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I do not know how to figure out what to put in the blank spots or what I have put in is close to being correct.

Submitted by kodihansen on Sat, 08/17/2013 - 14:40

I do not know how to figure out what to put in the blank spots or what I have put in is close to being correct.

Experiment 13 Assignment

Using the notes/data that you recorded while doing the experiment to help you complete the following report. Type you response in the space provided. Please be aware of proper spelling, punctuation, and sentence structure. Please refer to sample report at the end of experiment 3 if necessary on how this report is to be completed. When you have completed all the items to your satisfaction SUBMIT the report to your instructor for grading. Please be aware that it may be several days before your report is graded. When graded you can find your grade and any comments here in LateNiteLabs or in the course gradebook in Webadvisor.

 

 

Part 1 – Data from three best trials.

 

Trial 1

Trial 2

Trial 3

A.Initial Volume of NaOH in buret

23.47 mL

23.47 mL

23.47 mL

B.Final Volume of NaOH in buret

23.42 mL

23.42 mL

23.42 mL

C.Mass of primary standard used

.05 mL

.05 mL

.05 mL

 

 

 

Trial 1

Trial 2

Trial 3

Calculation Description

D.Total volume NaOH used to titrate the primary standard

    

E.Molarity of NaOH

    

F.Average NaOH Molarity

  

 

 

Part 2 – Data from three best trials.

Acid Unknown Number

2

 

Trial 1

Trial 2

Trial 3

G.Initial Volume of NaOH in buret

23.47 mL

23.47 mL

23.47 mL

H.Final Volume of NaOH in buret

23.07 mL

23.07 mL

23.07 mL

I.Volume of acid used, (unknown)

.4 mL

.4 mL

.4 mL

 

 

 

Trial 1

Trial 2

Trial 3

Calculation Description

J.Total volume NaOH used to titrate the acid (unknown)

    

K.Moles of NaOH used

    

L.Molarity of acid (unknown)

    

M.Average Molarity of Unknown

  

Suppose during the experiment you became confused about which of the two containers held the NaOH solution that you standardized during part 1 of this experiment and which held the acid (unknown) used during this experiment. What could or should you do to quickly identify each container?

What I learned while doing this experiment

Background

Solid sodium hydroxide (NaOH) is very hygroscopic (it draws moisture from the atmosphere), and therefore it cannot be used as a primary standard. In order to prepare an NaOH solution with an exact molar concentration, it must be standardized with an acid that is a primary standard.

For a substance to be a primary standard, the following criteria should be met, or at least approached:

1. It must be available in very pure form.

2. It must be reasonably soluble.

3. It must be stable in the pure form and in solution.

4. It must be non-hygroscopic and easily dried.

5. It must be a compound with a reasonably high formula weight.

In this experiment, dry benzoic acid (C7H6O2) is used as the primary acid standard for determining the molar concentration of a sodium hydroxide solution.

You are being asked to prepare a 0.1 M solution of NaOH from reagent grade solid pellets or flakes of NaOH. The molecular weight of NaOH is 39.997 g/mol, so 40g of solid NaOH dissolved in a liter of water should be very nearly a 1 M solution, and adding 4g NaOH to one liter of water should be very nearly 0.1 M.

That said, solid NaOH easily absorbs water from the moisture in the air. Depending on the storage conditions, the mass percent of water can be significant and there will be less NaOH in a given mass of the solid than expected. The actual concentration of such a solution must therefore be obtained by testing with other reagents whose quantities are precisely known.

Benzoic acid can be used as a primary standard because of its high purity, relatively large molar mass, and because it is not hygroscopic. In this experimental procedure, an accurately measured mass of dry benzoic acid is dissolved in deionized water. A prepared NaOH solution is then dispensed from a burette into the benzoic acid solution containing phenolphthalein indicator until the stoichiometric point is reached, as signaled by the indicator changing from colorless to pink.

The balanced acid-base reaction is:

C7H6O2+ NaOH →NaC7H5O2+ H2O

Procedures

Part 1: Preparing the Primary Standard

PLEASE NOTE: The procedures described in this lab assume that you have already done the Titration Tutorial and are familiar with the technique. If you have not yet done the Titration Tutorial Lab, please do it now.

1. Take abeaker from the Containers shelf and place it on the workbench.

2. Prepare a sodium hydroxide (NaOH) solution of approximately 0.1 M concentration by dissolving 0.8 g of solid sodium hydroxide in 200 mL of water from the Materials shelf.

3. Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.

4. Add 100 mL water from the Materials shelf to the Erlenmeyer flask.

7. Add 0.244g of benzoic acid (the primary standard) to the Erlenmeyer flask. The 100 mL of water will be enough to dissolve the solid benzoic acid.

8. Add 2 drops of phenolphthalein indicator to the Erlenmeyer flask.

9. Coarse Titration:

(a) Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50 mL of the prepared sodium hydroxide solution. Record the initial burette reading.

