TRATION: ANALYSIS OF VINE

Transcription

1 Experiment 10 Name: 22 Ti TRATION: ANALYSIS OF VINE 31 Ga R In this experiment, you will learn the concept and technique of titration. You will determine the concentration of acetic acid in commercial vinegar in terms of molarity and mass percent. Note that vinegar is a dilute solution of acetic acid in water. Titration is a technique used to determine the concentration of a solution by allowing it to react with a solution of known concentration. For this experiment, you will determine the concentration of acetic acid in vinegar by neutralizing it with a NaOH solution of known concentration. In this case, vinegar is called the analyte and NaOH is called the titrant or the standard. In an acid-base titration, the titrant is dispensed slowly from a buret into a flask containing the analyte and the indicator. Since NaOH will be titrated from the buret, NaOH is the titrant. The indicator signals the reaction completion by changing colors when its environment switches between acidic and basic. In this experiment, phenolphthalein is used as indicator; it is colorless in acid and pink in base. As soon as a change in color is observed (and remains permanent), titration is stopped. The endpoint, the point when the indicator changes color, serves as a visually detectable close approximation to the equivalence point, the point when the amounts of acid and base are stoichiometrically equal. One must ensure that this approximation holds by carefully performing the titration, adding the titrant dropwise while closely watching for the endpoint. The last drop of the titrant added must be the drop that brought about the permanent change in color. The concentration of the analyte is determined from the amount of titrant added required to reach the endpoint. Indicators are chosen based on the ph at which they change color. Different indicators change color at different ph values. For example, phenolphthalein changes color around a ph = 9. Phenolphthalein is used in this experiment because the neutralization of acetic acid by sodium hydroxide occurs around a ph of 9. Analysis of Vinegar Vinegar is an aqueous solution of acetic acid, HC 2H 3O 2. In this experiment, a sample of commercial vinegar is titrated with a standard NaOH solution to phenolphthalein endpoint to determine the molar concentration of acetic acid in vinegar and the mass percent. The chemical equation is: HC 2H 3O 2(aq) + NaOH(aq) NaC 2H 3O 2(aq) + H 2O(l) For example, if a mL vinegar sample is titrated with ml of the M standard NaOH solution, the molarity of HC 2H 3O 2 in the vinegar is calculated as follows: mol NaOH L NaOH soln ( ) ( mol HC 2H 3 O 2 L soln mol NaOH ) = mol HC 2H 3 O 2 Note: from the balanced chemical equation, acetic acid and NaOH are 1:1 mole ratio. 1

2 M = mol HC 2H 3 O L solution = M HC 2 H 3 O 2 solution Mass percent concentration is given by: mass % = mass solute mass solution 100 To convert the concentration in molarity to mass percent, one must consider the volume of the solution used. A ml sample of a M HC 2H 3O 2 solution contains moles of the solute HC 2H 3O 2. Note that M = mol/l mol L HC 2H 3 O 2 ( L)= mol HC 2 H 3 O 2 The number of moles of HC 2H 3O 2 is converted to mass using the molar mass: mol HC 2 H 3 O 2 ( 60.0 g HC 2H 3 O 2 mol HC 2 H 3 O 2 ) = g HC 2 H 3 O 2 The mass of the ml solution is calculated using the density of vinegar, which is 1.01 g/ml: The mass % is: 1.01 g solution ml solution ( ) = g solution ml solution mass % = g HC 2H 3 O g solution 100 = 4.03 % PROCEDURE A. Preparation of the Buret 1. Obtain ~100 ml NaOH solution in a dry, clean beaker. You will use this beaker (and a funnel if you find it necessary), to transfer NaOH solution to the buret. You may refill the beaker with NaOH as needed. 2

3 2. Before using a buret for analysis, it needs to be conditioned to minimize errors due to contaminants by rinsing the inside walls of the buret with the titrant, the NaOH solution. Start with the stopcock of the buret closed, or in horizontal position. Make sure the stopcock on the buret is closed (horizontal) when pouring the NaOH solution into the buret. 3. Fill the buret halfway with the titrant (NaOH solution). You may remove the buret from the stand to fill it. Empty the buret by slowly pouring out the solution into a separate waste beaker while turning the buret so that the solution makes contact with all the inner surface of the buret. 4. Clamp the buret to the stand, making sure it is perfectly vertical. Make sure the stopcock on the buret is closed (horizontal) when pouring the NaOH solution into the buret. Fill with NaOH solution just above the zero mark and place the waste beaker below the buret tip. Open the stopcock to vertical position by turning it 90. Drain some solution to fill the buret tip, from below the stopcock to the very tip, making sure there are no air gaps or bubbles. Note that the stopcock can control how fast the solution flows between the horizontal and vertical positions. The buret is now ready for analysis. At this time, the liquid level should be below the zero mark, within the gradations. You do not need to start with the initial buret reading at zero. Reading the buret. A buret is read at the bottom of the meniscus at eye level. In a buret, the zero mark is at the top and the number markings count up going down towards the tip, as shown in Figure 2a. (Keep in mind that this is opposite to the graduated cylinder where zero is at the bottom.) For example, the liquid level in Figure 1a is between 1 and 2 ml, but closer to 2 ml; the buret reading is 1.88 ml. For practice, read the buret in Figure 1b. (a) (b) reading = 1.88 ml reading = ml Figure 1: How to read a buret 3

