Acid Base Extraction Key PDF

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Acid Base Extraction Complete Answer Key...

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Chem 546: Lab 2: Acid-Base Separation of a Mixture Answer Key Pre-Lab Assignment (1 point) Descriptive Title of the Experiment Acid-Base Separation of a Mixture (1 point) Objectives The objective of this experiment is to separate a mixture of benzoic acid, 4-tert-butylphenyl, and biphenyl using a series of acid-base extractions. (3 points) Structures Provide the structures, names, and pKa values (if applicable) for the three compounds of your mixture.

(3 points) Table of Reagents Density g/mL

Hazards

0.71

Highly flammable

Substance

Wt. or Vol.

BP or MP, °C

Benzoic acid

1.0 g

123

4-tert-Butyl-phenol

1.0 g

101

Biphenyl

1.0 g

69

tert-Butyl methyl ether

35 mL

35

Sat. Aq. NaHCO3

18 mL

12 M HCl

4.5 mL

1.5 M NaOH

20 mL

1

Sat. Aq. NaCl

15 mL

 1.2

1 Strongly corrosive Strong base, corrosive

Na2SO4 Pre-Lab Questions (2 points) 1. Why would ethanol (EtOH) be a poor choice of solvent for extraction of an aqueous solution? EtOH is totally miscible with water and there would only be a single layer. Therefore extraction would be impossible.

(2 points) 2. What is the drop test, and how and why is it used? A test to determine which layer is the aqueous layer. Drop a small amount of water into the neck of the separatory funnel or test tube containing the two immiscible liquids. Watch it carefully: if it remains in the upper layer, that layer is the aqueous layer. If it sinks to the bottom of the upper layer, the bottom layer is the water layer. The test can also be carried out using a drop of the organic solvent. (1 point) 3. Write a balanced equation for the reaction of sodium bicarbonate with hydrochloric acid. NaHCO3 + HCl  NaCl + CO2 + H2O

4. A solution of 4 g of organic compound X in 300 mL of water was extracted with 300 mL of ethyl acetate. At equilibrium the concentration of X in the two phases was: 4.6 g / 100 mL of ethyl acetate; 0.8 g / 100 mL of water. (2 points) a) Calculate the partition coefficient, Kd, for compound X in this system. Show your calculations for full credit. Kd = 4.6 g / 0.8 g = 5.75 (3 points) b) What percentage of the original sample of X was extracted? Show your calculations for full credit.

(1 point) 5. When using a drying agent (sodium sulfate for example) to dry an organic layer, how do you determine is your layer is dry? The drying agent should not clump together upon swirling. (It should be granular and flow like sand when the flask is swirled.)

Chem 546 Report Form Lab #2 - Acid-Base Separation of a Mixture Name Date (1 point)

TA

Product Information 1. Benzoic acid

Weight

MP

2. p-tert-Butylphenol

Weight

MP

3. Biphenyl

Weight

MP

(3 points) Percent Recovery Calculate the percent recovery of the three components of the mixture. Show your calculations to the right for full credit. Benzoic acid

Starting amount of benzoic acid = mass of mixture / 3 % rec = isolated mass / starting mass

p-tert-Butylphenol

Biphenyl

Reactions (4 points) 1. Write a balanced equation for the acid-base reaction that occurs when the carboxylic acid component of your mixture was extracted from the ether solution. Write a balanced equation for the acid-base reaction required to recover the carboxylic acid from the aqueous layer.

(4 points) 2. Write a balanced equation for the acid-base reaction that occurs when the phenol component of your mixture was extracted from the ether solution. Write a balanced equation for the acid-base reaction required to recover the phenol from the aqueous layer.

Results and Discussion (3 points)

**Answers to these questions should be written in complete sentences, following the style for technical writing. See the handouts on myCourses for further instruction. Be specific with feedback. Indicate why points are lost and comment when something is done particularly well. All writing should be written in passive voice.

