Chemistry 201 (Winter 2019): Exp 2: Lemonade Contents Investigation PDF

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The second experiment ...

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Experiment 2 Investigating the Contents in Lemonade, Determining the amount of Ascorbic Acid (Vitamin C) and Citric Acid present What laboratory techniques and skills will you practice? • • •

Performing a titration Using volumetric flasks and pipets Building data tables

Pre-lab preparation 

Read through and write your own notes in the margins of the document.

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Complete and pass (>50%) the online pre-lab quiz in d2l.

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Complete the Experiment #2 Flowchart template on D2L.

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Attempt to answer margin question prompts, as they will prepare you for the worksheet

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Complete all the grey boxes withing the Introduction questions.

completed in the lab.

What chemical concepts will you apply? • • • •

Determine the molar mass of a chemical species Writing a balanced chemical reaction given the reactant(s) and product(s) Converting between moles, concentration (mol/L), and volume Performing dilution calculations and determining the consequences of using dilute vs. concentrated solutions

What to bring to lab    

**Lab coat & goggles Non- programmable calculator Pen, pencil, eraser Appendix D, E, and F.

What communication and reporting skills will you use? • • • •

Preparing an introduction for a formal report Analyzing data to determine results Interpreting results in discussion section Comparing experimental results to literature values

Background In Experiment #2 you will determine the amount of citric and ascorbic acid present in Lemonade. Then for one of the acids, ascorbic acid, you will confirm the percent recommended daily allowance (% RDA) that is present in a serving of Lemonade. Previously, in Experiment #1, you focused on understanding how one can use stoichiometry to determine the limiting reagent in a set of reactions where the concentration of one reactant varied.

Citric acid

Ascorbic acid

The goals of Experiment #2 is to: 1. expand on your understanding of stoichiometry with two different procedures quantifying different things and 2. see how the different experimental procedures A & B will have different reactivities with the different functional groups present in citric and ascorbic acid (e.g. C(O)OH groups in citric acid). We will come back to the ideas of what are functional groups and how different functional groups will have different reactivities when we talk about bond polarity within Lewis structures and reaction mechanisms (arrow pushing) in Lecture. CHEM 201 – WINTER 2019

A functional group is a specific organization/grouping of atoms into a common moieties. Each common moiety will have similar reactivities. For a complete list of functional groups please see: http://www.chem.ucalgary.ca/courses/351/ WebContent/orgnom/functional/func.html but remember you will only be responsible for knowing those in the course syllabus. Using the link above can you find the following functional groups in the structures above: alcohol, alkene, carboxylic acid & ester? Make sure to check your answers with your TA.

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The laboratory technique used for these two procedures is Titration, which yields quantitative information about a substance (analyte, in our case the analytes are citric acid and/or ascorbic acid) through the controlled addition of second substance (titrant, varies depending on procedure) whose quantitative information is known. The quantitative information is obtained at the equivalence point or the point at which exactly enough titrant (titre) has been added to react with all of the analyte. The equivalence point will be detected using a visual endpoint, which occurs when a carefully chosen indicator changes colour as it reacts with excess titrant. Two different procedures for titration will be used. Procedure A will rely on the acid/base behaviour of the functional groups present in both acids and Procedure B on the redox behaviour of the functional groups present in ascorbic acid (see Figure 1). Because of the different reactivity in each procedure, different indicators will be used for each procedure. In Procedure A you will use Thymol Blue that will turn the solution blue once it becomes basic (excess NaOH present). In Procedure B excess I2 will react with I– (from the scoop of KI added) to form I3–. I3– then forms a dark green/blue coloured complex with thyodene (see equations to the right).

A Titrant is dispensed using a buret. Its concentration and volume are known.

An Analyte is placed in an Erlenmyer flask. Its volume is known but concentration unknown. It’s moles are determined using the stoichiometric number of moles of Titrrant

Visual Endpoints

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Given the relationship between a visual endpoint and equivalence point, what experimental limitation(s) exist(s) for a visual titration?