(b) Place the Erlenmeyer flask on the lower half of the burette — this will connect them.

(c) Perform a coarse titration, adding large increments of the sodium hydroxide solution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you add the sodium hydroxide solution, check the volume remaining in the burette. As the sodium hydroxide is added to thebenzoic acid (C7H6O2) solution the pH increases. Watch for a gradual change of the phenolphthalein color in the Erlenmeyer flask. The pink color will appear in Erlenmeyer flask when the endpoint is either reached or crossed.

(d) Record both the last volume where the solution was colorless and the first volume where the solution was pink. This gives you the range within which to do the fine titration.

(e) Place the Erlenmeyer flask and burette in the recycling bin underneath the workbench.

10. Fine Titration:

(a) Set up the tiration as before: (i) An Erlenmeyer flask filled with 0.244g ofbenzoic acid, 50 mL of water and 2 drops of phenolphthalein.(ii) A burette filled with 50 mL of the prepared sodium hydroxide solution. Record the initial burettereading. (iii) Place the Erlenmeyer flask on the lower half of the burette.

(b) Click and hold the black knob of the burette to quickly add enough standard sodium hydroxide solution to just get into the range of the coarse titration: 1 mL BEFORE the color change. This is near, but not yet at, the titration's endpoint.

(c) Add sodium hydroxide solution in small increments, down to one drop at a time. Record the pH and volume until several drops after the endpoint is reached.

(d) Place the Erlenmeyer flask and burette in the recycling bin.

(e) Repeat the fine titration twice more, and record the results in your Lab Notes.

11. With the total volume of sodium hydroxide solution used to titrate the benzoic acid primary standard, calculate the concentration of the sodium hydroxide solution in your Lab Notes (remember to save them.) The rest of the sodium hydroxide solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the three titrations.

Part 2: Using the Secondary Standard to Determine the Concentration of an Acid

1. In an Erlenmeyer flask, add 25 mL of acetic acid unknown (CH3COOH) from Materials and 2 drops of phenolphthalein indicator. Be sure to record the number of the acid unknown in your LabNotes

2. Coarse Titration:

(a) Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50 mL of the standardized sodium hydroxide solution. Record the initial burette reading. Place the Erlenmeyer flask on the lower half of the burette.

(b) Perform a coarse titration, adding large increments of the sodium hydroxide solution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you add the sodium hydroxide solution, check the volume remaining in the burette. As the sodium hydroxide is added to theacetic acid solution the pH increases. Watch for a change of the phenolphthalein color in the Erlenmeyer flask. The pink color will appear in Erlenmeyer flask when the endpoint is either reached or crossed.

(c) Place the Erlenmeyer flask and the burette in the recycling bin.

3. Fine Titration:

(a) Set up the tiration as before: (i) An Erlenmeyer flask filled with 25 mL of unknown #1 concentration ofacetic acid and 2 drops of phenolphthalein indicator.(ii) A burette filled with 50 mL of the standardized sodium hydroxide solution. Record the initial burette reading. (iii) Place the Erlenmeyer flask on the lower half of the burette.

(b) Click and hold the black knob of the burette to quickly add enough standard sodium hydroxide solution to just get into the range of the coarse titration: 1 mL BEFORE the pink endpoint. This is near, but not yet at, the titration's endpoint.

(c) Add sodium hydroxide solution in small increments, down to one drop at a time, record both the pH and volume until the you have added several drops past the pink endpoint.

(d) Place the Erlenmeyer flask and the burette in the recycling bin.

(e) Repeat the fine titration twice more, and record the results in your Lab Notes. Remember to save your notes.

When You Finish Your Experiment

Now that you are done with the procedure for this lab, you need to make sure you understand everything you did.

When you are ready, exit this lab (the Exit Lab button is in the lower left) and go to Assignments. Complete the items indicated in Assignments.

Notes

Falsk 100.3 mL

Beaker 200.6 mL

1st 23.47 mL Burette

9. Flask and Burette

turned pink- 100.3 mL --> 23.42 mL

10. 1st flask --> 50.3 mL --> 23.47 mL burette

1. 23.42 mL --> 50.3 mL

2. 23.42 mL --> 50.3 mL

3. 23.42 mL --> 50.3 mL

11. With the total volume of sodium hydroxide solution used to titrate the benzoic acid primary standard, calculate the concentration of the sodium hydroxide solution in your Lab Notes (remember to save them.) The rest of the sodium hydroxide solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the three titrations.

Part 2

#2

23.47 burette

25.1 flask

23.22 mL - nothing

23.07 mL - pink

25.2 mL flask

#1

23.47 mL --> 25.1 mL

23.32 mL ---> 25.3 mL -- pink

several drops after

21.93 mL --> 27.7 mL


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