equation for the reaction between KI and Pb(NO3)2 is Pb^2+ + 2I^- → PbI2(s), representing the formation of a yellow precipitate.
⚖️ **Net Ionic Equation**: It is used to simplify the reaction by showing only the reacting species, omitting the spectator ions like K^+ and NO3^-.
📉 **Drying and Weighing**: The process of drying and weighing the filter paper with the precipitate multiple times ensures that all water is removed, providing an accurate mass of the precipitate.
🌊 **Excess Reagent**: The use of excess Pb(NO3)2 ensures that all I^- reacts, and the concentration of NO3^- is higher than K^+ in the solution.
🧮 **Calculating Moles**: The number of moles of precipitate is calculated by subtracting the mass of the filter paper from the total mass and then converting to moles using the molar mass of PbI2.
📊 **Mass Percent Calculation**: The mass percent of I^- in the tablet is determined after calculating the moles of PbI2, converting to moles and mass of I^-, and using the mass of the tablet.
⏳ **Water Volume Impact**: Changing the volume of water used to dissolve the tablet does not affect the mass percent of I^- determined, as the limiting reactant and product formation are not influenced by the water volume.
🔗 **Substitution of Reagents**: Using AgNO3 instead of Pb(NO3)2 will result in the precipitation of I^- as AgI due to the low solubility of AgI, indicated by its Ksp value.
📐 **Precision Limitation**: With a balance that measures to the nearest 0.01 grams, it's not possible to determine the mass of AgI produced to three significant figures if the mass of the precipitate is less than 1 gram.
📈 **Additional Resources**: For further review, the video suggests specific topics on AP Daily, covering net ionic equations, stoichiometry, solubility equilibria, and significant figures.

Q & A

What is the task given to the student in the video?
-The student is tasked with determining the iodide (I-) content of tablets that contain potassium iodide (KI) and an inert water-soluble sugar as a filler.
What is the significance of using distilled water to dissolve the tablet?
-Distilled water is used to dissolve the tablet because it is free from impurities and dissolved substances that could interfere with the chemical reaction or the subsequent analysis.
Why is an excess of 0.20 molar Pb(NO3)2(aq) added to the solution?
-An excess of 0.20 molar Pb(NO3)2(aq) is added to ensure that all the iodide ions (I-) react to form a yellow precipitate of lead(II) iodide (PbI2), leaving essentially no I- ions in the solution.
What is the balanced net ionic equation for the reaction that forms the precipitate?
-The balanced net ionic equation for the reaction is Pb2+(aq) + 2I-(aq) → PbI2(s), representing the formation of the solid precipitate from lead and iodide ions.
Why is it important to dry and weigh the filter paper with the precipitate multiple times?
-Drying and weighing the filter paper with the precipitate multiple times is crucial to ensure that all the water has been evaporated, allowing for an accurate measurement of just the precipitate's mass.
In the filtrate solution, is the concentration of K+ greater than, less than, or equal to NO3-?
-The concentration of K+ is less than that of NO3- because Pb(NO3)2 is added in excess, ensuring a higher concentration of nitrate ions in the solution.
How is the number of moles of precipitate produced in the experiment calculated?
-The number of moles of precipitate is calculated by first determining the mass of just the precipitate (subtracting the mass of the filter paper), and then dividing this mass by the molar mass of PbI2.
What is the mass percent of I- in the tablet?
-The mass percent of I- in the tablet is calculated by converting moles of PbI2 into moles of I-, then into mass of I-, and finally dividing by the mass of the tablet and multiplying by 100 to get a percentage.
If a tablet is dissolved in 55.0 mL of water instead of 50.0 mL, will the experimentally determined mass percent of I- be affected?
-No, the experimentally determined mass percent of I- will not be affected because the amount of water used only serves as a medium for the reaction and is later removed by filtration and drying, leaving the same mass of precipitate.
Will the substitution of AgNO3 for Pb(NO3)2 result in the precipitation of I- from the solution, given the Ksp value for AgI?
-Yes, the substitution of AgNO3 for Pb(NO3)2 will result in the precipitation of I- from the solution because AgI has a very low Ksp value (8.5 × 10^-17), indicating it is insoluble and will precipitate under these conditions.
Can the student determine the mass of AgI produced to three significant figures with a balance that measures to the nearest 0.01 grams?
-No, the student cannot determine the mass of AgI produced to three significant figures with such a balance because the mass of the precipitate is less than 1 gram, and the balance's precision limits the measurement to two significant figures.
What are some topics on AP Daily that could help a student struggling with the concepts presented in the video?
-Topics that could help include net ionic equations (topic 4.2), stoichiometry (topic 4.5), solubility equilibria and Ksp (topic 7.11), and gravimetric analysis and significant figures, which should be part of the student's lab notebook.

