Chem 101L: Pre-Lab Experiment 4: Conductometric Titrations - Fall 2019 PDF

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experiment 4 lab report...

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CHEM 101L Pre-Lab 4

Fall 2019

Name

Grayson Garcia

UNC onyen

graygar

Experiment 4: Conductometric Titrations Experiment #4 Pre-Lab Assignment Instructions: 1. When answering questions, do not repeat or copy from the lab manual. You are encouraged to use the Lab Manual as a reference, but plagiarized answers will not be graded. 2. Do not ever work to answer these questions with a partner - avoid plagiarism and cheating. 3. Do not exceed the limit given for each question. 4. Do not remove instructions or any bolded text. This section will help prepare you for writing an introduction for future experiments: 1. (8) What is the objective of this lab and how will you accomplish this objective? (what reaction are you monitoring; what are you measuring; how are you monitoring the reaction; why can you monitor the reaction this way; what species will be dissolved and what species will react; hypothesize what you think will happen in this reaction) (4-5 sentences) In experiment 4, we are using level of measured conductivity to control a titration of silver nitrate with lithium chloride. We begin the titration using a solution of Ag+ (aq) and NO3 - (aq) ions and titrate in a LiCl solution: the product of the reaction is AgCl, an insoluble salt precipitate. We measure when the end point of the titration has been reached using a conductivity sensor because AgNO3 is a conductive solution (as the reaction takes place, the concentration of Ag+ ions decreases, decreasing the conductivity of the solution that is observed by the sensor, and through this decline in conductivity, end point of the titration can be monitored/calculated). I hypothesize that the AgCl precipitate will form quickly and that it will not take long for the conductivity of the ions in solution to decrease and the titration to reach its end point.

2. (2) How does stoichiometry relate to this lab? Why can we not use the volume of the silver nitrate solution to determine how much lithium chloride to add? Stoichiometry is relevant to this lab because, in general, molecules and atoms react with others molecules and atoms based on the mole numbers or ratio. (1 mole = 6.022*10^23 molecules, so volume is not considered in stoichiometric reactions.)

CHEM 101L Pre-Lab 4

Fall 2019

3. (2) Summarize this experiment: Provide a bulleted list of tasks for this experiment, using 1 phrase or sentence per bullet. You can use a maximum of 4 bullet points to describe Experiment 4. • •

• •

Measure/record initial: concentrations of stock solutions, volume and conductivity of the silver nitrate solution and the lithium chloride solution using appropriate tools. Stir silver nitrate solution and titrate in 1.0 mL increments of lithium chloride, waiting 1 minute periods between additions for conductivity to stabilize, adding smaller volumes of titrant as the volume nears the predicted endpoint volume Record conductivity and volume each addition (take 4-5 measurements at endpoint) Repeat for two more trials using smaller increments to measure the exact conductivity curve and refine predicted titrant volumes-this increases data accuracy and precision

This next section is designed to prepare you for the experimental portion of the experiment, as well as your discussion an analysis after the experiment. Answer each question carefully. You may need to reference the lab manual or textbook to answer these questions. For some questions, you will likely need to think carefully about the reasoning (the answer won't be exactly written in a book!). 1. (2) Look up (use an internet search engine) and reference the SDS (Safety Data Sheet) for silver nitrate and lithium chloride. Report the health hazard ratings, one potential hazard and one prevention method for both compounds below. o Silver nitrate § § § §

o

Health Hazard Information: eye corrosion (Cat 1), skin corrosion (Cat 1B), acute and chronic hazards to aquatic environment (Cat 2) Potential health hazard: can cause skin irritation/ severe skin burns Hazard Statement: toxic to aquatic life with long lasting effects Prevention: wear protective clothing/gloves and avoid release to the environment

Lithium chloride § § § §

Hazard Identification: irritant with acute toxicity-oral, dermal, inhalation (cat 4) Hazard statement: causes skin and serious eye irritation Precautionary statement: wash skin thoroughly after handling Prevention: : do not eat or drink while handling lithium chloride

2. (2) What experimental uncertainty is reduced by rinsing the buret with the lithium chloride solution before filling it? (1 sentence)

CHEM 101L Pre-Lab 4

Fall 2019

If the buret has remnants of water or any other solvent or substance, they will be rinsed away by the lithium chloride solution before filling it. leading to more accurate conductivity results.

3. (2) What experimental measure will change if water is added to immerse the conductivity sensor in the silver nitrate solution? Will this affect the end point of the titration? The end point of a titration depends on the concentration of the solution: the addition of water decreases the concentration of the solution and affects the end point of the titration.

4. (2) How is the data improved by adding smaller volumes near the predicted end point? Is there any advantage to making small volume additions all the time? Is there a disadvantage to always making small volume additions? (1-2 sentences) Near the end point of a titration, the addition of smaller volumes helps to increase the accuracy of the end point and reduces the chance of adding excess reagent. The addition of smaller values near the end of the titration is an advantage, but if those small volumes are used for the entire titration, it will take much longer to reach the end point.

(2) How is the data improved by obtaining at least 4-5 measurements after the end point? The purpose of obtaining at least four or five conductivity measurements after reaching the end point of the titration is to minimize uncertainty—it is insurance that reduces the chance that the single conductivity measurement is inaccurate and ruins data analysis and also gives a range that balances out any random errors as the final conductivity stabilizes.

This last section of the pre-lab assignment is designed to prepare you for the conceptual components of the experiment and to highlight specific key concepts you should think about during the experiment. 1. Write out what you predict to be the following equations for the reaction between silver nitrate and lithium chloride. All equations must be balanced and include states of matter to receive credit: •

(2) Balanced Molecular Equation AgNO3 (aq) + LiCl (aq) => AgCl (s) + LiNO3 (aq)

•

(2) Net Ionic Equation

CHEM 101L Pre-Lab 4

Fall 2019

Ag+ (aq) + Cl- (aq) => AgCl (s)

•

•

(5) If the concentration of AgNO3 is 0.00503 M and the concentration of the LiCl solution is 0.00387 M, use the stoichiometry of your equation to predict the volume of lithium chloride needed to react with 9.98 mL of the silver nitrate solution. 1.29*10mL => 1.29*10 -2 L

o

volume LiCl =

o

Set up this calculation in Excel using good labels and equations so you can change the numbers accordingly in lab. (Paste screen capture of the excel sheet showing formulas in the designated space.) Failure to do this will result in a 5point deduction in your lab grade for the day.

(4) If the end point of the titration was calculated to have occurred at a volume of 17.32 mL and the concentration of the LiCl solution is 0.00387 M, calculate the moles of lithium chloride that reacted: o

moles LiCl =

TA ONLY: Late Penalty

6.70*10-5

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