Titrating Acid In this lab you will first determine the concentration of sodium hydroxide in a stock solution that you prepare. You will then use that stock sodium hydroxide solution to titrate a solution of acetic acid, enabling you to determine the concentration of acetic acid in that unknown solution. Equipment You will need a ring stand with a burette clamp, a rubber bulb, your wash bottle full of D.I. water, and your goggles. You will also need one 50 ml burette, a 400 ml beaker, a 10.00 ml volumetric pipette, and enough 250 ml or 125 ml Erlenmeyer flasks so that combined with the ones in your locker you have three Erlenmeyer flasks to work with. Chemicals You will need about 10 ml of the 6 M NaOH solution and about 3.5 grams of KHP. You will need your unknown. Safety Be careful not to get the acid or base in your eyes. If you get the acid or base on your skin wash it off with soap and water. Wear you goggles at all times. All solutions may go down the drain. If you discard any 6M NaOH dilute it with tap water before pouring it down the drain. Introduction In part A of this experiment you will make a solution of sodium hydroxide, your stock solution, by diluting some approximately 6M sodium hydroxide solution. You will then determine the concentration of sodium hydroxide ([NaOH]) by titrating a known mass of an acid, potassium hydrogen phthalata, or KHP. The balanced chemical equation is: NaOH(aq) + KHP(aq) KNaP(aq) + H 2 O(l) The important thing is that there is a 1:1 mole ratio between sodium NAME: 1 of 12
hydroxide and KHP. You will use an indicator (phenolphthalein) in the KHP solution. The purpose of the phenolphthalein is to let you know when you've added just enough NaOH to react with all of the KHP in the solution in your Erlenmeyer flask. Phenolphthalein is clear when there is acid present (such as KHP). When all of the acid is used up the phenolphthalein turns the solution pink. Because you will weigh out the KHP, knowing the molar mass of KHP allows you to calculate the moles of KHP. Because the mole to mole ratio between NaOH and KHP is 1:1, at the endpoint (when the solution first turns pink) the moles of KHP = moles of NaOH, so you know how many moles of NaOH you added. Because you used the burette to measure how much of your stock sodium hydroxide solution you added, you now know the volume and moles of NaOH. You will titrate your stock solution three times. You will calculate the molarity of NaOH in your stock solution from each of these three trials and take the average. The average molarity of sodium hydroxide in your stock solution, [NaOH] avg, is what you will use for the calculations in part B. In part B of this experiment you will determine the concentration of acetic acid, HC 2, in your unknown. To do this you will use a volumetric pipette to measure 10.00 ml of your unknown into an Erlenmeyer flask, add some water and phenolphthalein, and titrate your unknown with your stock solution of sodium hydroxide. The equation is: NaOH(aq) + HC 2 (aq) NaC 2 (aq) + H 2 O(l) This reaction also has a 1:1 mole ratio between the base (NaOH) and the acid (HC 2 ). At the endpoint the moles of NaOH that you added from your burette is equal to the moles of acetic acid in your unknown. You can now find the [HC 2 ] in your unknown by dividing the moles of acetic acid by the volume. The volume of your unknown is 10.00 ml, or 0.01000 L. NAME: 2 of 12
