Lab 05 Introduction Reactions Pre Lab Problems (answer on separate paper)

Transcription

1 Lab 05 Introduction Among the many types of quantitative chemistry techniques, volumetric analysis is a timehonored classical method. The characteristic feature of volumetric analysis is measuring the volume of a solution that is required to carry a chemical reaction from inception to completion. One way to observe when a reaction reaches completion is to include in the reaction mixture an indicator, a substance whose color changes when the reaction is completed. The occurrence of this completion is called the end point, and the procedure by which the volume is measured is called a titration. If the solution whose volume is being measured is of precisely-known concentration, it is referred to as a standard solution. The ultimate basis for standardization is always a primary standard, which is a substance of known and stable composition and purity. The chemist may dissolve a precise mass of a primary standard in a solvent to make a definite volume of solution, thereby formulating a standard solution, or they may standardize a solution by using it to titrate a definite mass of a primary standard. The data of the titration are used to calculate the precise molarity of the solution, which is then considered standardized. In any case, a standard solution is eventually used to titrate an unknown, and the observed volume of the standard solution required to reach the end point is used to calculate the quantitative composition of the unknown. As you follow the procedures of this experiment, these perhaps abstract concepts will become clear realities to you. (I hope!) Reactions In this experiment, you will learn to perform titrations. You will standardize a solution of a base (NaOH) against a primary standard acid (potassium hydrogen phthalate, KHP, or KHC 8H 4O 4). This standard has a very high molecular weight, and allows you to precisely weigh out a very small number of moles. KHC 8H 4O 4 + NaOH NaKC 8H 4O 4 + H 2O You will then titrate household vinegar with the standard NaOH solution to determine the percent by mass of acetic acid. HC 2H 3O 2 + NaOH Na C 2H 3O 2 + H 2O In addition, you will titrate samples of an unknown HCl solution to determine its molarity. Pre Lab Problems (answer on separate paper) 1. Find the molecular weight of potassium hydrogen phthalate (KHP). 2. A chemist weighed out g KHP, dissolved it in ml of water, and titrated with a NaOH solution of unknown molarity ml of the NaOH solution were required to reach the end point. Calculate (a) the moles of KHP titrated; (b) the moles of NaOH used in the titration; (c) the molarity of the NaOH solution. 3. If g of a primary standard acid required ml of NaOH titrant to reach the end point, what volume of titrant would be required to reach the end point in a second titration using g of the primary standard acid? (This problem can be solved by a simple proportion.) 4. A ml sample of dilute HCl is titrated to the end point with ml of M standard KOH solution:

2 HCl + KOH KCl + H 2O What is the molarity of the acid solution? Materials &Equipment NaOH solution, approximately 1.25M Vinegar Phenolphthalein indicator Distilled water Buret, stand, and clamp Weighing paper or tin Potassium hydrogen phthalate HCl solution of unknown molarity 250 ml Volumetric flask 50 ml Graduated cylinder Metal spatula or scoop 250 ml Erlenmeyer flasks Procedure In preparing for this experiment, be sure to read Appendices A and B, which contain important information about the use of a buret and the procedure for titrating. Bring a calculator with you to the lab. Contamination of your buret and flasks by an acidic or basic material from previous experiments can seriously affect your results. It is therefore advisable that you rinse your glassware with distilled water prior to use. To conserve distilled water, you should first rinse copiously with tap water, and then use several small portions of distilled water from a wash bottle for the final rinsing. 1. Preparation of NaOH solution (a) (b) (c) Rinse a 250 ml volumetric flask with distilled water, but do not dry it. With a graduated cylinder, measure approximately 50 ml of 1.25M NaOH and empty it into the flask. Immediately rinse the cylinder with tap water so that it will not be etched by the fairly concentrated NaOH solution. Fill the flask to the mark with distilled water. It is not necessary to be precise about filling to the mark, because you will determine the molarity of the solution later. Stopper the flask, hold the stopper in with the thumb of one hand, and thoroughly mix the contents by inverting the flask completely several times and moving the bulbous body in a rotary motion. The NaOH solution will be the titrant (the solution measured with a buret) in the titration. Clean and rinse a buret, support the buret with a stand and clamp (be careful that the buret is clamped securely, but not tightly enough to snap the glass), and rinse and fill the buret using about 60 ml of NaOH solution. 2. Standardization of NaOH solution (a) (b) Use a spatula to place 0.4 to 0.5 g of KHP crystals on a piece of clean, dry paper, and record the weight of paper plus acid. Precision is important in this step. Weigh the empty paper after transferring the sample to a clean (not necessarily dry) 250 ml Erlenmeyer flask. The difference in the two weights is the sample weight. Repeat with a second sample, making sure to somehow differentiate between the two. Avoid losses by spills. With distilled water rinse down any crystals on the inside walls of the flasks. Bring the total volume in each flask up to about 20 ml. Add 2 to 3 drops of 2