Procedure B:

→ I3-(aq) + thyodene → thyodene-I3– comple

Excess I2(aq)

I3–(aq)

+ I-(aq)

(Colourless Solutions)

As both procedures are time consuming, you will work with a partner, and each of you will be responsible for doing only one of the procedures. Once the total acid content and the amount ascorbic acid are known, you will come together as pairs to determine the amount of citric and ascorbic acids in Lemonade. This experiment requires you have to a strong understanding of the concept of stoichiometry.**

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Even though you will only be performing one of these procedures, you must be familiar with both in order to write-up your formal laboratory report. Knowing the amounts of acids in Lemonade is one goal but the ascorbic acid (or Vitamin C) in Lemonade has a recommended daily allowance. The RDA values from the Health Canada Website are shown in Table 1 below.

(Dark green/dark blue colour)

You could be assigned to do Procedure A, where you will be titrating a sample of Lemonade with NaOH to determine the total acid content (total H+), or you could be assigned to perform Procedure B, where you will be titrating a sample of Lemonade with KIO3 to determine the amount of ascorbic acid (Vitamin C).

Table 1. Age Group 4-8 year old children 9-13 year old children 19-30 year old Adult Females 19-30 year old Adult Males

RDA mg/day 25 mg 45 mg 75 mg 90 mg

Your second goal is to use the amount of ascorbic acid you determined to calculate the % RDA, for all age groups, that is present in a serving of the Lemonade (see equation [1] below). ascorbic acid in a serving (in mg)

%฀฀฀฀฀฀ ฀฀฀฀฀฀ ฀฀฀฀ ฀฀฀฀฀฀ ฀฀฀฀฀฀฀฀฀฀ =RDA � for that age group

CHEM 201 – WINTER 2019

� x 100%

[1]

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PROCEDURE A Overall balanced reaction: _ H+ + OH → Η2Ο _ Titrant OH

C6H 7 O6 C6H5O7

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The two acids of the analyte contribute different stoichiometric ratios of H+. How many equivalents of H+ do each contribute?

+ __?__ OH

_ + __?__ OH

_

Analytes in Lemonade Titrant

Analyte

Citric Acid C6H8O7

Ascorbic Acid C6H8O6

PROCEDURE B Is a two step process, first reactants form an intermediate *Note: and this intermediate* reacts 2 is an intermediate. with our IItanalyte is actually I2 and not the titrant

IO3- + 5I- + 6H+ → *3I2 + 3H2O

First step:

Second step:

+ *I2(aq)

+ *I2(aq) → 2I-(aq) + C6H6O6 + 2H+

Titrant KIO3

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*Note: Although KIO3 (IO 3–) is the titrant, It is actually the intermediate I2 after the first step that reacts directly with the analyte.

Overall balanced reaction:

C6 H 6O6 NO REACTION

IO3- (aq) + 3C6H8O6→ I-(aq) + 3C6H6O6 + 3H2O

Figure 1. Reactions taking place during procedures A and B.

CHEM 201 – WINTER 2019

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General Procedure – Preparing the Lemonade solution You will work in pairs to prepare the Lemonade solution, but will perform the titrations individually. Make sure your data is initialled by your TA before you leave the lab. 1. Start by recording relevant literature information about the reagents you use: • The ingredients on the canister of Country Time® Lemonade • The %RDA of Vitamin C claimed in one serving of Lemonade • The mass (in g) of Lemonade powder in a single serving, as reported by Country Time® Lemonade • The precise concentration of KIO3 titrant prepared for your lab section • The precise concentration of NaOH titrant prepared for your lab section 2. Add four rounded tablespoons of Lemonade powder to a 250 mL beaker and weigh (Figure 2). Determine the amount of Lemonade powder added to the beaker (You should have between 45-55 g). Record these masses and your observations of the physical properties of the powder in the corresponding results table of the worksheet.

Figure2. A visual representation of how much Lemonade powder to measure in each scoop.

? 3. Use a dry utility funnel and add the powder to a 500mL volumetric flask. Take approximately 100 mL of RO water and rinse the 250 mL beaker and the funnel, adding the rinsings to the 500 mL volumetric flask. Add another 100 mL of RO water to the flask and swirl it for 5 minutes to dissolve the contents of the flask. (Your flask will not be filled to the line at this point.) 4. Once the solution appears homogeneous, dilute to the line marked on the neck of the volumetric flask with RO water. Stopper the flask and repeatedly invert and shake the flask at least 20 times to mix the solution. If there appears to be bubbles at the dilution line, invert gently and revert to the upright position once. This should get rid of the bubbles. Keep your volumetric flask stoppered when not in use.

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Why is it important for all the Lemonade powder to dissolve?

