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Objectives ACID-­‐BASE TITRATION LABORATORY EXPERIMENT 1. Use burets to accurately measure volumes of solution. 2. Recognize the end point of a titration. 3. Determine the molarity of an unknown solution. Introduction Titration 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, H3O+, 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 your teacher in the event of a spill. • Never put broken glass into a regular waste container. • Dispose of all solutions as instructed. Materials • 0.100M HCl – standard solution • Graduated cylinder • Two 50-­‐mL burets & buret clamp • NaOH solution of unknown molarity • One beaker • Phenolphthalein indicator • One Erlenmeyer flask • Ring stand • One funnel • Water wash bottle
Procedure – Part A – Setup 1.
Set up the burets, double buret clamp, and ring stand as shown. 2.
Place a beaker underneath the acid buret. Pour about 15 mL of HCl (acid) into a graduated cylinder. Now pour ~5 mL of this acid into the buret with the aid of the funnel. Allow the solution to drain though the stopcock and into the beaker. Rinse the buret two more times in this manner using the remaining acid. (This is called “priming 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.
Repeat steps 2-­‐4 using the NaOH solution in the “base” buret. 5.
Dispose of the beaker solution in the waste container and wash the beaker with water. 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. Note: The buret measures volume dispensed, NOT volume contained. 7.
Draw out 8-­‐10 mL of HCl from the buret into the Erlenmeyer flask. 8.
Add 2 drops of phenolphthalein solution as an indicator to the flask. (The solution should remain clear.) 9.
Now, slowly add NaOH solution from the “base” buret to the flask. 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. Do not refill the burets, use the final volume reading of the last trial as the initial volume reading for the next trial. Conduct a third trial if time and solutions still remain. Try starting with a volume of NaOH this time and titrate with HCl. 14. Once you are satisfied with your results you can clean up. If you have leftover solution in your burets, simply mix them into the Erlenmeyer flask and dump it into the waste container. Rinse the burets with lots of water and flip them upside down in the buret clamps with the valves open. Rinse all other equipment used with lots of water. ACID-­‐BASE TITRATION 19
DATA TABLE LABORATORY WORKSHEET Trial 1 Acid Buret Starting Volume Buret Final Volume Volume Used (the difference) Trial 2 Trial 3 Base Acid Base Acid Base 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 to find mols of NaOH c. Hint 3: Use mols of NaOH and volume of NaOH to calculate molarity of NaOH. d. Calculate the molarity of NaOH from each trial and average them. 3. Why do we “prime” the burets before we start the lab? 4.
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Why is it OK to dump the contents of the titrations trials down the sink? What does the color change indicate in this lab? Write a summary of what you learned from this lab. What are some possible sources of error in this lab? Watch Mrs. Paige’s Titration video online: http://www.youtube.com/watch?v=SBHGaY3kA4U Now try some more problems using the 4-­‐step math technique. a. What is the molarity of an HCl solution if 5.00mL of the acid solution requires 28.6mL of a 0.145M NaOH solution to reach the endpoint? b. A solution of 0.162M NaOH is used to neutralize 25.0mL of an H2SO4 solution. If 32.8mL of the NaOH solution is required to reach the endpoint, what is the molarity of the H2SO4 solution?