Experiment 4, Calculation of Molarity of H 3 PO 4 by Titration with NaOH Chemistry 201, Wright College, Department of Physical Science and Engineering

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1 Name Date Experiment 4, Calculation of Molarity of H 3 PO 4 by Titration with NaOH Chemistry 201, Wright College, Department of Physical Science and Engineering Molarity is a common unit within the chemical laboratory to denote concentration of a solution. The units of molarity are moles of solute per liter of solution, mol/l, or commonly, M. In this experiment, you will determine the concentration (molarity) of an unknown phosphoric acid solution, H 3 PO 4. The concentration of the phosphoric acid solution will be determined via titration using NaOH with a known molarity. The balanced chemical equation will follow, H 3 PO 4 (aq) + NaOH (aq) > NaH 2 PO 4 (aq) + H 2 O (l) At the end point, the moles of H 3 PO 4 will equal the moles of NaOH. You will use Bromocresol Green as the indicator. Bromocresol Green is yellow colored in acidic solution and is blue in basic solution. The end point of the reaction will be a green solution. PRELAB ASSIGNMENT 1. What are the units of molarity? 2. What is the acid used in today s titration? 3. What is the base used in today s titration? 4. What is the purpose of today s experiment? 5. Which indicator will be used in this experiment? 6. What color is the indicator in H 3 PO 4 solution? 7. What color will the indicator be at the end point of the reaction? 1

2 PROCEDURE All waste can be put down the sink. 1. Obtain two burets and label them with a wax crayon. One burette will hold the phosphoric acid (H 3 PO 4 ) solution and the other burette will hold the sodium hydroxide (NaOH) solution. Be very careful with the burets, they are very expensive. 2. To prepare the H 3 PO 4 burette for titration: obtain about 200 ml of H 3 PO 4 solution. Use about 25 ml of the H 3 PO 4 solution to rinse out the H 3 PO 4 burette. Don t forget to release some of the H 3 PO 4 solution out the bottom of the burette. This is waste, put it down the sink. 3. To prepare the NaOH burette for titration: obtain about 200 ml of NaOH solution. Use about 25 ml of the NaOH solution to rinse out the NaOH burette. Don t forget to release some of the NaOH solution out the bottom of the burette. This is waste, put it down the sink. Record the molarity of the NaOH solution (1). 4. Fill each burette up close to the zero line with the respective solutions and record the initial volumes (2 & 5). Remember to read the burette to 0.01 ml. 5. Deliver between 10 and 15 ml of H 3 PO 4 solution into a clean 250 ml Erlenmeyer flask. Read the H 3 PO 4 buret to the 0.01 ml and record the final volume (3). Calculate the volume of H 3 PO 4 in the Erlenmeyer flask by subtracting the final minus initial H 3 PO 4 volumes (4). 6. Add about 25 ml of deionized water and 3 drops of Bromocresol Green indicator to the Erlenmeyer flask. Swirl the Erlenmeyer flask to make sure all components are mixed. 2

3 7. Place a piece of white paper under the NaOH burette. This will be used to observe colors and color changes. Bromocresol Green indicator appears yellow in acidic solution and appears blue in basic solution. The end point for this titration is a green color (right between yellow and blue). 8. Start adding NaOH to the Erlenmeyer flask containing the H 3 PO 4 solution. The solution starts out as yellow (due to the phosphoric acid), but you should start to see a blue color as the base (NaOH) is added. Start slowing down how fast you are adding the NaOH, as you don t want to overshoot the green endpoint. Add the NaOH in small increments, or even, dropwise, when you are close to the endpoint. Stop adding NaOH when the color of the solution is green. 9. When you have reached the endpoint, read the burette to the 0.01 ml and record the final volume of NaOH (6). Calculate the volume of NaOH in the Erlenmeyer flask by subtracting the final minus initial NaOH volumes (7). 10. The amount of H 3 PO 4 in the solution is related to the amount of NaOH necessary to reach the endpoint. Convert the ml of NaOH used to L of NaOH (8). Show your calculation here. 11. Use the molarity of the NaOH to convert L of NaOH to moles of NaOH (9). Show your calculation here. 12. Calculate the moles of H 3 PO 4 that reacted (10). Consult the coefficients in the balance chemical reaction to obtain the mole ratio. Show your calculation here. 13. Convert the ml of H 3 PO 4 to L of H 3 PO 4 (11). Show your calculation here. 14. Finally, calculate the molarity of the H 3 PO 4 solution by dividing the moles of H 3 PO 4 by the liters of H 3 PO 4 (12). Show your calculation here. 15. Refill the burettes with their solutions and repeat the titration 2 more times. 3

4 Calculation of molarity of H 3 PO 4 Report Sheet 1. Molarity of the NaOH solution mol/l Trial 1 Trial 2 Trial 3 2. Initial H 3 PO 4 volume ml ml ml 3. Final H 3 PO 4 volume ml ml ml 4. Volume of H 3 PO 4 added to flask ml ml ml 5. Initial NaOH volume ml ml ml 6. Final NaOH volume ml ml ml 7. NaOH volume used for titration to reach green end point ml ml ml 8. NaOH volume used for titration L L L 9. Moles of NaOH used for titration mol mol mol 10. Moles of H 3 PO 4 that reacted mol mol mol 11. Volume of H 3 PO 4 added to flask L L L 12. Molarity of H 3 PO 4 mol/l mol/l mol/l 13. Average molarity of H 3 PO 4 mol/l 4

5 QUESTIONS (Show your work) 1. Calculate the molarity of the solution prepared from 0.78 g CuSO 4 in 350. ml of solution. 2. According to the reaction below, how many grams of precipitate will be produced from ml of M CaCl 2 solution? Assume excess Na 3 PO 4. 3 CaCl 2 (aq) + 2 Na 3 PO 4 (aq) > Ca 3 (PO 4 ) 2 (s) + 6 NaCl (aq) 5

6 3. According to the equation below, how many milliliters of 0.25 M Na 3 PO 4 will react with ml of 0.57 M AgNO 3? Na 3 PO 4 (aq) + 3 AgNO 3 (aq) > Ag 3 PO 4 (s) + 3 NaNO 3 (aq) 4. Using the following reaction, H 2 SO 4 (aq) + 2 NaOH (aq) Na 2 SO 4 (aq) + 2 H 2 O (l) calculate the molarity of the H 2 SO 4 solution if ml of NaOH was necessary to reach the endpoint of a titration. The molarity of the NaOH solution was M and ml of H 2 SO 4 was added to the Erlenmeyer flask. Updated MV 6