PART A Procedure 1.) Rinse your 400 ml beaker with D.I. water three times. 2.) Transfer about 10 ml of 6 M NaOH solution to your 400 ml beaker. Add about 190 ml of D.I. water to the flask. Swirl the solution to mix it. This is your stock solution. 3.) Label your 3 Erlenmeyer flask as #1, #2, and #3. 4.) Weigh between 1 and 1.2 grams of KHP into each Erlenmeyer flask, taring each flask first. Record the mass of KHP in each flask to 3 places past the decimal in the data section (A1), (A2), & (A3). 5.) Add about 50 ml of D.I. water (use your graduated cylinder) to all three 250 ml flasks. Swirl to dissolve the KHP. 6.) ADD 3 4 DROPS OF PHENOLPHTHALEIN TO EACH ERLENMEYER FLASK! 7.) Rinse your 50 ml burette three times with D.I. water. 8.) Rinse your 50 ml burette with a few milliliters of your stock solution. 9.) Fill the 50 ml burette with your stock solution. Make sure that you remove all air bubbles from the tip and that the level is below the 0.00 ml mark at the top. 10.) Record the initial volume of stock solution in the burette in the data section (A4). 11.) Titrate the KHP solution in flask #1 with the stock solution in the burette to its endpoint. This means carefully add the stock solution from the burette to the flask, a little at a time, swirling the flask after each addition. You will reach a point where the KHP solution will turn pink briefly then will go back to clear after swirling. At this point, add the stock solution one drop at a time, swirling after each addition. NAME: 3 of 12
Stop when the first drop of stock solution turns the KHP solution pink and it does not turn clear with swirling after 30 seconds. Record the final volume of stock solution in the burette in the data section (A5). 12.) Refill your burette with stock solution. 13.) Record the initial volume of stock solution in the burette in the data section (A6). 14.) Titrate the KHP solution in flask #2 with the stock solution in the burette to its endpoint. 15.) Record the final volume of stock solution in the burette in the data section (A7). 16.) Refill your burette with stock solution. 17.) Record the initial volume of stock solution in the burette in the data section (A8). 18.) Titrate the KHP solution in flask #3 with the stock solution in the burette to its endpoint. 19.) Record the final volume of stock solution in the burette in the data section (A9). Part B 20.) Empty the three 250 ml Erlenmeyer flasks down the drain. Rinse each three times with D.I. water. 21.) Use the volumetric pipette to transfer 10.00 ml of your unknown solution into each flask. 22.) Add about 50 ml of D.I. water to each flask. 23.) ADD 3 4 DROPS OF PHENOLPHTHALEIN TO EACH ERLENMEYER FLASK! 24.) Refill your burette with stock solution. NAME: 4 of 12
25.) Record the initial volume of stock solution in the burette in the data section (B1). 26.) Titrate the unknown acetic acid solution in flask #1 with the stock solution in the burette to its endpoint. 27.) Record the final volume of stock solution in the burette in the data section (B2). 28.) Refill your burette with stock solution. 29.) Record the initial volume of stock solution in the burette in the data section (B3). 30.) Titrate the unknown acetic acid solution in flask #2 with the stock solution in the burette to its endpoint. 31.) Record the final volume of stock solution in the burette in the data section (B4). 32.) Refill your burette with stock solution. 33.) Record the initial volume of stock solution in the burette in the data section (B5). 34.) Titrate the unknown acetic acid solution in flask #3 with the stock solution in the burette to its endpoint. 35.) Record the final volume of stock solution in the burette in the data section (B6). NAME: 5 of 12
