CM1011 Exp6_Titration_Fa23

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 1 Acid-Base Titration In your text (Chang and Goldsby 7 th Ed): 4.3 Acid-Base Reactions 4.6 Solution Stoichiometry Extra reading: 17.3 A closer look at Acid-Base Titrations 17.4 Acid-Base Indicators Background: Titration is a method of quantitative analysis in which a solution of known concentration (the standard solution or titrant ) is gradually added from a burette to a measured quantity of unknown solution ( analyte ) until the reaction is complete ( equivalence point ). The equivalence point is also called the stoichiometric point, as it is where the number of moles of titrant is in the stoichiometric ratio (from the coefficients in the balanced reaction equation) with the number of moles of the analyte present in the sample. The titrant is typically delivered from a burette, which accurately measures volume. If the analyte is a solid, it must be weighed, then dissolved in the solvent. A small amount of an indicator is added (if applicable), and then the solution of the analyte is titrated. Since the titration will determine the number of moles of analyte, typically the mass of the solid and the number of moles of solid is used to determine the molar mass. If the analyte is a solution, a known volume must be used, a small amount of indicator is added (if applicable), and then the solution is titrated. Since the titration will determine the number of moles of solute in the analyte, typically the volume of the solution and the number of moles of the solute are used to determine the concentration. In acid-base titration, an acid-base neutralization reaction occurs between the titrant, typically a strong base, and the analyte, usually an acid with unknown concentration or molar mass. When the number of moles of the added strong base is at the stoichiometric ratio with the number of moles of the acid, the reaction is complete and the equivalence point is reached. An instrument or a visual change must be used to indicate the endpoint of the titration (when one stops adding titrant). In acid-base titration, an indicator, in very small amount, is added to the analyte solution to indicate the endpoint. An indicator is a compound that have different colors when it is in an acid solution or base solution. When the indicator changes color, the amount of the base added is just slightly more than the amount needed to reach the equivalence point. At this point the reaction is just slightly beyond completion, so the titration should end (thus, the endpoint). If the endpoint is carefully identified, it is very close to the equivalence point. Overview of the Procedure: You will be given a NaOH solution, whose concentration is only known approximately (about 0.1 M). The first step, in Part I, is to standardize the NaOH, i.e. titrate the NaOH solution against a well-characterized acid, to determine the concentration of the NaOH to 4 significant figures. Once this determination is made, your calculated molarity of NaOH is used in the calculations of the titrations in Parts II and III, where the total acidity in a sample of vinegar and a sample of fruit juice are determined, respectively.

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 2 Part I. KHP (potassium hydrogen phthalate, KHC $ H % O % , MW 204.2 g/mol) is a weak acid that is used to make solutions of very well-characterized concentrations, and hence it is a useful primary standard . It is a solid under normal lab conditions, it can be highly purified, it does not easily oxidize, and it has a high MW, resulting in good precision and accuracy when weighing convenient sample sizes. KHP is a monoprotic acid, reacting 1:1 mole ratio with NaOH: KHC $ H % O % (aq) + NaOH (aq) à KNaC $ H % O % (aq) + H ) O (l) Sample calculation of a standardization run: A student is given a starting solution of NaOH to standardize, known to be approximately 0.1 M. It took 19.98 mL of the approximately 0.1 M NaOH solution to reach the equivalence point with a solution of KHP that was prepared with 0.4168 g KHP. What is the concentration of the NaOH, to 4 s.f.? 0.4168 g/ (204.2 g/mol) = 2.041 x 10 -3 moles KHP 2.041 x 10 -3 moles KHP x [ 1 mol NaOH/ 1 mol KHP] = 2.041 x 10 -3 moles NaOH 2.041 x 10 -3 moles NaOH/ 0.01998 L = 0.1022 M NaOH Note: You cannot leave out the 1:1 mole ratio in the calculation because that is how moles of KHP are converted to moles of NaOH. Once the concentration of the NaOH titrant is known, the titrant is used to titrate samples of vinegar and fruit juice, to determine the unknown concentration of total acid in each of these samples. Part II. White vinegar will be titrated with the standardized NaOH. White vinegar is a dilute solution of a single acid (acetic acid, HC ) H * O ) , a monoprotic acid), so the data from the titration can be used to determine the concentration of acetic acid in the vinegar. Hydrogen ions from the HC ) H * O ) react with hydroxide ions from the NaOH in a 1:1 mole ratio to produce water and sodium acetate in the overall reaction: HC ) H * O ) (aq) + NaOH (aq) à H ) O (l) + NaC ) H * O ) (aq) A general form for this type of reaction, a monoprotic acid (HA) with NaOH is: HA (aq) + OH - (aq) à H ) O (l) + A , (aq) or HA (aq) + NaOH (aq) à H ) O (l) + NaA