4 The readings from a buret are not actual volumes. Buret readings are taken before and after dispensing the titrant during a titration analysis, so the volume of titrant used is the difference between the readings. If Figures 1a and 1b show liquid levels at the beginning and at the end of a titration analysis, respectively. Based on the buret readings shown in Figure 1, the volume of titrant used was30.28 ml. II. Analysis of Vinegar 1. Record the molarity of the NaOH standard used. 2. Record the initial buret reading. 3. Obtain ~50 ml vinegar in a clean beaker. Condition a ml pipet with the vinegar by drawing vinegar above the mark in the pipet then draining it into the waste beaker. (For a complete review of pipet use, refer to the experiment Density.) 4. Using the pipet, transfer ml of vinegar into a clean 125-mL Erlenmeyer flask. Add ~25 ml distilled water into the flask. Add one drop of phenolphthalein. Stir using the magnetic stirring plate at a low speed (avoid splashing of the solution) and the heat if OFF. The volume of the water added to the flask does not need to be known accurately. The calculation will be based on mL volume of vinegar being analyzed. Do not forget to add the indicator. Without the indicator, you cannot see when you reached the endpoint. 4. Open the stopcock to start delivering NaOH to the flask. Initially, the drops can be added quicker, but slow the flow when the pink color begins to appear. When getting close to the endpoint, drops of the NaOH solution will turn pink when they first enter the KHP solution, then quickly turn colorless. Add the NaOH dropwise nearing the endpoint, making sure the pink color disappears before adding the next drop. Record the final buret reading. You are aiming for a light pink, not a bright pink. If your solution is bright pink, you overshot the end point. You can place a white sheet of paper underneath your flask to help determine if your solution is pink or colorless. If you emptied the entire buret of NaOH and there was no color change, you probably forgot to add the phenolphthalein indicator. Repeat the trial. 4

5 Do not allow the liquid level to go below the calibrations at the bottom of the buret. If this is about to occur, record a final reading. Refill the buret, record another initial reading and then another final reading when titration is completed. The total volume of NaOH used is the sum of the volumes used for the first and second sets of readings. 5. Perform two more trials. 6. Rinse the buret with water when all titrations are completed and invert the buret in the clamp. 7. Calculate the acetic acid concentration of vinegar in molarity for each trial. 8. Convert each molar concentration to mass percent. Assume the density of vinegar to be 1.01 g/ml. Report the average mass percent. CLEAN-UP Dispose of wastes in the large jug in the front hood. Wash all glassware used. Rinse the pipet. Rinse the buret with water and drain by clamping it upside down. Return materials where they belong. Return the stir bar on top of the heat/stir plate. 5

6 Name: Partner s Name: Date: TITRATION: ANALYSIS OF VINEGAR Report all your measurements and answers in the correct number of significant figures and units. Molarity of standard NaOH solution = Trial 1 Trial 2 Trial 3 volume of vinegar initial buret reading (NaOH) final buret reading (NaOH) volume of NaOH solution molarity of HC 2H 3O 2 mass % of HC 2H 3O 2 average mass % Calculate the molarity and mass percent of acetic acid in vinegar for the three trials and report the results and the average mass % in the table below. Show the complete calculations for Trial 1 only. Observe correct number of significant figures and units. Trial 1 calculation for molarity of HC 2H 3O 2 in vinegar: 6

7 Trial 1 calculation for mass % of HC 2H 3O 2 in vinegar (Density of vinegar is 1.01 g/ml): POST-LAB EXERCISES Show clearly the complete calculations with correct number of significant figures and units. 1. In the titration analysis, a 10.0-mL sample of household ammonia solution required ml of a M HCl solution (Density of ammonia solution is g/ml). NH 3(aq) + HCl(aq) NH 4Cl(aq) (a) Calculate the molarity of this ammonia solution. 7

8 (b) Calculate the mass % of ammonia in the solution. 8