Experiment:Acid/Base Titration (Preparation & Standardization of NaOH) (Coursework Sample)

EXPERIMENT 7 - Acid/Base Titrations (%H2SO4)
The Experiment
In this experiment you will make up about 500 mL of 0.2 M NaOH solution. You will then standardize your base solution by titrating it with three weighed samples of solid oxalic acid. The standardized NaOH solution will be used to analyze an unknown sulfuric acid solution. Do not throw away the standardized NaOH solution until you have finished these additional titrations.
Experimental Procedure
Part 1. Preparation and Standardization of NaOH solution.
Preparation of NaOH solution: Add 16-17 mL of 6M NaOH to a 500 mL bottle and fill it to the shoulder with distilled water. (The 6M NaOH is not made up with quantitative accuracy, so your diluted solution will be approximately, but not exactly, 0.2 M.) Close the bottle with a rubber stopper and mix very thoroughly by repeatedly inverting and swirling the solution. Label the solution, keep it stoppered, and save for the titrations.
Preparation of the Oxalic acid samples: Weigh a clean, dry 250 mL flask to the nearest 0.0001g on an analytical balance. Now weigh a plastic weighing boat to the nearest 0.01g on the top loading balance. Add about 0.3 to 0.4 g of oxalic acid crystals, H2C2O4. 2H2O, to the weighing boat. Transfer this oxalic acid to the pre-weighed 250 mL flask. Now go back into the analytical balance and determine the weight of the flask plus the oxalic acid to the nearest 0.0001g. At no point should you be pouring oxalic acid near the analytical balances!
Repeat this weighing procedure two more times. Each time, transfer the oxalic acid sample into a clean, dry, pre-weighed 250 mL flask. Make sure you label your flasks so that you know which oxalic acid sample is which!
Standardization of the NaOH solution: Thoroughly clean one buret, using warm water and a little detergent solution. Water should flow freely down the walls of the buret, without forming droplets over the inside surfaces. Rinse the buret with tap water and allow as much of the water as possible to drain out with the buret upside down. Then rinse the buret with a 5 mL portion of your 0.2M NaOH solution, discard the rinsings and drain. Rinse a second time with your base solution and discard the rinsings by letting them run through the tip. Now fill the buret with your NaOH solution. You can remove any air bubbles that may be in the tip of the buret by opening the stopcock and allowing a little of the base solution to run out. Adjust the level of the liquid in the buret to (or slightly below) the zero mark. Remove any drops adhering to the tip by touching the tip to the side of a beaker. Record the initial buret reading on your data sheet. When you read both the initial and final buret readings, estimate the volume to the nearest 0.01 mL. To do this, you need to estimate where the liquid level is in between the smallest division lines on the buret.
Take one of your weighed oxalic acid samples and dissolve it by adding about 50 mL of distilled water.(The actual volume is not critical. Why?) When you are adding the water, be sure to rinse down any crystals that are clinging to the sides of the flask. If it doesn't dissolve readily, try warming the solution slightly. Add two drops of phenolphthalein (a pH indicator) to this oxalic acid solution and you are ready to begin the titration.
Begin to add NaOH solution from the buret to the flask containing the oxalic acid, swirling the solution as the base is added. Add the NaOH solution rapidly at first, until you near the endpoint. You can tell when the endpoint is approaching because the pink color will not disappear as rapidly as you swirl the flask. As you near the endpoint, rinse down the sides of the flask with a little distilled water, and begin to add the NaOH slowly, drop by drop, until the first drop of base that leaves a faint pink color, even after swirling. When you think you have reached the endpoint, rinse down the sides of the flask again and check that the faint pink color persists for about 20 seconds. If the color fades, add one more drop of base solution and check again. Be sure to touch off any drop hanging from the buret tip into your titration flask and rinse it down into your titration mixture. Record the final buret reading on your data sheet. (Note: If you go past the endpoint, you need to weigh out another sample of oxalic acid and start over.)
Fill up your buret with more of your NaOH solution and adjust the level to zero. Repeat the titration procedure with your two other oxalic acid samples.
Calculations: Add up the molecular weight of oxalic acid dihydrate, H2C2O4 . 2H2O (be sure to include the water of hydration in your molecular weight ). From their masses, calculate the number of moles of acid in each of your three oxalic acid samples. From the titration data, calculate the molarity of your NaOH solution. Be sure to include the mole ration between NaOH and oxalic acid.
Part 2. Titration of an Unknown Sulfuric Acid Solution.
Your unknown in this experiment is a dilute solution of H2SO4 which is used as an etching solution during electroplating. Prior to electroplating, a base metal will be dipped into an etching solution so the sulfuric acid can remove any oxide coating on the metal and roughen up the surface. This will allow the new metal to adhere well to the base metal during the electroplating process. The analysis you will do in Part 2 is similar to that done by an analytical chemist working in quality control at an electroplating plant. You will determine the concentration of the sulfuric acid solution to insure that it falls within the concentration range required for proper etching, which is 0.4% to 1.6% H2SO4 To accomplish this analysis you will titrate the unknown sulfuric acid solution with the standardized NaOH you prepared in Part 1.
Titration of the H2SO4 Solution: Give your instructor a clean, dry, stoppered 250 mL Erlenmeyer flask labeled with your name. Your instructor will fill it with an unknown sample of an H2SO4 solution. Rinse, fill, and zero two burets- one containing your H2SO4 unknown and the other containing your standardized NaOH. Be sure to record the initial volume readings for both burets. Deliver approximately 25 mL of the acid solution from the buret into a clean (but not necessarily dry) 250 mL flask. Make sure to record the volume of H2SO4 used. Add two drops of phenolphthalein to the acid solution in the flask and then titrate with your NaOH solution, using the same technique as Part 1, until the first drop that produces a pink color that last for at least 20 seconds. From the volume of NaOH solution needed to reach this endpoint, calculate the molarity of the H2SO4 solution.
Fill and re-zero both your acid and base burets and repeat the titration process. Collect data for a minimum of three titrations.
Determination of the Density of the H2SO4 Solution: You must determine the density of your unknown solution in order convert the molarity into weight percent. To determine the density of the H2SO4 solution weigh a clean, dry, stoppered 250 mL Erlenmeyer flask to the nearest 0.01 g using the top loading balance (not the analytical balance). Using your buret, deliver about 40-50 mL of your unknown H2SO4 solution into the flask, making sure that you record the initial and final buret readings. Stopper the flask and weigh it to the nearest 0.01g using the top loading balance. Calculate the density of the H2SO4 solution from the measured mass and volume.
Calculations: Using the average molarity value of your H2SO4 solution and the density you measured, convert the concentration of the H2SO4 solution from molarity into weight percent (percent by mass). Your answer should fall within the range of 0.4% to 1.6% H2SO4.
EXPERIMENT 7, TITRATIONSNAME _Simon Manukyan
Lab REPORT
Part 1. Standardization of NaOH Solution
DATA Sample # ______2___________

Trial 1

Trial 2

Trial 3

Mass of flask

82.06g

120.28g

142.07g

Mass of flask + oxalic acid

82.37g

120.70g

142.43g

Mass of oxalic acid

0.38g

0.38g

0.36g





NaOH buret, initial reading

4mL

3mL

1mL

NaOH buret, final reading

47mL

36mL

30mL