Acid-Base Titration Lab

Transcription

1 Acid-Base Titration Lab Name Objectives: - To apply knowledge of molarity to properly dilute a concentrated base - To apply knowledge of solution stoichiometry in order to correctly determine the unknown concentration of a strong acid - To measure the ph of a reaction vessel in order to construct a titration curve of volume of titrant vs. ph Directions: In this lab you will perform an experiment that is often employed by chemists called an acid-base titration. The goal of the lab is to correctly identify the concentration of hydrochloric acid by adding a known base titrant and use LoggerPro to construct a titration curve from ph and volume data. The following pre-lab questions will assist you in successfully completing the experiment and answering the post-lab questions. Pre-lab Questions: 1. Construct and balance the double displacement reaction between sulfuric acid and barium hydroxide. 2. What is the volume in ml of 1.0 M Ba(OH) 2 that would you need to dilute in order to end up with 75 ml of 0.50 M Ba(OH) 2? Show your work. 3. If it takes 25 drops of the barium hydroxide to achieve 1.00 ml, what would be the volume of 112 drops of barium hydroxide in ml? 1

2 4. Based on the molarity and volume from the previous question, what would the concentration of the H 2 SO 4 be if its volume was 15 ml? 5. Recall that acids have a low ph (1-6), neutral substances have a neutral ph (~7) and that bases have a high ph (8-14). Initially the reaction vessel only has the acid with unknown concentration. Also, once after reaction has gone to completion (all the acid has reacted with the base) you are adding excess base that cannot react. From this information, construct a sketch of what you think a titration curve would look like for the chemicals inside the reaction vessel: Titration of M Ba(OH) 2 with M H 2 SO 4 14 ph 7 0 Volume of titrant (ml) Explain why your titration curve looks this way: 2

3 Procedure: a) Show all work and record data on a separate page. Goggles must be worn at all times (no exceptions). If you spill any of the chemicals on your skin wash the skin for at least 5 minutes. Wash your hands thoroughly when you are all finished the lab. b) Divide the work load for the following steps in order to work efficiently and finish in time. c) Calculate the volume of 3.0 M NaOH necessary to make ml of 0.50 M NaOH. d) Obtain the volume of NaOH found in the previous step, put the solution in a 250 ml beaker and add water to the base in order to achieve ml. e) Obtain a laptop, LabPro and ph meter and set up LoggerPro for ph measurements. When the ph meter is not being used it should be submerged in a beaker of clean water at all times. When the ph meter is done being used it should be rinsed with water and should be stored in the beaker of clean water until it is dried and packed back into its box. f) Obtain 15.0 ml of the HCl of unknown concentration and place 2 drops of phenolphthalein indicator in it. g) You need to determine the volume of one drop from the plastic dropper pipette. To achieve this, count the number of drops of water it takes to fill 2.0 ml using a 10 ml graduated cylinder. Use this volume of one drop in the upcoming steps. h) To the first portion of HCl add enough droppers filled to the line with 0.50 M NaOH until you see the color change permanently. The reaction is now complete and the resulting products are in a basic environment. i) Using the amount of droppers of NaOH that you titrated the HCl with, determine the volume of NaOH that you added in order to make the color change permanent. This is a rough estimate of the volume of NaOH that you should use in the 2 nd trial. j) Subtract 2 droppers worth (not drops) of volume from your answer to the previous step. This is how much NaOH you should add to the 2 nd trial before switching to a drop wise technique. k) For the subsequent trials add the ph meter and run LoggerPro to record the original ph of the acid before you add any base. The ph meter will be used to show the current ph of the unknown in real-time. 3

4 l) Add the volume of 0.50 M NaOH that you found from Step K to the HCl. Record the volume in ml and ph. m) Add NaOH drop wise until the color change becomes permanent. Remember, this is the real titration so be careful not to overshoot the equivalence point (when the color change occurs)! n) Record the ph right after the color change becomes permanent. After this has been recorded, add about 20 more drops of NaOH and record the ph in the same fashion as Step m. o) Save your ph graph in LoggerPro so that you can overlay the next trials on the same graph. p) Repeat for as many trials as you can in the provided amount of time. q) Print this graph to the GHS 111 Printer and attach it to this packet to be turned in. Post-lab Questions: 1. Construct and balance the double displacement reaction between hydrochloric acid and sodium hydroxide (phase labels should be included). 2. Based on the volume of 0.50 M NaOH that you added to reach the equivalence point, calculate the number of mol of HCl present during the good titrations. 3. Based on your answer from the previous question, calculate the number of moles of HCl present in your original sample. 4

5 4. Based on your answer from the previous question and the fact that you started with 15.0 ml of the HCl solution, calculate the molarity of the original sample of HCl (you may use an average if you like). 5. Look at the titration curve you constructed. How does it compare with your prediction from Pre-lab Question #5? 5