How to calculate percent recovery for recrystallization

Purifying Acetanilide By Recrystallization

Purifying Acetanilide by

Recrystallization

1. The solubility of benzoic acid in water is 6.80 g per 100 mL at 100°C and 0.34 g per 100 mL at 25°C.

Show your calculations for the questions below.

(a) Calculate the minimum volume of water needed to dissolve 1.00 g of benzoic acid at 100°C.

(b) Calculate the maximum theoretical percent recovery from the recrystallization of 1.00 g of benzoic

acid from 15 mL of water, assuming the solution is filtered at 25°C.

2. The solubility of acetanilide in your recrystallizing solvent is 5.0 mg per mL at 10°C. (10ºC is the

approximate temperature of an ice/water bath). Show your calculations for the questions below.

(a) Calculate the theoretical maximum percent recovery in this experiment, assuming a 15.0-mL

recrystallizing solution is filtered at 10°C.

(b) Calculate the percent recovery of the acetanilide produced in your experiment.

(c) How do your results compare to the theoretical maximum percent recovery? Explain any difference

you may have observed. Why is the theoretical maximum percent recovery (calculated in 2a above) not

necessarily applicable your experiment? Aside from technical or human error (such as spillage,

incomplete transfers, loss on the filter paper, loss due to excessive washing, etc.) or slight differences involume of solvent or temperature of the ice bath, what assumptions or estimations were made in this

calculation which may not apply to your results?

3. A student rushed through this experiment. Describe the effect that the following procedural changes

would have on the percent recovery of acetanilide; would the % recovery by higher, or lower? Briefly

explain the basis of each answer.

(a) Rather than adding 0.5-mL portions of boiling solvent to the acetanilide, the student added 5-mL

portions of boiling solvent.

(b) The student forgot to cool 5 mL of solvent in Part 4 and washed the crystals with room-temperature

solvent.

Separating Acids and Neutral Compounds by

Solvent Extraction

1. Based on the amounts of p-toluic acid and acetanilide you recovered, estimate the

composition of the original mixture, assuming that you lost equal amounts of each

compound. Show your calculations. Express the composition as percentages of each

component: For example “the original mixture was 30% p-toluic acid and 70%

acetanilide”.

2. What product would you obtain if you evaporated the water from the NaOH layer prior to

acidifying the layer?

3. Suppose that you used dichloromethane instead of diethyl ether as the nonpolar solvent in

this experiment. What changes in the procedure would you make in view of the fact that

dichloromethane is more dense than water?

4. Benzoic acid (C6H5—COOH) is a weak acid and naphthalene is neutral, neither acidic or

basic. Prepare a flowchart for the separation and recovery of benzoic acid and

naphthalene.

Benzoic Acid Naphthalene

solubility in water: poor solubility in water: poor

solubility in ether: good solubility in ether: good

5. After comparing the melting points of each of your compounds to their respective

literature values, comment on the purity of each compound. (You may skip this question if your

instructor did not have you acquire melting points).

O OH

1. Briefly describe the hazards you should be aware of when you work with:

(a) diethyl ether

(b) 3M HCl

2. Briefly explain or describe the following:

(a) How would you determine which layer is the aqueous layer after you add NaOH solution to

the ether solution of your compounds?

(c) What visible evidence(s) of reaction will you see when you acidify the NaOH extract with

HCl solution?

(d) In which layer would p-toluic acid be more soluble if p-toluic acid were added to a two-layer

mixture of diethyl ether and water?

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Solvent Extraction, Pre-lab page 2

(e) How would the results differ if you added sodium p-toluate instead of p-toluic acid to the

two-layer mixture of diethyl ether and water?

4. How many milliliters of 3.0 M HCl would be required to neutralize 30. mL of 0.50 M

NaOH? (Show your work). (hint: this is a general chemistry question! Remember that the

acid HCl and the base NaOH react in a 1:1 molar ratio. The number of moles of acid need to

equal the number of moles of base to effect complete neutralization).

5. Briefly explain how you will isolate p-toluic acid after it is extracted it into NaOH solution.

6. Write the equation for the chemical reaction of the toluate ion that will occur when you

add HCl solution to the NaOH extract in part 3.

SN1 Reaction: A Kinetic study

Post-Laboratory Questions (attach your responses on a separate sheet)

1. From the experimental data, prepare a table of the following values. Record all calculated

results to the proper number of significant figures. Note that V and V¥ are the total volumes of

NaOH solution delivered; they are not burette readings (unless, of course, your initial buret

reading was 0.00 mL). Base your calculations on the actual value of V¥ that you measured in lab

- not the theoretical value! Time, t, is the elapsed time in seconds (its OK to use minutes if you

prefer). Use Equation 10 to calculate the values of k.

t (sec) V (mL) V/V¥ (1 - V/V¥) ln(1 - V/V¥) k

2. Prepare a graph of ln(1 – V/V¥) versus time, t. Using a computer program or a graphing

calculator, determine the best straight line through your data points. (See the supplement to this

lab for detailed instructions on how to determine best fit if you are unfamiliar with this process).