Outlines

00:00

🔍 Session 8: Long Free Response Questions Overview

Kristen Cacciatore introduces session 8 of AP Daily Practice, focusing on long free response questions, which are a part of the AP exam. She provides a link for viewers to download a PDF of the questions to practice independently. The session involves a scenario where a student is tasked with determining the iodide (I-) content in tablets containing potassium iodide (KI) and a soluble sugar filler. The process involves dissolving a tablet in water, adding an excess of lead nitrate (Pb(NO3)2) to form a yellow precipitate, which is then filtered, washed, and dried. The discussion covers writing a balanced net ionic equation for the reaction, explaining why a net ionic equation is used, the purpose of drying and weighing the precipitate, and the concentration comparison of potassium (K+) and nitrate (NO3-) ions in the solution.

05:02

🧪 Calculating Moles and Mass Percent of Precipitate

The video continues with a step-by-step calculation to determine the number of moles of the precipitate produced in the experiment. It involves subtracting the mass of the filter paper from the total mass to find the mass of the precipitate alone. The molar mass of lead(II) iodide (PbI2) is calculated using the molar masses of lead and iodine. With the mass of the precipitate, the number of moles of PbI2 is determined. The calculation proceeds to find the mass percent of iodide in the tablet by converting moles of PbI2 into moles of iodide (I-), then into mass of I-, and finally performing a percentage calculation using the mass of the tablet. The video also addresses a hypothetical scenario where a different volume of water is used and predicts the effect on the experimentally determined mass percent of I-. It concludes with a discussion on whether AgNO3 can be substituted for Pb(NO3)2 to precipitate I- from a solution, considering the solubility product constant (Ksp) of silver iodide (AgI).

10:03

📊 Precision in Measurement and Additional Study Tips

The final paragraph addresses the question of whether a student can determine the mass of silver iodide (AgI) produced to three significant figures with a balance that measures to the nearest 0.01 grams. It is concluded that with the mass of the precipitate being less than 1 gram and the balance's precision limit, only two significant figures can be accurately determined. The video concludes with study tips for reviewing the content covered, including topics on net ionic equations, stoichiometry, solubility equilibria, and Ksp. It also mentions additional AP Daily practice sessions and exam reviews from the past two years as resources for further study.

Mindmap

Keywords

💡Net Ionic Equation

A net ionic equation is a chemical equation that includes only those species that are involved in a chemical reaction and excludes the spectator ions, which do not participate in the reaction. In the video, it is used to represent the reaction between KI and Pb(NO3)2, resulting in the formation of PbI2 precipitate. The net ionic equation is essential for understanding the chemical process taking place and for calculating the stoichiometry of the reaction.

💡Stoichiometry

Stoichiometry is the calculation of quantities in a chemical reaction based on the law of conservation of mass. It is used in the video to determine the number of moles of precipitate produced and to calculate the mass percent of I- in the tablet. The process involves converting moles of PbI2 into moles of I-, and then into mass, which is essential for determining the iodide content of the tablet.

💡Gravimetric Analysis

Gravimetric analysis is a quantitative technique that involves weighing the mass of a substance before and after a chemical reaction to determine the amount of a particular component. In the video, it is used to find the iodide content in a tablet by weighing the precipitate formed after the reaction with Pb(NO3)2. The process is crucial for accurately determining the mass of the precipitate and, subsequently, the iodide content.

💡Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is a measure of the maximum amount of a compound that can dissolve in a saturated solution. It is used in the video to justify the precipitation of I- when AgNO3 is used instead of Pb(NO3)2. The low Ksp value for AgI indicates that it is insoluble and will precipitate from the solution, which is a key factor in the experiment's outcome.

💡Significant Figures

Significant figures are the digits in a number that carry meaning contributing to its precision. They are used in the video to round the final percentage of iodide in the tablet to three significant figures, which is based on the precision of the measurements taken. The concept is important for maintaining the accuracy of scientific results and ensuring that the results are reported to a precision that reflects the quality of the data collected.

💡Excess Reagent

An excess reagent is a chemical used in a reaction that is present in a quantity greater than the stoichiometric amount required for the reaction. In the video, Pb(NO3)2 is added in excess to ensure that all I- ions react and form a precipitate. The use of an excess reagent is a common practice in chemistry to drive the reaction to completion and to ensure that the limiting reactant is completely consumed.

