Experiment 10 Titration v2

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Experiment #10 Volumetric Analysis The Titration of Acids and BasesPurposeThis experiment will allow you to gain practical experience in the preparing standard solutions, using a pipette and a buret, and performing standard titrations. You will use this experience to experimentally determine the concentration of acetic acid in an unknown solution. There are three parts to this experiment, they are 1. Preparation a standard solution of oxalic acid (~0.07 M) 2. Preparation a sodium hydroxide solution (~0.1M) which will be standardized using the standard oxalic acid solution. 3. Determination of the concentration of acetic acid in an unknown vinegar solution. (0.1-0.2M)

IntroductionStandard solutions are solutions with known concentrations, generally to four significant figures. There are two different ways to make a standard solution. We can make a primary or a secondary standard. A primary standard is prepared directly by dissolving a known mass of sample to make a known volume of solution. A secondary standard is prepared by dissolving an approximate amount of sample into a volume of solvent and determining its exact concentration through titration experiments. Primary standards are prepared from compounds that are at least 99.9% pure, have a definite composition, are water soluble, are easily weighed, and do not change composition on contact with air. Oxalic acid dihydrate (H2C2O42H2O) fits these criteria and therefore may be used as a primary standard. Sodium hydroxide absorbs water when it comes into contact with air and therefore it is difficult to obtain a pure, dry sample to weigh. For this reason the sodium hydroxide solution will be titrated with the oxalic acid standard to become a secondary standard. In the first part of this experiment you will prepare a solution of known concentration of oxalic acid. The oxalic acid crystallizes with two water molecules per oxalic acid in the crystalline network. For this reason, we will weigh out an appropriate amount of oxalic acid dehydrate to dissolve in water. The water molecules in the crystal network will become part of the water of solution once it is dissolved. For this reason the molar concentration of oxalic acid dihydrate will be the same as the molar concentration of oxalic acid.

Sample calculation for determining concentration of oxalic acid solution.Data Mass oxalic acid dihydrate Final volume solution Mol oxalic acid dihydrate5.364 g H 2 C 2 O 4 2H 2 O

5.364 g 500.0 mL1 mol H 2 C 2 O 4 2H 2 O = 0.04255 mol H 2 C 2 O 4 2H 2 O 126.07 g H 2 C 2 O 4 2H 2 O.

Molarity of oxalic acid 0.04255 mol H 2 C 2 O 4 2H 2 O 1 mol H 2 C 2 O 4 1000 mL solution = 0.08510 M H 2 C 2 O 4 500 .0 mL solution 1 mol H 2 C 2 O 4 2H 2 O 1 L solution

In order to standardize the sodium hydroxide solution you will perform a titration. Sodium hydroxide reacts with oxalic acid according to the reaction below: H2C2O4(aq) + 2 NaOH(aq) 2 H2O(l) + Na2C2O4(aq) You will measure a 25.00 mL aliquot of the oxalic acid solution into a flask and add an indicator. An An aliquot is an exact indicator is a substance that changes color quantity of a substance when a solution changes from acidic to basic. or solution. The common indicator used for acid base titrations is phenolphthalein. Phenolphthalein is colorless in a solution that is acidic and bright pink in a solution that is basic. In this titration the oxalic acid solution is acidic and therefore phenolphthalein will be colorless. The sodium hydroxide solution will be added drop wise from a buret into the flask containing the oxalic acid and indicator. As the sodium hydroxide is added to the flask it will react with the oxalic acid and be neutralized. At the point where all of the oxalic acid is reacted, the next drop of sodium hydroxide will make the entire solution basic and it will turn pink. At this point you have completed the titration.

Figure 10.1

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Sample calculation for determining molarity of sodium hydroxide standardData Volume oxalic acid solution 25.00 mL Volume sodium hydroxide to titrate 19.43 mL Mole of sodium hydroxide reacted 1 L H 2C2O2 0.0851 mol H 2 C 2 O 2 2 mol NaOH 25 .00 mL H 2 C 2 O 2 1000 mL H 2 C 2 O 2 1 L H 2C 2O 2 1 mol H 2 C 2 O 2 = 0.004255 mol NaOH Molarity of sodium hydroxide0.004255 mol NaOH 1000 mL solution = 0.2190 M NaOH 19 .43 mL solution 1 L solution

In order to determine the concentration of acetic acid in the vinegar solution you will titrate it with the standardized sodium hydroxide solution. The equation for this reaction is HC2H3O2(aq) + NaOH(aq) H2O(l) + NaC2H3O2(aq)