This graph should be properly labeled and drawn to scale. Do the data support the SN1

mechanism? Briefly explain.

3. Calculate the value of k from the slope of the line from the graph in 2.

4. Using the values of k which you determined above (in question #1), calculate the average

value of k, and estimate its uncertainty. One way to estimate its uncertainty is to

(1) calculate an average value of k

(2) calculate the absolute value of the deviation of each value from the average value

(3) calculate the average of these deviations.

Prepare a table which shows the deviation for each datum, and report the average value of k and

the average deviation. It may be convenient for you to include these data in the table you prepare

for post-lab question #1, above.

Note: What you are doing here is a standard statistical way of measuring reproducibility.

Theoretically, the value of k should be the same for all trials. In practice, however, you almost

certainly will observe some variation in the calculated values of k. The average deviation is a

way of expressing in a single number how much variability you have in your data. Low average

deviations mean your numbers are all close together; high average deviation means you have a

lot of “noise” or “scatter” in your data.

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Pre-Laboratory Assignment SN1 Reaction: A Kinetic Study

Name ________________________________________________________________

Note: you may wish to copy this pre-lab before you submit it. (The results of pre-lab question

#2 will be helpful for your lab calculations).

1. Describe briefly the hazards associated with the reaction mixture and the safety precautions

you must take when performing the experiment.

2. Assume that you use 1.00 mL of 2-chloro-2-methylpropane. Calculate the following

quantities. Be sure to show your calculations; you will NOT receive credit if work is not

shown!

a. the number of moles of 2-chloro-2-methylpropane used. The density of the liquid is

0.851 g/mL.

b. the number of moles of HCl produced by complete solvolysis of 1.00 mL of 2-chloro-2-

methylpropane.

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SN1 Reaction: A Kinetic Study, Pre-lab page 2

c. the volume in milliliters of 0.350 M NaOH required to neutralize the HCl produced by

complete solvolysis of 1.00 mL of 2-chloro-2-methylpropane. (This is referred to as V¥ in your

lab manual).

d. the volume in milliliters of 0.350M NaOH required to neutralize the HCI produced when

solvolysis of 1.00 mL of 2-chloro-2-methylpropane is 50% complete. (This is referred to

as V50% in your lab manual; the volume of base consumed when the reaction is 50%

complete).

Note: you may wish to record the values of V¥ and V50% separately before you turn in your

pre-laboratory assignment; you will need these values during the experiment.

Studying SN1 Reactions:

Nucleophilic Substitution at

Saturated Carbon

Post-Laboratory Questions

1. When you compared 2-bromo-2-methylpropane and 2-chloro-2-methylpropane (in part

one), what were the relative rates of the two reactions?

2. Based on your answer to question 1, which is the better leaving group, Br- or Cl- ?

HBr is a stronger acid than HCl. Are these results consistent with the relative

basicities of these two ions? Explain your answer.

3. Which compound, 2-bromo-2-methylpropane or 2-bromopropane, reacted faster in

your SN1 experiment? How are the reactivities of 2-bromo-2-methylpropane and

2-bromopropane related to the stabilities of the carbocations produced as intermediates in

the reaction? Explain your answer.

4. Which of the two solvent mixtures, 40% 2-propanol or 60% 2-propanol, is more

polar? Explain your answer.

5. In which of the two solvent mixtures did 2-bromo-2-methylpropane react faster?

Account for your results in terms of the effect of solvent polarity on the rate-determining

step in this SN1 reaction.

6. Use your results to explain which variable — leaving group, alkyl structure, or solvent

polarity — has the greatest impact on the rate of an SN1 reaction. In other words, which

variable had the most pronounced effect on the observed times?

9

Pre-Laboratory Assignment Studying SN1 reactions

Name _________________________________________________________________

1. Why do tertiary alkyl halides typically undergo SN1 substitution reactions more rapidly

than do primary or secondary alkyl halides?

2. Show a complete and balanced reaction for the solvolysis of

2-bromo-2-methylpropane in water. Also show a complete and balanced reaction for the

solvolysis of 2-bromopropane in water. (It is NOT necessary to show intermediates or

mechanisms, just the overall reactions). Which of these reactions is kinetically favored?

3. How does the SN1 reaction in this experiment cause the acid/base indicator,

phenolphthalein, to change color? Briefly explain.

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4. Why is it important that the volume of 0.5 M NaOH be measured exactly? What

would happen if you put different amounts of NaOH in the different reaction vessels,

what error would this cause in your results?

5. Which is the better leaving group, Cl-1 or Br-1 ? Briefly explain why one is better than

the other.

Extract caffeine from i tea

Post Laboratory Questions (attach your responses on a separate sheet)

1. Calculate the mass percent of caffeine in your instant tea sample.

2. On the average, people use one teaspoon (2.5 g) of instant tea to make an 8.0-oz glass of iced tea. The

average glass of iced tea contains 10. mg of caffeine per oz.

