Lab 6 - Titration of Acids and Bases (KHP)-1

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CHEM 1412 Lab – 6 Dr. Pahlavan - 1 - Acid – Base Titration Determination of the Purity of KHP (Potassium Hydrogen Phthalate) Please Watch This Video : https://www.youtube.com/watch?v=sFpFCPTDv2w Purpose: a) Determine the percent of potassium acid phthalate (KHP) in an unknown sample by means of an acid base titration with NaOH. b) To standardize a sodium hydroxide solution with potassium hydrogen phthalate. c) To determine the molar concentration and mass/mass percent concentration of acetic acid in an unknown vinegar solution. d) To gain proficiency in the laboratory technique of titration. INTRODUCTION: In this experiment, a NaOH solution of known concentration will be used to titrate measured masses of a sample containing an unknown amount of potassium hydrogen phthalate (abbreviated KHP but the formula is K H C 8 H 4 O 4 ). KHP and the strong base NaOH undergo an acid/base reaction with the following molecular equation: KHC 8 H 4 O 4 + NaOH → H 2 O + KNaC 8 H 4 O 4 which corresponds to the following net ionic equation? HC 8 H 4 O 4 - + OH - → H 2 O + C 8 H 4 O 4 2- Since there is only one acidic hydrogen in KHP (the one that is shown in boldface in the above formula), an equal number of moles of the strong monoprotic base NaOH and the amphiprotic KHP will react.

CHEM 1412 Lab – 6 Dr. Pahlavan - 2 - (Although we are primarily interested in the acidic properties of KHP in this experiment, since KHP is amphiprotic it can also react with acids, in which case a diprotic acid named phthalic acid, H 2 C 8 H 4 O 4 is produced.) The volume of your NaOH solution needed to react with the KHP will be determined by titration. Phenolphthalein, which changes from colorless to pink when enough NaOH has been added to react with all the KHP present, is used as an indicator for the titration. The number of moles of sodium hydroxide used in each titration reaction will be determined from known molarity and measured volume of the sodium hydroxide solution. The moles NaOH used in the reaction can be converted to moles of KHP used in the reaction, and then the grams of KHP present in the previously weighed sample can be determined. This mass of KHP along with the mass of sample used in the titration allows calculation of the percent KHP in the unknown. (a ) (b ) (c) Figure -1 Apparatus for the Titration of an Acid (KHP) with a Base (NaOH) (a) Read the initial volume of NaOH in the buret (10.45 mL). (b) A flash pink indicates an approaching endpoint. (c) A permanent(light) pink color signals the final endpoint. Read the final volume of NaOH in the burette (40.55 mL). The volume of NaOH used in the titration is: 40.55 mL - 10.45 mL = 30.10 mL

CHEM 1412 Lab – 6 Dr. Pahlavan - 3 - After standardizing the sodium hydroxide solution, we will then determine the KHP concentration in an unknown KHP. A sample of KHP will be titrated with the standardized sodium hydroxide to a phenolphthalein endpoint. The equation for the reaction is; KHP + NaOH ➔ KNaP + H 2 O Example (1) A 0.905 g sample of KHP (204.23 g/mol) is dissolved in water and titrated with 19.90 mL of NaOH solution to a phenolphthalein endpoint. Find the molarity of the NaOH solution. Solution : Referring to the preceding equation for the reaction and applying the rules of stoichiometry, we have M (NaOH) = (g KHP) (1 mol KHP / 204.23 g) (1 mol NaOH / 1 mol KHP) / (V L of NaOH) (0.905 g KHP) (1mol KHP__ ) (1mol___ ) = 0.00443 mol NaOH 204.23 g KHP 1mol KHP The molarity of the NaOH is found as follows: 0.00443 mol NaOH__ x (1000 ml) = 0.223 mol NaOH = 0.223 M NaOH 19.90 ml solution 1 L 1 L solution In this example, the concentration of the standard NaOH solution is 0.223 M . Example (2) The titration of a 10.0 mL vinegar sample requires 29.05 mL of standard 0.223 M NaOH. Calculate the (a) molarity and (b) mass/mass percent concentration of acetic acid. Solution : We can calculate the moles of acetic acid from the moles of NaOH solution: (29.05 ml solution) (0.223 mol NaOH /1000 ml) ( 1 mol HC 2 H 3 O 2 / 1 mol NaOH) = 0.00648 mol (a) The molar concentration of HC 2 H 3 O 2 is (0.00648 mol HC 2 H 3 O 2 /10.0 ml solution) (1000 ml/1 L) = 0.64 M (b) To calculate the m/m % concentration, we must know the density of the vinegar (1.01 g/mL) and the molar mass of acetic acid (60.06 g/mol). (0.648mol HC 2 H 3 0 2 ) x ( 60.06 g_HC 2 H 3 O 2 ) x ( 1 mL solution__ ) x 100 = 3.85% HC 2 H 3 0 2 1000 mL solution 1 mol 1 HC 2 H 3 O 2 1.01 g solution

