Unit 10 Acid/Base Titration Lab

Transcription

1 Getting Started Name Unit 10 Acid/Base Titration Lab Titration is a technique used to determine the amount (or concentration) of a substance. A solution with a known concentration is used to titrate (react with) another solution with a known volume and an unknown concentration. Some type of indicator is used to signal when all the unknown substance has been reacted. This is commonly called the endpoint of the titration. The most common type of titration is one involving an acid and a base reacting with one another. In this part of the experiment, we will titrate a base with an acid. The unknown base is a sodium hydroxide solution. We will know the volume titrated, but NOT the molarity. We will titrate with 0.10-molar hydrochloric acid (HCl). A good place to start is to examine the ph nature of the solution through the use of ph test paper and the addition of an indicator solution. An indicator is a compound similar to that in litmus strips and ph paper that changes color based on ph. We will use phenolphthalein indicator solution. The indicator has a fuchsia (pink) color if the solution is basic and no color if the solution is acidic. The indicator will allow us to keep track of what is happening in the solution. Goals As you complete this investigation you will: 1. Become familiar with the lab technique of titration. 2. Prepare a standard HCl (aq) solution. 3. Titrate a solution of sodium hydroxide with a strong acid to the endpoint, using an indicator. 4. Determine the molarity of the unknown solution of sodium hydroxide. 5. Report your results in the usual scientific manner. Safety 1) Make sure to wear goggles and aprons during the entire course of the lab, which includes all clean-up time. 2) This lab involves handling glassware, which is breakable. Handle all glass with proper care, making sure to not drop or knock over any pieces. 3) This lab involves handling acid and base solutions, which can splash if dropped. Handle all solutions with proper care, making sure to protect yourself from spills. 4) Should a spill occur, inform your teacher AT ONCE. Materials Ring stand Double buret clamp 50-mL buret 125-mL Erlenmeyer flask distilled water bottle stirring rod 10-mL graduated cylinder small plastic funnel 400-mL beaker (for waste) 100-mL beaker Chemicals Labeled bottle of.100 M hydrochloric acid (HCl) Labeled bottle of sodium hydroxide (NaOH) solution unknown concentration Phenolphthalein indicator solution 1

2 Preparation of the buret assembly (1-2 people) 1) Attach the double buret clamp to the ring stand and collect approximately 75 ml of M HCl in a 100-mL beaker. 2) Attach a 50-mL buret to the buret clamp and rinse it with about ml of HCl solution. Follow the discussion on rinsing and other techniques from class. Ask if you have questions. 3) When rinsed and drained, use a funnel to fill the buret to within 5 ml of the zero mark. Make sure to drain 2-3 ml of solution through the tip before you start in order to rinse the tip and eliminate air bubbles that would give a false volume reading. Read the initial buret volume and record this in your data table (Estimate to nearest hundredths place!). 4) Arrange the buret and flask for the titration. Have a clean stirring rod and filled distilled water bottle handy. Preparation of the base sample (1-2 people) 5) Rinse a 125-mL Erlenmeyer flask with some distilled water. Swirl the water in the flask and discard the water. Don't dry the flask. 6) Record the letter of your assigned NaOH solution. Also record the letter in your lab notebook. 7) Use a dry 10-mL graduated cylinder to measure about 10-mL of NaOH solution. (If it is 9.90 or 9.85, that's OK, just make sure to record the EXACT volume of NaOH solution used by estimating to the hundredths place.) 8) Pour the NaOH solution into the 125-mL flask. Do not spill any base solution. Use 2-5 ml of distilled water to rinse the 10-mL graduated cylinder. Add this water to the 125-mL flask. Do this twice. 9) Add 2-3 drops of the indicator solution, phenolphthalein, to the flask. The indicator should give a pink color in the presence of the basic solution. Rinse the graduated cylinder thoroughly in the sink and set in the drying rack. Set the flask aside in a safe place and continue. Studying the Reaction 1) Start out by testing the ph of your solution. Record an approximate value. 2) Add 3-4 ml of acid from the buret and test the ph again. 3) Continue adding acid to the base solution until the solution changes color, test ph. 4) Add a few more milliliters of acid and test the ph again. 2