Sample calculation for determining molarity of vinegar solutionData Volume vinegar solution 25.00 mL Volume sodium hydroxide to titrate 22.84 mL Mole of acetic acid reacted 1 L NaOH 0.2190 mol NaOH 1 mol HC 2 H 3 O 2 22.84 mL NaOH 1000 mL NaOH 1 L NaOH 1 mol NaOH = 0.005002 mol HC 2 H 3 O 2 Molarity of acetic acid 0.005002 mol HC 2 H 3 O 2 1000 mL solution = 0.2001 M HC 2 H 3 O 2 25.00 mL solution 1 L solution

In order to get the best precision possible, you should repeat each titration until you get 3 trial that are within 1% of each other. Experiment #10 Titration 3

ProcedureSafety Notes Wear safety glasses at all times. Preparation of Standard Oxalic Acid Solution Carefully weigh a 100 or 150 mL beaker and record its mass. Measure Remember to use a between 2.1 and 2.3 g of pure oxalic acid dehydrate crystals into the milligram balance and beaker and weigh again. Add 30-60 mL of deionized water to the record all masses to the beaker and dissolve the crystals. You may gently heat the solution to nearest 0.001 g. speed up this process. Transfer the solution quantitatively into a clean 250-mL volumetric flask. Rinse the beaker with 15-20 mL of deionized water and pour this solution into the volumetric flask and repeat. This will ensure that all of the oxalic acid is transferred into the volumetric flask. Fill the volumetric flask to within about 2 cm of the mark and allow it to sit for a minute. This will allow any water clinging to the edges of the neck to drain into the flask. Using an eyedropper, fill the flask to the mark with water. Stopper the flask and mix the solution by repeated inversion and swirling. This requires about 30 inversions and takes close to 1 minute. Next you will transfer this solution into one of the clean 500 mL bottles in your drawer. To transfer, first pour a small amount (~10 mL) of the standard solution into the bottle and rinse the inside walls of the bottle. Discard this wash solution. Do this two times to prevent the standard solution from being diluted with any pure water that might have remained in the bottle after washing. Figure 10.2 Calculate the molarity of the oxalic acid solution using the mass of acid used and the volume of the 0.5318 M Oxalic Acid volumetric flask and record the concentration of 7/27/04 Tina your solution on the bottle. You label should look Titration like the one pictured in figure 10.2. Preparation of Sodium Hydroxide Solution Calculate the volume of concentrated sodium hydroxide solution you must use to prepare 500 mL of approximately 0.1-0.15 M sodium hydroxide solution. (Check the label on the reagent bottle to determine its approximate concentration. The concentration will be 6M) Measure an appropriate amount using your graduated cylinder. Try to measure to within 1 mL of the desired amount of reagent. The exact amount is not important because you will be standardizing this solution later. Pour the concentrated base solution into a clean (need not be dry) 500 mL bottle and fill the bottle up to the shoulder with deionized water. Shake the bottle well and label it as above recording the concentration to 1 significant figure.

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Titration 1. Pipette 25.00 mL of oxalic acid solution into a clean but not necessarily dry Erlenmeyer flask. (You may do three samples as you will be doing at least 3 titrations.)

Pipetting a LiquidVolumetric pipets are useful for quick delivery of liquids with greater accuracy and precision than a graduated cylinder. The volumetric pipet delivers a single specific aliquot or volume of liquid. 1. Preparation of the Pipet Obtain a clean pipet from the stockroom. A clean pipet should have no water droplets adhering to its inner wall. Inspect the pipet to ensure it is free of chips or cracks. Transfer the liquid that you intend to pipet from the reagent bottle into a clean, dry beaker; remember never to put a pipet directly into a reagent bottle. Dry the tip of the pipet with a paper towel. Draw 2-5 mL of solution into the pipet using a pipet bulb. This will be used to rinse the inner walls of the pipet with the liquid and ensure that it is not diluted by the water adhering to the pipet walls. Repeat this rinse step one or two more times making sure that you dispose of the rinse liquid. 2. Filling the Pipet Place the pipet tip well below the surface of the liquid. Using a pipet bulb, draw the liquid into the pipet until the level is 2-3 cm above the mark on the pipet. Remove the bulb and quickly cover the top of the pipet with your finger. Wipe any liquid clinging to the outside of the pipet with a dry towel and allow some liquid to drain out of the pipet until the meniscus is at the mark in the pipet. (You may wish to practice this step with deionized water until you are able to control the flow easily with your finger.) 3. Delivery of the Liquid Deliver the liquid to the receiving vessel by releasing the finger from the top of the pipet. Hold the pipet tip to the wall of the receiving vessel to avoid splashing. Do not blow or shake the last remaining bit of liquid out of the pipet. Do not blow any remaining solution out with the pipette bulb, as these pipettes are designed to retain a small amount of liquid in the tip when they have dispensed the appropriate amou