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 3 Sample calculation of a run to determine the concentration of a monoprotic acid: A 10.00 mL sample of an unknown monoprotic acid solution is titrated, using the standardized 0.1022 M NaOH solution. It took 20.04 mL of the NaOH to reach the equivalence point. What is the concentration of the monoprotic acid, HA, to 4 s.f.? 0.1022 M NaOH x 0.02004 L = 2.008 x 10 -3 moles NaOH 2.008 x 10 -3 moles NaOH x [ 1 mol HA / 1 mol NaOH] = 2.008 x 10 -3 moles HA 2.008 x 10 -3 moles HA/ 0.01000 L = 0.2008 M HA Note: You cannot leave out the 1:1 mole ratio in the calculation because that is how moles of NaOH are converted to moles of HA. Part III Lemon juice will be titrated with the standardized NaOH. Fruit juices typically have multiple organic acids, such as citric, ascorbic (vitamin C), tartaric, oxalic, malic, etc. However, citric acid ( H * C - H . O / , a triprotic acid) is the dominant acid in lemon and lime juice. The calculations in this titration will assume that the determination of total acidity is due only to citric acid. This approximation will give the maximum amount of citric acid in the lemon juice, or total acidity as equivalent to citric acid. The three hydrogen ions from the citric acid react with hydroxide ions from the NaOH in a 1:3 mole ratio to produce water and sodium citrate in the overall reaction: H * C - H . O / (aq) + 3 NaOH (aq) à 3 H ) O (l) + Na * C - H . O / (aq) A general form for this type of reaction, a triprotic acid ( H * A) with NaOH: H * A (aq) + 3 OH - (aq) à 3 H ) O (l) + A *, (aq) or H * A (aq) + 3 NaOH (aq) à 3 H ) O (l) + Na * A (aq) Sample calculation of a run to determine the concentration of a triprotic acid: A 2.00 mL sample of the unknown citric solution is titrated, using the standardized 0.1022 M NaOH solution. It took 18.67 mL of the NaOH to reach the equivalence point. What is the concentration of the triprotic acid, H 3 A to 3 s.f.? 0.1022 M NaOH x 0.01867 L = 1.908 x 10 -3 moles NaOH 1.908 x 10 -3 moles NaOH x [ 1 mol H * A/ 3 mol NaOH] = 6.360 x 10 -4 moles H * A 6.360 x 10 -4 moles H * A/ 0.00200 L = 0.318 M ࠵? ࠵? A Note: These sample calculations for monoprotic and triprotic acids are for illustration. They are not clues to the actual concentrations in the samples.