(a) Based on this information, calculate the mass percent of caffeine in a glass of iced tea.

(b) Are your experimental results consistent with these facts? Briefly explain.

(c) If not, explain why your experimental results may differ from these data.

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Pre-Laboratory Assignment Isolating Caffeine from Tea

Name _________________________________________________________

1. Why should you use a fume hood when working with dichloromethane?

2. Why is gentle shaking necessary during the extraction of caffeine from tea?

3. Why is it important to not overheat the dichloromethane while extracting the caffeine?

4. During recrystallization, why should the 2-propanol be kept hot while dissolving the crude crystals?

Synthesize acetylsalicylic acid (aspirin)

Post-Laboratory Questions (attach your responses on a separate sheet)

1. Calculate the % yield of aspirin in your experiment. Be sure to fully show your calculations

for theoretical yield and % yield; you will NOT receive credit if work is not shown!

2. a. Other than human error, impurities in the starting materials, or equipment

malfunctions, can you think of any reason why the aspirin might not be completely pure?

What steps could be taken to improve the purity of your product?

b. If you determined the melting point of the aspirin sample, comment on the purity of

your product.

3. Using your textbook or another appropriate reference, show a detailed mechanism for the

esterification reaction in this experiment.

Pre-Laboratory Questions Synthesis of Aspirin

Name _________________________________________________________________

1. Calculate the theoretical yield of aspirin, in grams, for this experiment, assuming

that you start with 2.0 grams of salicylic acid and 5.0 mL of acetic anhydride. The

density of acetic anhydride is 1.08 g/mL. You must show ALL of your calculations to

receive credit on this question and question #2. (You may want to make a copy of this

calculation and result before turning in the pre-lab; you will need this value for the

post-lab questions, also).

2. If 1.9 grams of aspirin were obtained in this experiment, what would be the percent

yield?

3. Write the equation for the reaction which occurs when acetic anhydride is mixed with

water. (please show structures or structural formulae in your response).

Oxidation of an Alcohol:

Oxidizing Methoxybenzyl Alcohol to

Methoxybenzaldehyde Using

Phase-Transfer Catalysis

Post-Laboratory Questions (attach your responses on a separate page)

1. Calculate the percent yield of your product. [Note: if the amount of methoxybenzyl alcohol

you used differs from the amount listed in the “Reagents and Properties” section of the lab, you

will have to re-calculate the theoretical yield, based on the amount of alcohol actually used].

Make certain to show your work for this calculation.

2. Calculate Rf values for methoxybenzyl alcohol and methoxybenzaldehyde, using the 20-min

spotting time. Why does the alcohol have a lower Rf value than the aldehyde?

Note: you should submit the TLC plates! They belong in the results section. Tape them onto a

sheet of paper and label them appropriately. If you worked with a partner, make a note if your

TLC plates are with your partner’s report.

3. The ethyl acetate layer was extracted with NaOH to remove any traces of methoxybenzoic

acid that may have been produced by over-oxidation of the alcohol.

a. Draw the structure for methoxybenzoic acid.

b. Show the balanced reaction of NaOH and methoxybenzoic acid.

c. If you wished to isolate and collect methoxybenzoic acid from the NaOH solution,

explain how this could be done. (hint: methoxybenzoic acid is not soluble in water).

4. Explain how TLC could be used to monitor the reduction of methoxybenzoic acid to

methoxybenzaldehyde acid using LiAlH4 as a reducing agent.

5. In this experiment, students often note that on the TLC plates, the spot for the standard

p-methoxybenzaldehyde has a very faint second spot with a very low Rf. You may have noticed

this on your own TLC plates - it is due to a contaminant in the benzaldehyde. The Rf of this spot

is lower than that of p-methoxybenzyl alcohol; what do you think this spot may be? (Hint: the

contaminant appears after p-methoxybenzaldehyde has been stored for a long period of time in

an oxygen atmosphere such as air; the contaminant is NOT observed if the

p-methoxybenzaldehyde is stored away from oxygen).

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Pre-Laboratory Assignment Oxidizing Methoxybenzyl Alcohol

Name ___________________________________________________________________

1. What safety precautions must be observed when using

(a) ethyl acetate?

(b) dichloromethane?

2. Using the data in the Reagents and Properties table, calculate the theoretical yield of

methoxybenzaldehyde, assuming you start with 1.50 g of the alcohol. (Assume the alcohol is the

limiting reactant). Be sure to show your work; you will NOT receive credit if work is not

shown. You may wish to make a copy of this calculation to assist you with your post-lab

questions.

3. Explain why a phase-transfer catalyst is used when oxidizing methoxybenzyl alcohol using

NaOCl.

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Pre-Laboratory Assignment Oxidizing Methoxybenzyl Alcohol

Name ___________________________________________________________________

4. Draw the structure of the expected organic product for each of the reactions shown below: