Experiment 9: Acids and Bases Adapted from: Chemistry, Experimental Foundations, 4th Ed. Laboratory Manual, by Merrill, Parry & Bassow.

Transcription

1 Chem 121 Lab Clark College Experiment 9: Acids and Bases Adapted from: Chemistry, Experimental Foundations, 4th Ed. Laboratory Manual, by Merrill, Parry & Bassow. Content Goals: Increase understanding of the existence of acids and bases in our everyday lives. Make and test a buffered solution. Explain how a buffer works to resist change in ph. Process Goals: Make qualitative observations and comparisons about the nature of solutions. Required Reading: Chapter 9, Smith. Part I: Household Acids and Bases Many common household solutions contain acids and bases. The acidity of a solution can be expressed using the ph scale. Acidic solutions have ph values less than 7, basic solutions have ph values greater than 7, and neutral solutions have a ph value of 7. Acidbase indicators, such as universal indicator, turn different colors in acidic and basic solutions. The universal indicator contains a number of indicators that change color at various values of ph, allowing you to observe a ph value within a unit or so of that which you can obtain using a ph meter. In this experiment, you will use universal indicator solution to determine the ph values of many household substances. Experimental Procedure: 1. Label 9 test tubes with the numbers 19 and place them in a test tube rack. 2. Add about 3 ml of water to test tube #1. Add peasized piece amount of crushed AlkaSeltzer tablet to tube. Wait until fizzing has ceased and most of the solid has dissolved (a little shaking may be required) 3. Add about 3 ml of vinegar into test tube #2. Name: Partner: Instructor initials: 4. Refer to the data table on the next page and fill test tubes #3 #9 with the respective solution (you need about one inch of solution in each tube). 5. Add two drops of universal indicator to each solution and record the colors that are produced in the table on the next page. 6. Determine the approximate ph of each solution by comparing the color produced in each test tube with the color and matching ph value listed on the cork stoppers of the standard solutions found in the test tube rack. Acids and Bases S11 SB & KB Page 1 of 5

2 Waste Disposal: Pour your Household Solutions into the labeled waste container Part I: Exploration of ph Test Tube Solution 1 Alkaseltzer Universal Indicator Color ph 2 vinegar 3 ammonia 4 lemon juice 5 soft drink 6 drain cleaner 7 detergent 8 baby shampoo 9 baking soda Questions 1. What color is universal indicator in acids? In bases? 2. According to the results of your tests with universal indicator solution what household chemical was most acidic? What household chemical was most basic? Acids and Bases S11 SB & KB Page 2 of 5

3 Part II: Exploration of Buffers An interesting result occurs when you mix a weak acid and its conjugate weak base together in solution: the solution acts like a buffer, resisting changes in solution ph. More specifically, the buffered solution of a weak acid and its conjugate base work together in an equilibrium reaction to counteract the addition or removal of free hydrogen ions (H + ) in the solution. In Part II, the buffer system we will examine is with acetic acid and sodium acetate, shown below: To explore the behavior of this buffer system, we will compare ph changes by adding strong acid and strong base to acetic acid and to a prepared buffer from acetic acid. We will use vinegar as our source of acetic acid (note: vinegar is a 5% (m/v) preparation of acetic acid in water). Procedure 1. Obtain 4 beakers, 2x100mL and 2x150mL. Label the 100mL beakers Acid A and Acid B. For the 150mL beakers, label one Buffer A and the other Buffer B. 2. Obtain 25mL of vinegar and pour it into the 100mL beaker marked Acid A. Repeat this for Acid B. Both beakers should now have 25mL of vinegar 3. Obtain 50mL of vinegar and pour it into the 150mL beaker marked Buffer A. 4. Weigh out about 5.7 grams of sodium acetate trihydrate (NaC 2 H 3 O 2 3H 2 O) on the balance. Add the solid to the beaker marked Buffer A. Note: Your lab MAY be using sodium acetate (NaC 2 H 3 O 2 ), NOT sodium acetate trihydrate. Check the bottle! If this is the case, use 3.4g instead! Label on bottle: MW given on bottle: 5. After the solid has dissolved, pour 25 ml of the solution from Buffer A into the beaker marked Buffer B. 6. Add six drops of universal indicator to each of the four beakers and record the color and ph in the table on the next page. 7. Add 20 drops of 3M HCl to the beakers marked as A and record the color and ph in the table on the next page. 8. Add 20 drops of 3M NaOH to the beakers marked as B and record the color and ph in the table on p Pour your waste into the labeled container in the hood. Acid A 25mL Vinegar Acid B 10. Continue to the next page and answer the followup questions. Buffer A 25mL Acetic Acid Buffer Buffer B 20 drops HCl 20 drops NaOH Acids and Bases S11 SB & KB Page 3 of 5

4 Results, Part II Added HCl AcidA BufferA Initial Color Initial ph Number of drops of HCl added 20 drops 20 drops Color after HCl added ph after HCl added Results, Part II Added NaOH AcidB BufferB Initial Color Initial ph Number of drops of NaOH added 20 drops 20 drops Color after NaOH added ph after NaOH added Questions 1. Which solution changed ph more readily upon addition of the strong acid, HCl? Did the ph go up or down? 2. Which solution changed ph more readily upon addition of the strong base, NaOH? Did the ph go up or down? 3. Comment on the differences in behavior between the buffered solution and the unbuffered solution. Acids and Bases S11 SB & KB Page 4 of 5

5 4. Which chemical species in the acetic acid buffer equilibrium below reacts when H + is added from HCl? Which direction does the equilibrium shift? 5. Which chemical species in the acetic acid buffer equilibrium below reacts when H + is removed by adding NaOH? Which direction does the equilibrium shift? 6. Commercial vinegar is about 5.0%(m/v) acetic acid. How many grams of acetic acid, HC 2 H 3 O 2, were contained in 50 ml of vinegar used to prepare you buffer? 7. Calculate the number of moles of acetic acid in 50mL of vinegar. 8. Based on the mass of sodium acetate you used, how many moles of sodium acetate were used to make your buffer? (Note: Use the molecular mass of the material you used in lab see step #4!) 9. Buffers are made by using equal molar amounts of a weak acid and its conjugate base. Was your buffer prepared correctly? Compare your results from #2 and #3 above. 10. Read about the blood buffer system in your textbook and explain how blood is buffered. Acids and Bases S11 SB & KB Page 5 of 5