💡Precipitate

A precipitate is a solid material that forms as a result of a chemical reaction in a solution. In the video, the reaction between KI and Pb(NO3)2 leads to the formation of a yellow precipitate, which is PbI2. The formation of the precipitate is central to the experiment as it allows for the determination of the iodide content in the tablet.

💡Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is used in the video to convert the mass of the precipitate (PbI2) into moles, which is a necessary step in stoichiometric calculations. The molar mass of PbI2 is calculated by adding the molar masses of lead and two iodide ions, which is crucial for determining the number of moles of the precipitate produced.

💡Mass Percent

Mass percent is a measure of the mass of a constituent in a mixture relative to the mass of the entire mixture, expressed as a percentage. In the video, it is used to calculate the percentage of iodide in the tablet by dividing the mass of iodide by the total mass of the tablet and multiplying by 100. This calculation is essential for determining the iodide content of the tablet.

💡Spectator Ions

Spectator ions are ions that are present during a chemical reaction but do not participate in the reaction. They are typically included in the full ionic equation but omitted in the net ionic equation. In the video, K+ and NO3- are spectator ions in the reaction between KI and Pb(NO3)2, as they do not react to form the precipitate and are not involved in the stoichiometry of the reaction.

💡Filtering and Drying

Filtering and drying are techniques used in chemistry to separate a solid precipitate from a liquid solution and to remove all traces of water from the solid, respectively. In the video, the yellow precipitate is filtered, washed, and dried to ensure that the mass measured is solely that of the precipitate and not water. This process is critical for obtaining accurate results in gravimetric analysis.

Highlights

Session 8 focuses on preparing students for long free response questions on the AP exam.

A data table presents a scenario where a student must determine the iodide (I-) content of tablets containing KI and a water-soluble sugar filler.

The tablet is dissolved in 50.0 mL of distilled water and reacted with an excess of 0.20 M Pb(NO3)2 to form a yellow precipitate of PbI2.

The balanced net ionic equation for the reaction is written as Pb2+ + 2I- → PbI2(s).

Net ionic equations are used to show only the reacting species and omit the spectator ions.

The purpose of drying and weighing the filter paper with the precipitate multiple times is to ensure all water has been removed, leaving just the mass of the precipitate.

In the filtrate solution, the concentration of K+ is less than NO3- because Pb(NO3)2 was added in excess.

The number of moles of precipitate produced is calculated by first finding the mass of just the precipitate, then using the molar mass of PbI2.

The mass percent of I- in the tablet is determined by converting moles of PbI2 to moles of I-, then to mass of I-, and finally calculating the percent using the mass of the tablet.

In a different trial where the tablet is dissolved in 55.0 mL of water instead of 50.0 mL, the mass percent of I- is predicted to be equal to the previous calculation since the amount of water does not affect the reaction.

If AgNO3 is substituted for Pb(NO3)2, I- will precipitate as AgI due to the very low Ksp value of 8.5 x 10^-17, indicating AgI is insoluble.

With only one KI tablet and a balance measuring to the nearest 0.01 grams, the student will not be able to determine the mass of AgI produced to three significant figures.

The video provides tips for reviewing the content, suggesting specific topics on AP Daily to focus on based on different parts of the problem.

Additional AP Daily practice sessions and AP exam reviews from the last two years are available as resources.

The importance of gravimetric analysis and significant figures in solving the problem is emphasized.

The video concludes by encouraging students to utilize the provided resources and thanking them for watching.

Transcripts

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Takeaways

Q & A

What is the task given to the student in the video?

What is the significance of using distilled water to dissolve the tablet?

Why is an excess of 0.20 molar Pb(NO3)2(aq) added to the solution?

What is the balanced net ionic equation for the reaction that forms the precipitate?

Why is it important to dry and weigh the filter paper with the precipitate multiple times?

In the filtrate solution, is the concentration of K+ greater than, less than, or equal to NO3-?

How is the number of moles of precipitate produced in the experiment calculated?

What is the mass percent of I- in the tablet?

If a tablet is dissolved in 55.0 mL of water instead of 50.0 mL, will the experimentally determined mass percent of I- be affected?

Will the substitution of AgNO3 for Pb(NO3)2 result in the precipitation of I- from the solution, given the Ksp value for AgI?

Can the student determine the mass of AgI produced to three significant figures with a balance that measures to the nearest 0.01 grams?

What are some topics on AP Daily that could help a student struggling with the concepts presented in the video?