Why is it important to keep the volumetric flask stoppered when not in use? What assumption do we make about this solution throughout the course of the experiment?

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Burette Volumes The first decimal place is explicitly known from the markings of the burette, while the second decimal place must be estimated (0-9).

18 5. Each partner should take approximately 130 mL of the Lemonade solution in a clean and dry 150 mL beaker to use for titrations. **You will be working individually from this point forward & using a different titrant than your laboratory partner** Make sure you know what titrant you are adding to your burette. Also read the side on how to read volumes in a burette.

CHEM 201 – WINTER 2019

ex)

18.6? 18.68 (8 must be estimated)

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Titration Procedure A – Determining the Total Acid (H+) 1. Rinse a 25.00 mL volumetric pipette with a small volume of Lemonade solution, and then pipette 25.00 mL aliquots into clean 250 mL Erlenmeyer flasks. In total, you will need four flasks with 25.00 mL Lemonade in each. Using the 100 mL graduated cylinder, add 50 mL of RO water to each flask (to help see the endpoint). 2. Obtain ~80 mL of ~0.1 M NaOH solution in a dry 100 mL beaker. Rinse the burette with a small volume of ~0.1 M NaOH solution1, and then fill with the NaOH solution (titrant). Record the concentration of the NaOH solution (to two decimal places) and also the initial burette reading (to ± 0.01 mL). 3. Add 5 drops of Thymol Blue indicator to one of the Lemonade solution aliquots and begin titrating, i.e. adding the sodium hydroxide solution to the Lemonade solution. Addition can be quite rapid at the start (~ 1-2 mL volumes). Continue titrating (rapidly with constant swirling) until the change in colour that first appears as the titrant is added takes a fair amount of swirling to disappear upon the addition of small (~0.5 mL) volumes. At this point, begin titrating more slowly (dropwise) until a permanent colour change is achieved. 4. Record the final burette reading (to ± 0.01 mL) and then calculate the volume of NaOH solution required to reach the end-point (the “titre”). Set this Erlenmeyer flask aside in order to compare its colour with the endpoints of subsequent titrations.

Sodium Hydroxide Sodium hydroxide is toxic and corrosive. Be careful to avoid skin contact, inhalation, and ingestion.

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NaOH Titrant Based on Footnote 1, why should you not leave NaOH exposed to air? What assumption do we make in this experiment? Consider researching this topic.

Thymol Blue Indicator Thymol blue has multiple colour stages shown in the pH ruler below. Your Lemonade solution may appear orange prior to titration due to the acidic nature of your sample.

pH

1.2

2.8

8.0

9.6

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Use the endpoint volume from your first trial to more efficiently complete trials 2-4 with accuracy and precision.

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Acceptable Titre Volumes

5. Repeat steps 3 & 4 with the other flasks. Refill your burette between titrations so you do not run out of titrant in the middle of your titration! 6. Determine the average and standard deviation of the NaOH volumes and discard any poor trials (see Appendix D for more details). As stated in Appendix D, one is justified in rejecting any experimental result which is not within ± 3 standard deviations of the mean. If you think you need to discard more than one titre, you should perform additional titrations until you have 3 to 4 titres that are consistent with each other.

� − (3 × ฀฀฀฀) ฀฀

� ฀฀

� + (3 × ฀฀฀฀) ฀฀

� = mean titre volume (mL) ฀฀ ฀฀฀฀ = standard deviation (± mL)

7. Get your TA to initial your recorded data in your blue laboratory notebook before leaving the laboratory. 8. Be sure to dispose of all wastes responsibly. Wash all glassware and rinse the burette carefully with water a few extra times. Return all equipment to your drawer and lock it securely

After calculating your standard deviation (± mL), create a numerical range of acceptability as shown below.

Waste Disposal

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Unused Lemonade goes down the sink. All other unused reagents and contaminated wastes are disposed of in their designated waste containers.

1 In a 150 mL beaker, keep no more than 80-100 mL of NaOH as a stock solution for refilling and rinsing the burette. Make sure to cover the beaker with a watch glass when not in use.

CHEM 201 – WINTER 2019

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Titration Procedure B – Determining the Amount of Ascorbic Acid 1. Obtain ~80 mL of ~0.001 M KIO3 solution in a dry 100 mL beaker. Rinse a clean burette with a small volume (...