(5 points) 1. Discuss, briefly, how you separated the components of this mixture. This should not be written like a procedure. Instead, discuss the methods used to separate components and why they worked (or didn’t). The procedure used two different base washes to separate components; discuss why this worked based on the pKa values. Also discuss how the compounds were finally isolated from solution. A complete answer will discuss how the acid-base extractions separated components. A weak base (NaHCO3) was used initially to separate benzoic acid (pKa = 4.17) from the mixture. A stronger base would have also deprotonated 4-tert-butylphenol (pKa = 10.17). Benzoic acid was isolated by adding HCl to protonate the salt. 4-tert-Butylphenol was then separated from biphenyl using NaOH and isolated by protonating the salt with HCl. Biphenyl was isolated by rotary evaporation. (3 points) 2. Comment on the melting points of your isolated compounds. How do they compare to the literature values provided? Do they confirm the identity of the two isolated samples? Why or why not? What do they say about the purity of the samples? A complete answer will comment on any decrease observed in melting point as well as the range. These indicate purity of the samples because impurities lower and broaden melting points. (2 points) 3. Percent recoveries for this type of experiment are usually slightly less than 100% due to miscellaneous mechanical losses. Comment on your percent recoveries and what they say about the success of the experiment. If the yields are significantly below 100% or above 100%, you should try and think of logical reasons why based on the operations you carried out; don’t just make a laundry list of excuses. Many sources of loss. A few of the main ones are:    

Incomplete extraction due to use of too little acid or base (not checking pH) Incomplete precipitation from aqueous layer due to slight solubility of cold in water Incomplete precipitation from aqueous layer due to too short a cooling period Mechanical loss of product on filter paper, funnels, flasks, etc.

(5 points) Laboratory Notebook Follow the requirements in the notebook guidelines. Notebook pages should contain enough information that you or a peer could easily repeat the experiment. Remember to record observations.

Notebook pages should be inspected to ensure proper formatting (See lab notebook guidelines). Be specific with comments so students know how to proceed with future experiments. Post-Lab Questions (3 points) 1. Benzoic acid is extracted from an organic solution using 20 mL of 2 M NaOH. How many mL of 3 M HCl are required to neutralize the NaOH in order to recover the benzoic acid? Show your work for full credit. (2 moles/L) x 0.02 L = 0.04 moles of NaOH 0.04 moles / (3 moles/L) = 0.0133 L = 13.3 mL (3 points) 2. The mixture you separated consisted of a neutral compound and two acidic compounds. What additional steps would you have to add to the procedure if your mixture also contained a water-insoluble, basic amine that you wished to isolate? Extract the amine into two portions of an aqueous acid, such as 1.0 M HCl. Combine the aqueous extracts and make strongly basic (pH  11), for example by use of 3 M NaOH. Cool and collected the precipitated amine by vacuum filtration, wash with cold water, and dry. 3. Describe how and why the following experimental mistakes might affect the results of your experiment. In each case specify the component(s) of the mixture whose percentage(s) would be too high or too low, and how the purity of the isolated products would be affected. (3 points) a) During the NaHCO3 extractions you failed to thoroughly mix the aqueous and organic layers. Not all of the benzoic acid will be converted to the sodium salt, so it will remain in the organic phase. Thus the reported percentage of benzoic acid will be low. The benzoic acid in the organic phase will be extracted with the subsequent NaOH extraction, and will contaminate the phenolic component. Thus, the phenolic component yield will be high and the purity will be low.

(3 points) b) While attempting to extract the phenolic component, you mistakenly extracted the ether solution with 1.5 M HCl rather than 1.5 M NaOH. HCl is an acid, not a base. This will result in very little phenol extracted into the aqueous layer. The phenol will therefore have a low yield, and the neutral component will have a high yield. In addition the neutral component will have low purity due to the contamination by the phenolic component.

(2 points) c) When recovering the benzoic acid component, instead of using pH paper you adjusted the pH to 7 using litmus paper. Little if any benzoic acid will precipitate at pH 7, so the reported percentage of benzoic acid will be low. 4. Partition Coefficient Calculations (3 points) a) The distribution coefficient, Kd, for organic compound Y between diethyl ether and water is 6.0. What weight of Y would be removed from a solution of 20 g of Y in 200 mL of water by a single extraction with 100 mL of diethyl ether? Show your work for full credit.

(3 points) b) What total weight of Y would be removed by two successive extractions, each using 50 mL portions of diethyl ether? Show your work for full credit....