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 4 Experimental Procedure: Review: Reading a burette, Exp. 1 ( http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.htmL) Fig. 1 Reading a burette Burette Read at the bottom of the meniscus. The smallest division on the burette is 0.1 mL. Therefore, our reading needs to have the estimated digit in the hundredth place. A good volume reading is 20.38 ± 0.02 mL. A reading of 20.35 mL ± 0.05 mL is also acceptable. Look at the pictures of a burette below. Note that the numbers get bigger as you go down the burette. This is different from the beaker or the graduated cylinder. This is because the liquid leaves the burette at the bottom Always read a burette at eye level. The rings on the burette aid in reading the meniscus without parallax error, or incorrect volume reading due to looking above or below eye level, as shown in Fig. 2 below. When reading volume with a meniscus, always read the bottom of the meniscus. Using either a white card with a black bar on it or a plain white card (moving up from underneath) behind the burette will help highlight the bottom of the meniscus, as shown in Fig. 3 below. Fig. 2 Fig. 3 Note that the ring in the back of the buret can be seen, looking up at the 5 mL mark and down at the 7 mL mark.

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 5 Fig. 4. Experimental setup ↑ Always wear goggles! – 5 pts safety deduction each time you are asked by the instructor or TA to put your goggles on. NaOH is corrosive and can damage eyes, skin, and clothing. If contact accidentally occurs, rinse the affected parts with large amounts of water. Funnel (optional) for filling. Remove the funnel after the burette is filled. In this experiment, NaOH is the ONLY substance that goes in the burette. Burette clamp. Use to adjust the height of the burette so meniscus is at eye level. Reading at an angle results in parallax error. Erlenmeyer flask, containing a) acid to be titrated b) 2- 3 drops phenolphthalein c) water, as diluent or solvent White paper, to enable better detection of the faint pink endpoint

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 6 Preliminaries You should first view the Burette Technique video posted along with this handout on the Brightspace course site. Set up the apparatus as in Fig. 4. Take a large beaker and, with a pencil, label it “WASTE” on the white patch on the side. Use this for on-bench disposal of burette rinses and finished titrations. Take a 150 mL beaker, label it “NaOH”, and use it to get about 100 mL of the approximately 0.1 M NaOH from the stock solution in the hood. Put the waste beaker under the burette instead of an Erlenmeyer flask, and rinse the burette twice, using about 25 mL each time. (Note that the “off” position for the stopcock is when the handles of the stopcock are perpendicular to the long axis of the burette). Swirl the funnel to wash the rinse NaOH down the sides of the burette. Any time you will fill a burette with a funnel, you might have to lift the funnel slightly so the liquid flows freely. Once the burette has been rinsed, with the waste beaker still under the burette (to collect any accidental overflow), fill it to your eye level (must be above the 20-mL mark), lifting the funnel to keep from overflowing. Turn the stopcock full on, to blow the air out of the tip. If there is a residual air bubble, open the stopcock so the liquid flows freely and tap or gently jiggle the burette, to knock the bubbles out. Ask your instructor for help if you are not able to get rid of the bubbles. Once the air bubbles are cleared, fill to back up to your eye level. Your burette is now ready to start titrating. As needed use your “NaOH” beaker to get refills. Part I. Standardization of NaOH solution. 1. The actual molarity of the approximately 0.1 M NaOH must be determined by titration to 3-4 s.f., remember the actual number of s.f. is dictated by the s.f. of the worst (lowest number of s.f.) piece of data. Obtain a small weighing boat (keep this boat to be reused for all the KHP trials). Tare the balance with the boat and weigh approximately 0.4 g of KHP (potassium hydrogen phthalate, KHC $ H % O % , MW 204.2 g/mol) on one of the analytical balances. The sample does not have to be exactly 0.4000 g, but the mass should be recorded to 0.0001 g (ex. 0.3976 g, 0.4010 g, etc.) record the mass on the Data Sheet, Part I, Step 1 . (Do the calculations of the number of moles later, at home). 2. Carefully transfer the KHP sample into a 250 mL Erlenmeyer flask, and use the DI water bottle to rinse any residual crystals into the flask. Add about 50 mL of deionized water and 2-3 drops of phenolphthalein indicator to the KHP solution. Swirl or stir with a glass rod until all of the solid is dissolved. Record the initial volume reading of the burette on the Data Sheet, Part I, Step 2. It is not critical what the starting point is, as long as the meniscus is on the burette scale and above the 20-mL mark, since you will need at least 20 mL for each titration.

