Titrations

Cynthia-Grace Clarkson

Date Completed: June 13, 2013

Date Submitted: June 17, 2013

Lab Partners: Matthew and Akshay

Abstract In this experiment, I performed a series of steps in order to calculate the concentration of an HCl solution, the mass percent of OH in a solid, and the mass percentage of analytes such as lemonade powder. In order to do so, we completed steps of titrations, experiments that involve adding a solution from a buret into a specific volume of a solution containing indicator drops until a color change is present. After doing so, I found that the molar concentration of HCl is .09568 M, the mass percent of OH in unknown A is 40.52% and 25.48% in unknown B, and the average mass percent of analyte with the lemonade was 5.85% and 33.64% with the clinitest sample.

Procedure Part I of our experiment was to determine the concentration of an unknown HCl solution. First, I calculated the mass of NaOH necessary to prepare 1.000 L of a .1000 M solution, which was 4.001 grams of NaOH. Wearing gloves, I placed a weigh-boat on the analytical balance and pressed zero, then transferred approximately 4.001 grams of NaOH to the weigh-boat. We made sure to record the exact mass to four decimal places in our notebook. We then transferred the NaOH to a 2 L volumetric flask, adding about 1 L of distilled water and swirling until dissolved. We added distilled water to the mark, and labeled the flask with the concentration to 4 decimal places. Using my 25 mL pipet, I put 25.00 mL of HCL solution to a 125 mL Erlenmeyer flask and added 5 drops of phenolphthalein indicator to it. We then continued this part of the experiment by setting up a titration station. We placed an Erlenmeyer flask on a piece of paper, and slowly added the NaOH to the flask while swirling, watching for traces of pink in the solution. When the pink color persisted for 15 seconds, we recorded the final volume of NaOH, which told us the volume of NaOH that was added to the flask. We calculated the number of moles of HCl present in the flask and then the molar concentration of HCl. We repeated these titration steps 3 times, and recorded the average concentration. Lastly, we made sure to save our NaOH flask to use in Part III. Part II of our titration experiment involved determining the mass percent of OH in an unknown solid. First, we filled a buret with HCL solution and recorded its initial volume. My partner and I each selected a different unknown. Using gloves, I obtained 3 clean 125 mL Erlenmeyer flasks. I calibrated a weigh-boat to zero and transferred a single pellet of unknown to the boat and recorded the exact mass. Then, I moved the pellet to a clean flask and repeated this process with the other 2 flasks. I added approximately 50 mL of distilled water to the first flask using a graduated cylinder and set up a titration station.
Like part I, I set up a titration station. I gently swirled the pellet until it was dissolved then added 10 drops of bromcresol green indicator. I titrated the pellet solution until the blue color disappeared and the green persisted. We recorded the final volume of HCl. We did our calculations, and repeated these steps with the other 2 flasks. Finally, we calculated the average mass percent of OH in our unknown.
In part III, we analyzed commercial solids such as lemonade powder; one partner chose powder containing citric acid and the other chose tablets containing NaOH. Each partner filled a buret with either HCL or NaOH. We then performed each titration in triplicate. For the powder, we transferred a little more than .1000 g to each of the flasks. For the tablets, we put an accurately weighed tablet to each flasks. We then dissolved the solid in each flask with approximately 50 mL of distilled water. We added the correct dye, and titrated as before. We repeated this process twice. At the end, we calculated the moles of analyte in the titration flask, the mass of analyte in the titration