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CHEM 1412 Lab – 6 Dr. Pahlavan - 4 - Example (3) Assume we used 25 ml of NaOH to reach the endpoint using 0.1000 mol/L NaOH for titration of unknown sample of KHP. Calculate the percentage KHP in the unknown sample mass of 0.9735 g. Solution - The number moles of acid the following formula is used: (0.1000 mol/L NaOH x 0.02500 L NaOH) x (1 mol KHP/1 mol NaOH )= 0.0025 mol KHP To determine the percent KHP in an unknown sample mass of 0.9735 g, the following calculation is used: (0.0025 mol KHP x 204.23 g KHP) / (1 mol KHP x 1/0.9735 g unknown) x 100% = 52.44% KHP Example (4) Assume 0.5472 g of 100% pure KHP is titrated with a NaOH solution of which the molarity is not known. Calculate the molarity of sodium hydroxide solution if 37.42 mL (0.03742 L) of sodium hydroxide were used to titrate the KHP. Solution - The following equation is used to calculate the molarity of the NaOH solution. Therefore: (0.5472 g KHP) ( 1 mol KHP/204.23 g KHP)( 1 mol NaOH/1 mol KHP)( 1/0.03742 L NaOH ) = 0.09771 mol NaOH/1 L NaOH = 0.09771 M NaOH Preparation Prepare a ~0.10 M NaOH solution and store it in a polyethylene bottle (not glass). You will standardize this solution, so the preparation need not be exact. Before use, be sure your NaOH solution is fully dissolved, and well mixed by shaking. Rinse your burette with your NaOH solution, and then fill it, being sure to remove any air bubbles. SAFETY CAUTION NaOH is strongly basic and can damage skin and clothing. Handle it carefully and clean up any spill immediately. Note : Carefully add NaOH solution to the funnel so as to not overfill the buret. Drain some NaOH through the tip of the buret to clear any air bubbles.

CHEM 1412 Lab – 6 Dr. Pahlavan - 5 - Procedure Part I - Standardization of NaOH Label three 250 mL Erlenmeyer flasks #1, #2, and #3. Accurately weigh by difference approximately 0.50-0.70 g of KHP (100%; NOT your unknown) into each of the flasks. Add approximately 50 mL of distilled water to each flask and swirl gently (may require heat) as necessary to dissolve the KHP crystals. Add two drops of phenolphthalein indicator to each flask. Position Erlenmeyer flask #1 under the buret as shown in Figure -1. Record the bottom of the meniscus as the initial buret reading. Titrate the KHP sample to a light pink endpoint. Record the final buret reading. Refill the buret, record the initial burette reading, add two drops of phenolphthalein indicator to flask #2, titrate the KHP sample, and record the final buret reading. Refill the buret, record the initial buret reading, add a drop of phenolphthalein indicator to flask #3, titrate the KHP sample, and record the final buret reading. Calculations 1. Use the mass of pure KHP added to each flask, and the volume of sodium hydroxide solution used in each of the three standardization titrations to calculate the molarity (or Normality) of the sodium hydroxide solution for each trial. calculate the concentration of sodium hydroxide used in each titration Trials). M b (NaOH) = ( g KHP)( 1 mol KHP / 204.23 g) ( 1 mol NaOH / 1 mol KHP) / (V L of NaOH) OR: M b (NaOH) = (g KHP) x (204.23 g/mole) / V b (L NaOH) ➔ Mb(1) , Mb(2), and Mb(3) 2. Calculate the average sodium hydroxide concentration , deviations , and the standard deviation of the results. Record the in the report form. Average Concentration: Mb(ave) = [ (Mb(1) + Mb(2) + Mb(3)] /3 Deviations: Δ Mb(1) = ꟾ Mb(ave) - Mb(1) ꟾ , Δ Mb(2) = ꟾ Mb(ave) - Mb(2) ꟾ , Δ Mb(3) = ꟾ Mb(ave) - Mb(3) ꟾ Average deviations ➔ Δ Mb(ave) = [ Δ Mb(1) + Δ Mb(2) + Δ Mb(3)] /3 Standard Deviation (SD):