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 7 Figure 5. Good Technique Figure 6. Poor Technique Begin the titration. Note in Fig. 5, Good Technique, the Erlenmeyer flask is held and swirled during the titration. In Fig. 6, Poor Technique, the Erlenmeyer is not swirled, and the titrant is introduced in bursts, not dropwise. Titrant can be added rapidly at the beginning. As the NaOH enters the solution, there is a locally high concentration, and it is quickly dispersed. Partway through the titration, you will start to see pink where the NaOH enters (Fig. 7a). Slow down the rate of titration. This local pink region will get darker, larger and more persistent as the endpoint nears (Fig. 7b), but it will disperse as you will swirl the flask. You should be adding NaOH one drop at a time as you near the endpoint, and at a good endpoint, the solution will go from clear to a very, very pale pink with the introduction of only one drop of NaOH. The pink of a good endpoint should be so pale that it is visible only when viewed through the thickest part of the solution, with white paper behind. Fig. 7c shows a reasonably good endpoint vs. an over-shot endpoint. (The “good” endpoint illustration is actually one drop over, because the very, very pale pink didn’t show up in the photo otherwise). Data from a dark pink endpoint is not usable, since the volume of NaOH that was used is too large.

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 8 VIEW THIS PAGE IN COLOR ON THE COMPUTER BEFORE PRINTING! Fig. 7a. Partway through the titration Fig. 7b. Getting close to endpoint Fig. 7c. Comparison, good vs. bad endpoint

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 9 3. If you have a good endpoint, record the volume on the Data Sheet, Part I, Step 3 . (otherwise, do it again). Dispose of the contents of the Erlenmeyer flask in the waste beaker, and rinse the flask with DI water twice (rinse can go into the drain). Refill your burette to eye level again, Repeat the procedure of Part I with a new sample of KHP and record that set of data under Determination 2. When you do repetitive titrations, you do NOT need to do anything more to your burette once you have set it up the first time (the Preliminaries procedure), other than refilling it with NaOH to your eye level. In Part I, it is only necessary to record the data in Steps 1, 2 & 3. The calculations should be done at home, as described in the sample calculation, Part I, Overview of Procedure , p.2. If you take time to do calculations, you might not have time to get all of the data. You have to do 6 titrations. (2 each from three parts). Part II. Molarity of Acetic Acid in White Vinegar 1. In this part of the experiment and in Part III, dispenser bottles are used to dispense fixed volumes of acids. Photos in Figure 8 show the proper use of the dispenser bottles. Figure 8a Figure 8b Figure 8c In Figure 8a, notice a slider knob indicated by the red arrow. In this photo, the dispensing volume is set to 3.00 mL. DO NOT move the knob! To dispense the liquid into your Erlenmeyer flask, gently lift the top white cylinder in one motion until it stops, as shown in Figure 8b. Then, use one hand to hold the flask at the sprout of the dispenser (where the solution is coming out), and use the other hand to gently push down the cylinder all the way. The liquid being delivered out of the spout is at the set volume (in this case, 3.00 mL).

