Name #: ________________________________________ Assignment #:__________

ACID-BASE TITRATIONLABORATORY EXPERIMENT

Objectives1. Use burets to accurately measure volumes of solution. 2. Recognize the end point of a titration. 3. Determine the molarity of an unknown solution.

IntroductionTitration is a process in which the concentration of a solution is determined by measuring the

volume of that solution needed to react completely with a standard solution of known volume and concentration. The process consists of the gradual addition of the standards solution to a measured quantity of the solution of unknown concentration until the number of moles of hydronium ion, H 3O+, equals the number of moles of hydroxide ion, OH-, The point at which equal numbers of moles of acid and base are present is know as the equivalence point. An indicator is used to signal when the equivalence point is reached. The chosen indicator must chance color very near or at the equivalence point. The point at which an indicator changes color is called the end point of the titration. Phenolphthalein is an appropriate choice for this titration. In acidic solution, phenolphthalein is colorless, and in basic solution, it is pink.

In this experiment, you will be given a standard HCl solution and told what its concentration is. You will carefully measure a volume of it and determine how much of the NaOH solution of unknown molarity is needed to neutralize the acid sample. Using the data you obtain you can calculate the molarity of the NaOH solution.

Safety Always wear safety goggles and a lab apron to protect your eyes and clothing. Do not touch any chemicals. If you get a chemical on your skin or clothing, wash the chemical off

at the sink. Do not taste the chemicals or items used in the laboratory. Call you teacher in the event of a spill. Never put broken glass into a regular waste container. Put any acid or base solution in appropriate waste container.

Materials 0.106M HCl (Acid) Two 50-mL burets One small beaker -to fill burets One Erlenmeyer flask-to collect

solutions One small funnel

Double buret clamp NaOH solution of unknown molarity

(base) Phenolphthalein indicator Ring stand Water wash bottle

Procedure – Part A – Setup

1. Flip the two burets at your station over. Identify the acid buret (A) and base buret (B). If burets are not identified, use masking tape to label them A or B. Place an Erlenmeyer flask underneath the acid buret.

2. Pour about 15 mL of HCl solution into your beaker. Now pour 5 mL of this solution into acid buret with aid of funnel. Allow the solution to drain though the stopcock and into the Erlenmeyer. Repeat this procedure two

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Name #: ________________________________________ Assignment #:__________

more times using the remaining acid in your beaker. (This is called “priming the buret.”) Why do we “prime” the buret?

3. With the stopcock closed, fill the “acid” buret with HCl solution to the top (close to the zero mark, it does not have to be exact!). Remove any bubbles trapped in the stopcock or in the tip of the buret (waste as little solution as possible.)

4. Now take the Erlenmeyer flask that has about 15 mL of acid in it and place under base buret B. Rinse your beaker and funnel with lots of water to remove acid from them.

5. Repeat steps 2 and 3 using the unknown NaOH solution. Place the contents of the Erlenmeyer flask (about 15 mL HCl and 15 mL NaOH) into waste container, then rinse out Erlenmeyer with lots of water. Both burets should now be filled with the appropriate solutions, 0.106 M HCl and the unknown base. You are ready to titrate!!!

Procedure – Part B – Titration Trials

6. In your data table, record the initial reading of each buret, estimating to the nearest 0.05mL. For consistent results, have your eyes level with the top of the liquid each time you read the buret. Always read the scale at the bottom of the meniscus. The buret measures volume dispensed, NOT volume contained.

7. Place Erlenmeyer flask under acid buret. Dispense 8-10 mL of HCl from the buret into the Erlenmeyer flask.

8. Add 2 drops of phenolphthalein solution as an indicator to the Erlenmeyer flask. (The solution should remain clear!)

9. Now, slowly add NaOH solution from the “base” buret to the flask with the 0.106 M HCl in it. As you add the base, gently swirl the solution in the flask. Stop occasionally and wash down the inside surface of the flask using the water bottle.

10. A pink color will appear and quickly disappear as the solutions are mixed. As more and more base is added, the pink color will persist for a longer time before disappearing. This is a sign that you are nearing the end point. Continue to add sodium hydroxide more slowly, until a single drop of base turns the solution a pale pink color that persists for 15–30 seconds.

11. If you overshoot the end point—that is, if you add too much base so the solution turns bright pink—simply add a few drops of acid from the acid buret to turn the solution colorless again. Approach the end point again, adding base drop by drop, until one drop causes the color to change to pale pink.

12. When you are sure that you have achieved the end point, record the final volume reading of each buret. Note: Do not allow the level of the solution in either buret to go below the 50–mL mark. If you do, you will have to discard your sample and begin again!

13. Discard the neutral solution down the drain, rinse the flask with water, and repeat steps 7-12 for a second trial. Try to use the same amount of acid as you did in trial #1. Do not refill the burets, use the final volume reading of the last trial as the initial volume reading for the second trial. Conduct a third trial if time and solutions still remain.

14. Once you are satisfied with your results you can clean up. If you have leftover solution in your burets, simply mix them into Erlenmeyer and dump into waste container. Rinse burets with lots of water and flip them upside down in buret clamps. Rinse remaining glassware with lots of water.

Name #: ________________________________________ Assignment #:__________

ACID-BASE TITRATIONLABORATORY DATA SHEET

Data TableTrial 1 Trial 2 Trial 3

Acid Base Acid Base Acid Base

Buret Reading #1

Buret Reading #2

Volume Used (difference between #1

and #2)ANALYSIS QUESTIONS 1. Write the balanced equation for the reaction between HCl and NaOH.

2. Calculate the concentration of the unknown solution of NaOH.

a. Hint 1: Find the number of mols of HCl used

b. Hint 2: Use the mole ratio (from your balanced equation in question #1) to find mols of NaOH

c. Hint 3: Use mols of NaOH and volume of NaOH to calculate molarity of NaOH.

d. Calculate the average molarity of NaOH. (Calculate molarity of NaOH from each trial and average them)

3) Why is it OK to dump contents from titration trials down the sink?

4) On back of this sheet, write a typical 5 point discussion that reflects your results from this lab.