AcidityofBeverages - LapReport AcidityofBeverages...

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Unformatted text preview: ​ Lap Report Acidity of Beverages Pre­lab questions 1.using the structural formula of citric acid shown in figure 1, determine the molecular formula of citric acid and calculate its molar mass(g/mole) answer:C 6H 8 O 7 6 C x 12.01 = 72.06 8 H x 1.01 = 8.08 7 O x 16.00 = 112.00 ================ Total = 192.14 g/mole 2.A 10.0­mL sample of pineapple juice was titrated with 0.100 M sodium hydroxide solution. the average volume of NaOH required to reach the endpoint was 12.8 mL a.Calculate the number of moles of sodium hydroxide required to reach the endpoint answer: ​MOLES OF NaOH: 1000.0mL of NaOH contains 0.1 mol Of NaOH 12.8mL of NaOH contains (0.1*12.8)/1000 moles of NaOH at the endpoint =1.28*10­3 mol of NaOH b.Using the mole ratio for the neutralization reaction shown in equation 1, determine the number of moles of citric acid in 10.0 mL of the of pineapple juice. answer:​MOLES OF CITRIC ACID: 3NaOH = 1 C6H8O7 −3 3 1.28*10 = (1*1.28*10 − )/3 −4 =​ ​4.2666*10 mol of C6H8O7/10.0mL Moles of citric acid/10.0mL=4.2666*10­4 mol of C6H8O7 c. Multiply the number of moles of citric acid by its molar mass to calculate the mass of citric acid in 10.0 mL of the juice. answer:​4.2666*10­4 mol of C6H8O7 × ​192.124 g/mol d. the concentration of acid in juices is usually expressed in grams of acid per 100 mL of juice.What is the concentration of citric acid in pineapple juice? answer:The concentration of citric acid in pineapple juice can also be expressed in grams of acid per 100 mL of juice because they contain acid and at the sametime they are both acids. 3.write a balanced chemical equation for the neutralization reaction of (a) hydrochloric acid water + answer: H Cl ­­­­­­­­>H ˆ (aq) + C l−(aq) b.acetic acid with sodium hydroxide. answer: ​HC2H3O2 + NaOH ­­­­­­> NaC2H3O2 + H20 4. The titration curves for hydrochloric acid and acetic acid with sodium hydroxide are shown below.distinguish between the strong and weak acid in terms of the initial PH ,the PH at the equivalence point and the overall shape of the titration curve. answer:The titration curve of CH3COOH +NaOH is the one with weak acid. The titration curve of HCl and NaOH is the one with strong acid. Introductory Activity Initial color mL of NaOH Color B Yellow 3mL Blue P Milky 2 mL Pink T Gold 3 mL Blue 2 mL of juice 3 mL of NaOH ​ Guided Inquiry 1.choose a suitable indicator for determining the endpoint in the neutralization of a weak acid with a strong base.explain your reasoning based on the evidence obtained above as well as the titration curve data discussed in the pre­lab question 4. answer:The most suitable indicator to use for the titration would be phenolphthalein as its pH transition interval is between 8.2­10.0 which is where the endpoint and equivalence point is most likely to be at. The color that indicates the endpoint would be pink since the pH is above 7 (basic) in a weak acid­strong base titration. 2.would you expect any differences in the choice of an appropriate indicator for the titration of a strong acid such as HCl? why or why not? answer:​no I won't because phenolphthalein has nothing to do with the acid. any acid­base indicator that would change color in the pH range of 4 to 10 can be used in strong acid­ strong base titration.Whenever a strong base is use to titrate an acid whether strong or weak phenolphthalein is the best choice as an indicator, because; when acid is added in phenolphthalein solution equilibrium will shift towards left as the number of hydrogen ions increase, so phenolphthalein appears to be colorless. As soon as the strong base is added the equilibrium shifts toward right. When the pH comes around in the range of 8 to 10 the phth­ ions appears to be pink. 3. acidic beverages generally contain weak acids, such as citric acid in the citrus fruit juices, tartaric or malic acids in other fruit juices, phosphoric acid in colas, and carbonic acid in seltzers.write a balanced chemical equation and determine the mole ratio for the reaction of each acid with sodium hydroxide Note that it is not necessary to draw their chemical structures answer: ​Reaction with phosphoric acid 3 NaOH + H3PO4 = Na3PO4 +3H2O + 3NaOH = 1 H3PO4 3:1:1:3:3: Reaction with carbonic acid 2NaOH +H2CO3 = Na2CO3 + 2H2O + 2NaOH = 1 H2CO3 2:1:1:2:2: Reaction with tartaric acid 2NaOH + C4H6O2 = Na2C4H4O2 + 2H2O + 2NaOH = 1 C4H6O2 2:1:1:2:2 4.The titrant used in a titration experiment is a standard solution.Explain what this means, identify the titrant and obtain the known molarity from your instructor. answer:A standard solution is one whose concentration is accurately known.You should ask and identify the tirant so that the concentration could be accurate. The amount of citric acid in citrus fruit juices can be determined by titration with a standard solution of sodium hydroxide. 