CHEM 1412 Lab – 6 Dr. Pahlavan - 6 - Part II - Percentage purity of KHP Repeat the experiment with unknown samples. Weigh by difference approximately 0.50 -0.70 g of unknown into each of three separate clean 250 mL Erlenmeyer flasks. Record all masses to 2 decimal places. Repeat the procedure by adding about 75-100 mL deionized water to each flask, and swirl gently to dissolve the sample. Add two drops of phenolphthalein indicator to each flask and titrate. Record the volume required. Calculations 1. Use the weight of impure KHP (unknown) added to each flask and the average volume of the NaOH solution (from part I) to calculate the percentage purity of the impure sample of KHP. Again, calculate the standard deviation of the results. Grams Weight of KHP ➔ (Wt. KHP) 1 = Mb(ave) . Vb (L) 1 . (204.23 g/mole) (Wt. KHP) 2 = Mb(ave) . Vb (L) 2 . (204.23 g/mole) (Wt. KHP) 3 = Mb(ave) . Vb (L) 3 . (204.23 g/mole) 2. Calculate the average KHP percentage from grams KHP (unknown) , deviations , and the standard deviation of the results. Record the in the report form. Percentage of KHP ➔ % (Wt. KHP) 1 = [(Wt. KHP) 1 ] x100/ g KHP(unknown) % (Wt. KHP) 2 = [(Wt. KHP) 2 ] x100/ g KHP(unknown) % (Wt. KHP) 3 = [(Wt. KHP) 3 ] x100/ g KHP(unknown) Average KHP percentage: % (Wt. KHP) (ave) = [ % (Wt. KHP) 1 + % (Wt. KHP) 2 + % (Wt. KHP) 3 ] /3 KHP Percentage De viations: Δ (%KHP) 1 = [%KHP(ave) - %KHP(1)] Δ (%KHP) 2 = [%KHP(ave) - %KHP(2)] Δ (%KHP) 3 = [%KHP(ave) - %KHP(3)] Standard Deviation (SD):

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CHEM 1412 Lab – 6 Dr. Pahlavan - 7 - Determination of the Purity of KHP REPORT FORM Name: _____________________________ Part I. NaOH Standardization mass of Erlenmeyer flask _______g ________g ________g flask + KHP _______g ________g ________g mass of KHP _______g ________g ________g initial burette reading _______ml ________ml ________ml final burette reading _______ml ________ml ________ml volume NaOH _______ml ________ml ________ml Molarity of NaOH _______M _________M __________M Average molarity of NaOH ____________M Trial # Grams mass unknown (KHP) Volume NaOH (mL) used Molarity of NaOH 1 2 3 Standard Deviation: ________________ (Show your work)

CHEM 1412 Lab – 6 Dr. Pahlavan - 8 - Part II. KHP Percentage determination ( Unknown # __ ) mass of Erlenmeyer flask _______g ________g ________g flask + unknown sample _______g ________g ________g mass of unknown sample _______g ________g ________g initial burette reading _______ml ________ml ________ml final burette reading ______ml ________ml ________ml volume NaOH _______ml ________ml ________ml gr Weight KHP _______g ________g ________g % KHP _______% ________% ________% Average % KHP ______________________ Trial # Sample mass (g) Volume NaOH (ml) Grams mass (KHP) Weight % KHP 1 2 3 Standard Deviation: ______________________ (Show your calculations)

CHEM 1412 Lab – 6 Dr. Pahlavan - 9 - Pre-Laboratory Review Questions and Exercises Due before lab begins. Answer on a separate sheet of paper. Name _______________________________________ 1. What is the difference between a primary and a secondary standard? Give an example of each. 2. Why must air bubbles be expelled from the burette tip? . 3. 0.3043 g of pure KHP was weighed out and titrated to an end point with 15.12 mL of a NaOH solution that was approximately 0.1 M . What is the exact concentration of the NaOH titrant? 4. 0.5366 g of an KHP sample of unknown purity was massed. The sample was dissolved in approximately 100 mL of distilled, degassed water and indicator was added. The end point was reached after 21.35 mL of 0.09854 M NaOH solution was titrated into the solution. What is the percentage of KHP in the original sample? 5. What is the molarity of a NaOH solution if 32.47 mL is required to titrate 0.6013 g of potassium hydrogen phthalate (C 8 H 5 O 4 K)?

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CHEM 1412 Lab – 6 Dr. Pahlavan - 10 - Post- laboratory Questions and Exercises Due after completing lab . Answer in the space provided. Name ___________________________________ 1. Write the formula unit and the net-ionic equations to describe the reaction between potassium hydrogen phthalate (KHP) and cesium hydroxide. Use C 8 H 5 O 4 K as the formula for potassium hydrogen phthalate, C 8 H 5 O 4 - to represent the hydrogen phthalate ion and so forth. 2. A student needs to determine by titration with NaOH the precise % KHP in an unknown sample that is thought to contain approximately 50% KHP. Approximately what mass of sample should the student use in order to use about 20 mL of 0.1005 M NaOH to reach the endpoint of the titration? 3. If the student in problem 2 used 0.847 g sample of the impure KHP and the endpoint was reached after 19.82 mL of the sodium hydroxide solution was added, what is the percent KHP in the unknown sample to four significant figures (Yes, we are abandoning rules of thumb for significant figures for this result)? 4. A 0.5123-g KHP (M.W. 204.23) sample was dissolved in about 25 mL of distilled water, and titrated to the phenolphthalein end point with 28.75 mL of a sodium hydroxide solution. Calculate the molar concentration of the hydroxide solution. 5. 19.80 mL NaOH is required to titrate 0.6013 g of potassium hydrogen phthalate (C 8 H 5 O 4 K). How many ml NaOH required to titrate 25.0 ml 0.10 M HCl solution?

Lab 6 - Titration of Acids and Bases (KHP)-1

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