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 10 2. At the front of the lab, dispenser bottles filled with white vinegar will be provided. The dispenser is already set up to dispense the sample of 2.00 mL. Dispense 2.00 mL of white vinegar into a 250 mL Erlenmeyer flask. Record the volume on the Data Sheet, Part II, Step 2 . Remember to include all three significant figures. 3. Add about 50 mL of deionized water and 2-3 drops of phenolphthalein indicator to the vinegar sample. Record the initial volume reading of the NaOH burette on the Data Sheet, Part II, Step 3 . It is not extremely critical what the starting point is, as long as the meniscus is on the burette scale, but do not start at 30 mL or greater, since you will need at least 20 mL for each titration. 4. As in Part I, titrate to a good (pale pink) endpoint, and record the final volume of NaOH on the Data Sheet, Part II, Step 4 . Dispose of the contents of the Erlenmeyer flask in the waste vessel in the hood by the door. Repeat the procedure of Part II again with a new sample and record that set of data under Determination 2. During the lab, in Part II, it is only necessary to record the data in Steps 2, 3 & 4 . The calculations should be done at home, as described in the sample calculation, Part II, Overview of Procedure , pp. 2-3. Part III. Molarity of Citric Acid in Lemon Juice 1. In the fume hood of the lab, a dispenser bottle filled with lemon juice will be available to deliver 5.00 mL. Dispense 5.00 mL of lemon juice into a 250 mL Erlenmeyer flask. Record the volume of juice on the Data Sheet, Part II, Step 1 . 2. Add about 50 mL of deionized water and 2-3 drops of phenolphthalein indicator to the juice sample. Record the initial volume reading of the NaOH burette on the Data Sheet, Part III, Step 2 . It is not extremely critical what the starting point is, as long as the meniscus is on the burette scale, but do not start at 30 mL or greater, since you will need at least 20 mL for each titration. 3. Unlike the colorless samples in Part I and II, this sample will be slightly yellow. As the endpoint approaches, the yellow color will brighten . Use this visual signal to slow down, as you are nearing the endpoint. Since there is a yellow background in this sample, the endpoint will be at the transition from the brighter yellow to peach (i.e. pale pink + yellow). As in Parts I and II, record the final volume of NaOH on the Data Sheet, Part III, Step 3 . Dispose of the contents of the Erlenmeyer flask in the waste vessel in the hood by the door. Repeat the procedure of Part III again with a new sample of juice and record that set of data under Determination 2. During the lab, in Part III, it is only necessary to record the data in Steps 1, 2 & 3 . The calculations should be done at home, as described in the sample calculation, Part II, Overview of Procedure , p.3.

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 11 Come to lab prepared to work efficiently and manage your time wisely! Make sure to complete at least ONE titration for each part. If you do not complete two titrations for each part, you will lose up to 10 points per part. There are 15 points of ACCURACY credit in this lab. Pre-Laboratory Assignment 1. Describe what the equivalence point is in an acid- base titration. How do we know when the equivalence point has been reached during the titration? 2. In the following two cases, what is the quantitative relationship between the amount of base added and the initial amount of the acid when the equivalence point is reached? Explain your statement with the support of a balanced reaction equation in each case. You can use the generic formula HA and H 3 A for the acids. a. A monoprotic acid titrated by NaOH b. A triprotic acid titrated by NaOH Grading Pre-Lab ______/21 Data ______/56 Post-lab ______/23 Safety (deduction) - ___ Legibility (deduction) - ___ Tardiness (deduction) - ___ TOTAL ______/ 100

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 12 Reminder: include the mole ratio in all calculations (even for 1:1) or make a statement of the mole ratio to support the calculation. 3. In the standardization of NaOH solution, about 0.4 g of KHP (potassium hydrogen phthalate, KHC $ H % O % , MW 204.2 g/mol) will be used for each trial. The NaOH solution that needs to be standardized in our experiment is about 0.1 M. What volume (in mL) of the NaOH solution is needed to reach the endpoint? Show your work and round your answer appropriately (to one significant figure based on “0.4 g” and “0.1 M”). Calculations: (include mole ratio!) 4. To standardize a solution of approximately 0.1 M NaOH, the KHP solution was made by dissolving 0.3948 g in water and titrated it with the NaOH. It took 19.10 mL of the NaOH solution to titrate the KHP solution to the endpoint. Molarity of the NaOH solution = _____________ Calculations: (include mole ratio!) 5. A total of 16.25 mL of 0.1012 M NaOH was required to reach the endpoint when titrating a 5.00 mL sample of fruit juice. Assuming that the only acid present in the juice is citric acid (a triprotic acid), calculate the molarity of citric acid in the juice sample. Molarity of citric acid = _______________ Calculations: (include mole ratio!)