5.Review the setup shown in figure 2 for a titration procedure. ● the buret should be cleaned and then rinsed with the titrant before beginning the titration.Explain why this is necessary ● answer: it is necessary to rinse the cleaned buret with the titrant before beginning the titration because it may affect the concentration of the titrant or performance of the titration will not be accurate. ● is it necessary to know the precise volume of beverage that will be titrated? Explain answer: yes,it is necessary to know the precise volume of beverage that will be titrated because if the volume of the beverage is unknown it will affect the titration curve or would not know how many mL of the acid/base we are supposed adding for the equivalence point to occur. or to reach the endpoint is measured and the used to calculate the concentration of citric acid in the juice. ● choose the type of volumetric glassware(flask, graduated cylinder or pipet)to measure the beverages that will be titrated in this experiment.Explain the choice. answer: Graduated cylinders are useful for measuring liquid volumes to within about 1%. if greater accuracy is needed, use a pipet or volumetric flask.so that we can get the best measurement for the beverage that will be titrated. ● it’s helpful to occasionally rise the side of the beaker or flask with distilled water during the titration procedure.Explain why it is not necessary to know the volume of rinse water. answer :it is not necessary to know the volume of the rinse water because it doesn’t affect the concentration of the titration and it also it does not affect the PH of the solution. 6. Examine a buret and explain how to read the volume of titrant in the buret.What precision(number of significant figures)is allowed in these measurements? an example answer: ​Assume that the burette is filled to the point indicated in the figure at the left. I would record the initial point as 3.30 ml; the ending point would be 3.90 ml. Therefore, the titration would have required 0.60 ml. Remember that we should read the number that is at the bottom of the ​meniscus. two number of significant figure is allowed in these measurements. 7. what data must be measured and plotted to obtain the titration curve for an acidic beverages? What is an appropriate volume interval for obtaining this data during titration?Explain your reasoning? answer:the volume of the titrant and the PH of the solution must be measured and plotted to obtain the titration curve for an acidic beverage. 8.write a detailed step­by step procedure for titrating a beverage to determine the centration of weak acid, if present include the reagents needed, the glassware and equipment that will be used, and appropriate measurements and observation that must be used answer Guided Inquiry Lab Procedure Safety Precautions: Keep phenolphthalein and bromthymol blue away from flames or things that can cause ignition. Wear safety goggles, gloves, and aprons during experiment to avoid contact with any chemicals that can cause skin irritation such as nitric acid Do not ingest any of the solutions as it toxic. Lab equipments: ● Indicators­Phenolphthalein, Bromthymol blue, thymol blue ● Solutions­fruit juice , sodium hydroxide. ● Distilled water ● Beakers­50, 150, and 250 mL ● Beral­type pipets ● Buret, 50 mL ● Graduated cylinders­10 and 100 mL ● Magnetic stirrer, stir bar, stirring rod ● pH sensor or pH meter ● Support stand, buret clamp ● Test tube rack and 4 test tubes Lab Techniques: ● Closing and opening buret ● Operating a magnetic stirrer ● Understand how pH meters work Lab Procedures: 1. Set up buret using a support stand and clamp. Make sure it is secured tightly and appropriately near a magnetic stirrer. A second setup is optional if it is preferred. 2. Secure a pH sensor/meter onto the support stand. 3. Measure 10 mL of the base/acid (Depends on what you want to titrate) with the unknown molarity using a 20 mL graduated cylinder into a 250 mL beaker. 4. Wash the pH meter with distilled water in another beaker before putting it inside the acid solution. The pH meter is covered in a buffer solution that, if not washed off, can throw off the titration experiment. 5. Put the beaker of the unknown acid/base onto the magnetic stirrer with the stir bar and pH meter inside the beaker. 6. Make sure there is space between the stir bar and pH meter so they do not interfere with each other during the titration. If the pH meter is not fully in the acid solution use distilled water to fill the beaker up until the bottom of the meter is immersed. 7. Before adding the titrant (Acid/base depending on your experiment) pour distilled water into the buret to eliminate any air bubbles that might be hiding. Air bubbles can mess up measurement and ruin your calculations. 8. Add the 50 mL of the titrant into the buret and secure it back onto the support stand. 9. Add 1­3 drops of a selected indicator from the ​materials section. 10. Turn on the magnetic stirrer to a suitable speed that is able to mix the acid­base solution during titration. 11. Record the initial pH of the acid solution before titration and after each 1 mL of the titrant added. Take note of when the solution changes colors. 12. Use distilled water to wash off any solution that is on the sides of the beaker. Distilled water will not mess with the experiment since it only dilutes the acid­base solution. The moles of acid and base still remain the same even when diluted. 13. Clean up the setup and dispose of any chemicals properly. Wash pH meter with distilled water before putting it back in the buffer solution bottle. 