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July 24, 2014

Homework Help: chemistry

Posted by Zach on Friday, April 15, 2011 at 11:14am.

PLEASE NOTE: Titration requires several steps in order to obtain exact results. The procedures described in this lab assume that you have already done the TITRATION TUTORIAL and are familiar with the technique.

1. Prepare the NaOH solution by dissolving approximately 4g of solid NaOH in 200 mL of water in a 250 ml beaker. Calculate the approximate NaOH concentration:
a. Convert grams of NaOH to moles of NaOH (MW = 40.00, moles = g/MW)
b. Calculate Concentration of NaOH (Molarity = moles/Liter, Remember: 1 Liter = 1000mL) Record this amount.

2. To prepare the primary standard, first calculate and record the expected mass of benzoic acid required to react in stoichiometric proportion with 20 mL of the NaOH solution:
a. From the balanced acid-base reaction,
C7H6O2 + NaOH -> NaC7H5O2 + H2O, you know that the acid to base ratio is 1:1
b. Find moles of NaOH (moles = Molarity x Volume(L))
c. Convert moles of NaOH to moles of C7H6O2 using ratio of 1:1
d. Convert moles of C7H6O2 to grams of C7H6O2. (grams = moles x Molecular Weight, MW = 122.12) Record this mass.

3. Add the calculated mass of benzoic acid to a clean Erlenmeyer flask and dissolve in 50 mL of water. Add 2 drops of phenolphthalein.

4. Fill a 50 mL burette with NaOH and record the initial volume.

5. Connect the Erlenmeyer flask containing the benzoic acid solution to the base of the burette stand. Open the Data window and click the Pushpin icon to keep the data display set to the flask.

6. Titrate the benzoic acid solution by adding NaOH solution in large increments at first and then in small increments. Open the Properties window and click on the buret. Enter the amount of titrate to add, and click either the drop or flow buttons to deliver the desired amount.

7. Once the solution in the Erlenmeyer flask turns pink, you have reached - and probably passed - the endpoint of the titration.

8. Repeat the analysis with another flask of
benzoic acid solution prepared EXACTLY as before. Refill the burette with NaOH solution. This time, add NaOH in large increments until you get NEAR the endpoint of the previous trial.

9. Change to adding drops instead of mL, and add NaOH solution until the endpoint is reached as exactly as possible. You should be able to determine the endpoint to within 0.02 mL.

10. With the total volume of NaOH solution used to titrate the benzoic acid primary standard, calculate the concentration of the NaOH solution.

11. Repeat the precise titration twice more and compare the 3 NaOH concentrations. They should be within 1% of each other.

12. The rest of the NaOH solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the 3 titrations.

Procedure 2 ( click to view assignments for this procedure )

Acetic Acid of Unknown Concentration

1. In an Erlenmeyer flask, add 25 mL of acetic acid of unknown concentration and 2 drops of phenolphthalein indicator.

2. Refill the burette with the NaOH solution (standardized in Procedure 1), and record the initial volume.

3. Titrate the acetic acid sample to the phenolphthalein's color change endpoint. Record the final volume of the NaOH in the buret. Calculate the concentration of the acid.

4. Repeat the titration 2 more times and calculate the acid concentration of each trial.

5. Give the final acid concentration as the average of the 3 results.

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