3 phenolphthalein indicator to each flask. DO THIS PART FIRST SINCE KHP CAN TAKE A LONG TIME TO DISSOLVE. (c) PERFORM THIS TITRATION LAST TO GIVE THE KHP LONGER TO DISSOLVE. Titrate both samples by the procedure described in Appendix B. For each sample, record the initial and final buret readings to 0.01 ml and calculate the titrant volume (the difference between the initial and final readings). Once you have titrated the first sample, you can estimate the titrant volume needed for the second sample by a calculation similar to Pre Lab Problem 3. (d) For each sample, divide the titrant volume by the sample weight to obtain the milliliters of titrant per gram of sample. For accurate results, the two volumes values of milliliters of titrant per gram of sample should agree to within 1%; that is, the ratio of these two values should be between 0.99 and If this is not true, prepare and titrate a third sample (or as many as are necessary to obtain reproducible results). Careful work will prevent you from repeating this part. 3. Titration of vinegar (a) At the front of the lab, you will find a buret containing household vinegar. Into clean 250 ml Erlenmeyer flasks, dispense duplicate samples of 7 to 8 ml vinegar. Record the volume of each sample to the nearest 0.01 ml. (b) Dilute each sample to about 20 ml with distilled water. Add 2 to 3 drops of phenolphthalein indicator to each flask. Titrate. If the values of milliliters of titrant per milliliter of sample do not agree within 1%, repeat with one or more additional samples until you obtain reproducible results. 4. Titration of unknown acid You will be assigned a HCl solution. Titrate duplicate samples by the procedure of Step 3. Repeat with one or more samples if the values of milliliters of titrant per milliliter of HCl do not agree within 1%. 5. Cleanup Since solutions of strong base can etch glass, it is imperative that you empty your buret and volumetric flask and thoroughly rinse them with tap (and distilled) water before you leave. Be sure to let the water flush out the stopcock and the top of the buret. When you are finished, clean your lab area and glassware before being signed out. Calculations Calculate the following to three significant figures. For each type of titration use the results of your best two trials (those which agree by 1%). 1. Calculate the molarity of the NaOH solution. 2. Calculate the molarity and percent by mass of the acetic acid in vinegar. Use a value of g/ml for the density of vinegar for conversions. 3. Calculate the molarity of the unknown HCl solution. Post Lab Questions 1. What effect would spilling a small portion of the KHP in Step 2 (after weighing) have on the calculated molarity of the acetic acid in the vinegar? 2. The minimum legal content of acetic acid in vinegar is 4% by mass. Does the vinegar that you titrated comply? 3

4 Report: Introduction to Acid-Base Titrations Data Vinegar: Name Lab Partner(s) Section Date performed Titration 1 Titration 2 Titration 3 Sample volume ml ml ml (NaOH) titrant delivered ml ml ml Unknown Acid Code: Volume of HCl solution ml ml ml Titrant delivered ml ml ml Standardization of NaOH titrant: Mass of KHP g g g Titrant (NaOH) delivered ml ml ml Calculations (detailed calculations are attached) NaOH M M M M average: M Acetic Acid M in Vinegar M M M (using avg. NaOH M) average: M % by Mass Acetic Acid in Vinegar % (using avg. HOAc M) Unknown HCl Molarity M M M (using avg. NaOH M) average: M 4

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