9. Review the hazard of the chemical used in the procedure and write appropriate safety precautions that must be followed during the experiment. answer: the fruit juice was hazardous when we drink it because it has expired.We have to wash our hands after using. the citric acid was also hazardous when we get it on our skin.We have to wear gloves and flash our hands with water after using. 11.choose an amount of beverage to be titrated that will require at least 10.10 mL but less that 20 mL of titrant.Explain why this range of titrant is optimum answer:when the range of the titrant is optimum it will be easy to predict the equivalence point of the titration curve.When the range of the titrant is not optimum the titration process will not be a occurrent.know the amount of beverage and set the range of the titrant to be optimum so that we can know how much we are adding. 12.carry out the titration to obtain the titration curve data.Record data in an appropriate data table 10 mL of Juice 14 mL of NaOH the initial PH of 10 mL of juice is 4.2 mL of NaOH PH 5 mL 5.53 10 mL 6.8 11 mL 7.1 12 mL 7.3 13 mL 7.5 14 mL 8.0 15 mL 8.18 16 mL 8.33 17 mL 8.56 analysis the result: note: the color changes at 14 mL which is the equivalence point. Ap Review Question 1.Why is phenolphthalein an appropriate indicator for titration of strong acid with a strong base?explain based on the PH at the equivalence point and the transition range for Phenolphthalein. answer:​Any acid­base indicator that changes colour between pH 4 and pH 10 is suitable to detect the endpoint for a strong acid ­ strong base titration. phenolphthalein could be used. Just one drop of the added base will bring about a change in colour of the indicator. 2.A 10.00 mL sample of HCl solution was transferred to an Erlenmeyer flask and diluted by adding about 40 mL of distilled water.Phenolphthalein indicator was added, and the solution was titrated with 0.215 M NaOH until the indicator just turned pink.The exact volume of NaOH required was 22.75 mL. Calculate the concentration of HCl in the original 10.00 mL sample? answer: 22.75 mL of 0.215 M NaOH is 0.02275 × 0.215 = 0.004891 Mol of NaOH, which may completely neutralize 0.004891 Mol of HCl. So the original molar concentration in 10 ml HCl is: 0.004891/0.01 = 0.489M. 3. one studently accidentally “overshot” the endpoint and added 23.90 mL of 0.215M NaOH.Is the calculated concentration of HCl likely to be too high or too low as a result of this error? answer: The relative error is (23.90 ­ 22.75)/22.75 = 1.15/22.75 = 5.05% The calculated concentration of HCl likely to be 5.05% high, because the concentration calculation of HCl is based on molar mass of NaOH used. 4.acid­base indicators are large organic molecules that behave as weak acid.This distinguishing characteristics of indicators is that the acid(HIn) and conjugate base(In^­) forms are different colors HIn(aq) +H2O → I n−(aq) H3O+(aq) (color A) (color B) − The color of an indicator solution depends on pH and the relative amount of HIn and In ˆ at a given pH.Consider the following indicators and their acidic and basic colors,as well as the pH transition range for each. − indicators HIn In ˆ pH transition Alizarin* Yellow red 5.5­6.8 Alizarin* Red purple 11.0­12.4 Bromthymol blue Yellow blue 6.0­7.6 Phenolphthalein colorless pink 8.2­10.0 a. the intermediate or transition color of bromthymol blue is green.What are the relative − proportions of HIn and In ˆ bromthymol blue is green?Explain. answer:Most of the indicator molecules exist in the form HIn and the color of the solution is essentially the color of HIn. Most of the indicator molecules exist in the form In– and the color of the solution is essentially the color of In–. The solution contains roughly equal amounts of the two forms and the resulting color is intermediate between that of HIn and In–. The exact concentrations of H3O+ at which cases 1–3 will predominate depend on the structure of the indicator and the equilibrium constant for the equation above. Different indicators change color in different pH ranges b. A colorless solution was tested with phenolphthalein, Bromthymol blue and alizarin,The solution was colorless with phenolphthalein, yellow with bromthymol blue and orange with alizarin.What is the pH of the solution? Explain. answer: The pH of the solution will be 5.5­6.8 because the alizarin has color of yellow and red and the combination of red and yellow makes orange. ​summary The purpose of this inquiry lab is to conduct acid­base titrations and determine the concentration of acid in common beverages such as orange juice or pineapple juice. The beverages contain weak acids, which will be titrated with a strong base, sodium hydroxide (NaOH). We used the titration that we performed last month as a model to design a titration procedure that will allow us to obtain a titration curve data and calculate the molar concentration of acid in a beverage. The titration curve was analyzed and the amount of acid in a typical serving size or bottle may also be determined. The identity of the acid in the beverage may be derived by reviewing the titration curve and reference information, and by consulting the ingredients label. this lab was so complicated that we could repeat it for several times before we could get the right reslut! ...
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