DATA Unknown Number STOCK SOLUTION PART A Mass of KHP in Erlenmeyer flask #1 (A1) Mass of KHP in Erlenmeyer flask #2 (A2) Mass of KHP in Erlenmeyer flask #3 (A3) Initial burette reading trial #1 (A4) Final burette reading trial #1 (A5) Initial burette reading trial #2 (A6) Final burette reading trial #2 (A7) Initial burette reading trial #3 (A8) Final burette reading trial #3 (A9) UNKNOWN SOLUTION PART B Initial burette reading trial #1 (B1) Final burette reading trial #1 (B2) Initial burette reading trial #2 (B3) Final burette reading trial #2 (B4) Initial burette reading trial #3 (B5) Final burette reading trial #3 (B6) NAME: 6 of 12
Analysis Show all work including significant figures and units. Record your answer in the blank provided to the correct number of significant figures with the correct units. PART A 1.) Calculate the moles of KHP in flask #1. moles KHP = grams KHP Molar Mass of KHP = (A1) 204.22 g/mol Moles KHP in flask #1 (A10) 2.) Calculate the moles of KHP in flask #2. moles KHP = grams KHP Molar Mass of KHP = (A2) 204.22 g/mol Moles KHP in flask #2 (A11) 3.) Calculate the moles of KHP in flask #3. moles KHP = grams KHP Molar Mass of KHP = (A3) 204.22 g/mol Moles KHP in flask #3 (A12) NAME: 7 of 12
4.) Calculate the volume, in liters, of NaOH required to titrate the KHP in flask #1. Volume of NaOH = (A5) (A4) 1000 ml/l Volume of NaOH needed to titrate KHP in flask #1 (A13) 5.) Calculate the volume, in liters, of NaOH required to titrate the KHP in flask #2. Volume of NaOH = (A7) (A6) 1000 ml/l Volume of NaOH needed to titrate KHP in flask #2 (A14) 6.) Calculate the volume, in liters, of NaOH required to titrate the KHP in flask #3. Volume of NaOH = (A9) (A8) 1000 ml/l Volume of NaOH needed to titrate KHP in flask #1 (A15) 7.) Calculate the [NaOH] in your stock solution from the titration of flask #1. moles of NaOH [NaOH]= liters of solution = (A10) (A13) [NaOH] 1 (A16) NAME: 8 of 12
8.) Calculate the [NaOH] in your stock solution from the titration of flask #2. moles of NaOH [NaOH]= liters of solution = (A11) (A14) [NaOH] 2 (A17) 9.) Calculate the [NaOH] in your stock solution from the titration of flask #3. moles of NaOH [NaOH]= liters of solution = (A12) (A15) 10.) Calculate the average [NaOH]. [NaOH] 3 (A18) Average [NaOH] = (A16) + (A17) + (A18) 3 Average [NaoH] in stock solution (A19) NAME: 9 of 12
PART B 11.) Calculate the volume, in liters, of NaOH required to titrate the unknown in flask #1. Volume of NaOH = (B2) (B1) 1000 ml/l Volume of NaOH needed to titrate unknown in flask #1 (B7) 12.) Calculate the volume, in liters, of NaOH required to titrate the unknown in flask #2. Volume of NaOH = (B4) (B3) 1000 ml/l Volume of NaOH needed to titrate unknown in flask #2 (B8) 13.) Calculate the volume, in liters, of NaOH required to titrate the unknown in flask #3. Volume of NaOH = (B6) (B5) 1000 ml/l Volume of NaOH needed to titrate unknown in flask #3 (B9) NAME: 10 of 12
14.) Calculate the moles of HC 2 in your unknown in flask #1. moles HC 2 = moles NaOH = Volume NaOH Molarity NaOH =(B7) (A19) Moles HC 2 (B10) 15.) Calculate the moles of HC 2 in your unknown in flask #2. moles HC 2 = moles NaOH = Volume NaOH Molarity NaOH =(B8) (A19) Moles HC 2 (B11) 16.) Calculate the moles of HC 2 in your unknown in flask #3. moles HC 2 = moles NaOH = Volume NaOH Molarity NaOH =(B9) (A19) Moles HC 2 (B12) 17.) Calculate the [HC 2 ] in your unknown solution from the titration of flask #1. [HC 2 ]= moles of HC 2 liters of solution = (B10) 0.01000 L [HC 2 ] 1 (B13) NAME: 11 of 12
18.) Calculate the [HC 2 ] in your unknown solution from the titration of flask #2. [HC 2 ]= moles of HC 2 liters of solution = (B11) 0.01000 L [HC 2 ] 2 (B14) 19.) Calculate the [HC 2 ] in your unknown solution from the titration of flask #3. [HC 2 ]= moles of HC 2 liters of solution = (B12) 0.01000 L [HC 2 ] 3 (B15) 20.) Calculate the average concentration of acetic acid in your unknown. Average [ HC 2 ] = (B13) + (B14) + (B15) 3 Average [HC 2 ] (B16) TURN IN PAGES 6 12 ONLY! NAME: 12 of 12