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 13 DATA SHEET Instructor’s Initials: _____ I. Standardization of NaOH Solution Determination 1 Determination 2 Determination 3 (if necessary) Step 1 Mass of KHP _____________ _____________ _____________ Moles of KHP _____________ _____________ _____________ Show the calculation of number of moles of KHP for each determination: Step 2 Initial burette reading, mL _____________ _____________ _____________ Step 3 Final burette reading, mL _____________ _____________ _____________ Volume of NaOH titrant in mL _____________ _____________ _____________ Molarity of the NaOH titrant (M or mol/L) _____________ _____________ _____________ Show the calculation of molarity for each determination (don’t forget the mole ratio) : Average molarity: _____________ (avg. of Determination 1 and Determination 2) Copy this value to the top of Data Sheet II and Data Sheet III

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 14 DATA SHEET Instructor’s Initials: _____ Average molarity of NaOH, from Part I. _______________ M II. Molarity of acetic acid in white vinegar Determination Determination Determination 1 2 3 (if needed) Step 2 Vinegar Volume from dispenser, mL ____________ ____________ ____________ Step 3 NaOH Initial burette reading, mL ____________ ____________ ____________ Step 4 NaOH Final burette reading, mL ____________ ____________ ____________ Volume of NaOH, mL ( Step 4 – Step 3) ____________ ____________ ____________ Molarity of acetic acid in vinegar ____________ ____________ ____________ Show the calculation of molarity for each determination (don’t forget the mole ratio) : Average molarity: __________________

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 15 DATA SHEET Instructor’s Initials: _____ Average molarity of NaOH, from Part I. _____________ M II. Molarity of citric acid in Determination Determination Determination lemon juice 1 2 3(if necessary) Step 1 Lemon juice Volume from dispenser, mL ____________ ____________ ____________ Step 2 NaOH Initial burette reading, mL ____________ ____________ ____________ Step 3 NaOH Final burette reading, mL ____________ ____________ ____________ Volume of NaOH, mL ____________ ____________ ____________ Molarity of citric acid in lemon ____________ ____________ ____________ Juice (M, or moles/liter) Show the calculation of molarity for each determination (don’t forget the mole ratio) : Average molarity: ___________________

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Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 16 Post-Laboratory Questions 1. The strong base used in the titration can be Ba(OH) 2 solution instead of NaOH solution. If the molarity of the Ba(OH) 2 solution is the same as the NaOH solution you standardized in the experiment, which measured quantity would be different in the titration of vinegar to determine the molarity of acetic acid? Would this quantity be lower or higher than that in the same titration you carried out? Write a balanced reaction equation and use it to explain your answer. 2. In all of your titrations, you took care to get a “good” endpoint (meaning very pale pink in color). a. If you had added NaOH in bursts, or did not swirl the flask during the titration, and thus over-shot the endpoint (getting a dark pink color), how would the volume of NaOH dispensed by the burette be affected? Too high or too low? Explain. b. You used the volume of NaOH for the overshot endpoint to calculate mole of acid in the flask that reacted with NaOH. Would the calculated mole of acid be affected? Too high or too low? Explain. c. Would the calculated concentration of the acid be too high or too low? Explain.

Name ______________________________________ Section __________________ Experiment 6 Acid-Base Titration CM1001 Fall 2023 17 3. In all three parts of the experiment, the acid - KHP, vinegar or lemon juice - was either dissolved in or diluted by about 50 mL of distilled water. Does the volume of distilled water have an effect on the titration results? In other words, if 40 mL of water was used, would the volume of NaOH added to reach the endpoint be affected? Why or why not? (Hint: What determines how much NaOH needed to completely react with the acid in the flask, and why?) 4. Acid-base titration can also be used to determine molar mass of an unknown acid. Suppose you are given a sample of an unknown solid monoprotic acid, describe how to determine the molar mass of this acid by the titration with a strong base (i.e. what measurements are needed and how the molar mass is calculated). Assume the acid is soluble in water and the base is standardized. (Hint: molar mass is mass divided by mole.)

CM1011 Exp6_Titration_Fa23

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