3 Unit 10 Acid/Base Titration Lab Titration Procedures cont. II. Titration Steps 1) Prepare a new base sample. 2) Record the initial volume of HCl in the buret. (Estimate to the nearest hundredths place!) 3) Run three to four ml of HCl solution into the flask and swirl the flask. Note that there is a color change very near where the solution goes in, but it quickly disappears, especially when the flask is swirled. You are NOT DONE yet. 4) Continue adding small amounts of HCl (about one ml at a time) and swirling. As the titration proceeds, note that the color changes linger longer and longer. Comment: You are looking for the very first, PERMANENT color change that pervades the entire solution in the flask. Students have misunderstood the color change both of two ways. (1) Upon first addition of HCl solution, they see a color change and stop. This is incorrect. The color change must be permanent and pervade the entire solution. This is why swirling is necessary: to mix the solution and allow things to react. (2) They run in a large amount of HCl and say "Oh look, it changed color. I'm done." This is also incorrect. It is the first change, done with the absolute minimum amount of HCl. We want to catch the EXACT moment of change. This is the equivalence 5) As the color change lingers longer, switch to adding HCl by drops. Stop adding drops when the color change lasts for thirty seconds. Use the distilled water bottle to rinse the inside walls of the flask, since swirling has put unreacted acid up on the walls. If the color changes back, add one more drop of HCl (just to be sure), then stop. Record the ending buret volume in your data table. (Estimate to the nearest hundredths place!) 6) Repeat for a second and third trial. III. Clean-up Steps 7) Pour all solutions into the waste beaker. Rinse the buret with about 20 ml of water at least 8-10 times. Make sure to run some distilled water out the tip of the buret also. Discard rinse water in the sink. Run tap water for 30 seconds to rinse sink. 8) Do not attempt to dry the inside of the buret. Open the valve of the buret and clamp it upside down. Make sure some paper towel is below it to catch any drips. 9) Dry all other equipment that is wet, including the tabletop. Put all materials onto a dry paper towel near the back of the table. 10) Make sure you have ALL the data. Do not depend on your partner being in class the next day. 3

4 Name Date Pd Unit 10: Acid/Base Titration Lab Report Problem Statement: Data Table Title: Volume of analyte, NaOH (ml) record exact volume from graduated cylinder Molarity of standard titrant, HCl Volume of HCl (ml) initial reading of buret Volume of HCl (ml) final reading of buret Volume of HCl (ml) delivered (calculated) Trial #1 Trial #2 Trial #3 Average Calculations & Critical Thinking Complete the following sections ON YOUR OWN PIECE OF PAPER and attach to this paper to complete your lab report. Calculations 1. Determine the molarity of the unknown NaOH solution as follows. (use average values) a. Write the balanced chemical equation for the reaction of hydrochloric acid with sodium hydroxide. b. Determine the average number of moles of HCl that reacted during the titration. c. Use stoichiometry to help you find the number of moles of NaOH that reacted in the titration. d. Calculate the molarity of the unknown NaOH solution. (Remember: M = mol/l or mmol/ml) 4

5 Critical Thinking Questions: 1. Does the amount of water that was added to the sodium hydroxide when rinsing the graduated cylinder affect the outcome of the investigation? (refer to step 7 of the procedure for preparation of base sample) Explain why or why not? 2. Titration is an analytical chemistry technique capable of yielding highly reproducible results, equivalent to +/- 1 drop of titrant. Explain how each of the following parts of the procedure contributes to this precision. a. Rinsing the buret with the acid to be used. b. Adding single drops of titrant (HCl) near the end of the titration. c. Washing down the inside of the reaction flask with water. 3. Explain why the chemical equation is necessary to complete the calculations for molarity of the base. 4. Give two possible sources of error in this lab and explain how they might have affected the results. 5