Titration of Oxalic Acid Lab - Sardis Secondary School

1 Name: Block: Date: Chemistry 11 Experiment 4: Lab Titration with Oxalic acid Abstract: Titration is a versatile analytical procedure that can be used for a wide variety of chemical analyses. For example, when your town s water supply is tested for purity, or pond water is tested for dissolved oxygen and contaminants, chances are a Titration is carried out. Some tests that are essential for a medical diagnosis require a Titration of various body fluids. A Titration makes use of a known reaction between two chemicals. A solution of unknown concentration is reacted with a precisely measured amount of another chemical.

2 An appropriate indicator must be used to determine when chemically equivalent amounts of each chemical are present, that is, when no excess of either reactant is present. This is known as the equivalence point. To measure solution volumes accurately, finely calibrated pipets and burets are used. titrations are commonly used to determine the strength of acids or bases. acid ( base titrations follow a relatively standard procedure for analysis of acid or base strength. The concentration of either an acid or base solution can be determined. A measured amount of acid is neutralized by reacting it with a basic solution titrated from a buret. Consider the following example: HCl + NaOH NaCl + H2O (known) (unknown) The example shows that the reaction is a special case of a double replacement reaction, where the products are water and a corresponding salt.)

3 At the equivalence point, all of the NaOH has been reacted with the HCl and a pH of 7 (a neutral solution) would result. Some reactions (ones with weak acids or weak bases) do not reach their equivalence point at a pH of 7 because of equilibria occurring in the solution. In this experiment, you will determine the concentration of a solution of NaOH by reacting it with a known concentration of Oxalic acid , H2C2O4 a weak acid , which will have an equivalence point at approximately pH 8. The indicator phenolphthalein will be used to find the endpoint of the Titration because it is colourless below pH 8, but turns pink just above pH 8. Purpose: To determine the concentration of a sodium hydroxide solution using the process of Titration .

4 Materials: safety goggles laboratory apron 2 Erlenmeyer flasks, 250mL balance Oxalic acid dihydrate (H2C2O4 < 2H2O) distilled water phenolphthalein wash bottle buret buret clamp ring stand sodium hydroxide solution (NaOH) 2 beakers, 100mL white paper Safety Warnings: Wear safety goggles at all times during the laboratory period. Ensure the top of the buret is below eye level when filling to avoid splashing solution into your face. Acids are corrosive and bases are caustic, so wipe up all spills immediately with a wet paper towel and then dry the area. Wash affected skin areas with cold water and notify the teacher. Burets are cumbersome and break easily handle them with care.

5 Procedure: Part A: Preparation 1. Put on your safety goggles (and lab apron if desired). While one partner prepares the Oxalic acid solution (steps 2 + 3), the other should prepare the buret for Titration (steps 4 9). Each group member should do one Titration ! 2. In a 250 mL Erlenmeyer flask weigh out approximately of Oxalic acid (this will be a very small amount!). Record the exact mass of Oxalic acid used in the data table. Repeat this step in another Erlenmeyer flask for trial 2. 3. Dissolve the Oxalic acid in the flask with approximately 50mL of distilled water. Add 2(3 drops of phenolphthalein indicator to the flask. 4. Obtain a buret and buret funnel. 5. Rinse the buret thoroughly with tap water, then rinse it once with a small amount of distilled water, draining the final rinses through the tip.)

6 Clamp the clean buret to the ring stand. 6. Obtain 50(60 mL of NaOH solution in a 100 mL beaker. Position the buret so that the top is below eye level and ensure it is closed! 7. Pour approximately 5 mL of the NaOH into the buret. Drain this solution through the tip to remove water and coat the inside of the buret with base . 8. Fill the buret to slightly above the zero line with NaOH. With a waste beaker below the buret, drain some of the base through the tip to clear the buret of air. Stop between and mL. Remove the hanging drop by touching the tip to the inside of the waste beaker. 9. Read the initial volume of the buret and record it in the data table. Part B: Titration : 10.)

7 Place the flask with acid and phenolphthalein under the buret. The buret tip should be down about 1cm inside the mouth of the flask to avoid losing any of the base . Place a sheet of white paper under the flask to help highlight the pink indicator colour. 11. Drip the base into the flask while swirling the flask to stir it. Add base quickly at first, but as the pink colour starts to last longer, slow the drip rate. It is better to take longer on this part than to need to start again! When the whole flask flashes pink before turning clear again, add only one drop at a time and swirl the flask before adding more. Occasionally rinse down the splashes on the side of the flask using a little distilled water from a wash bottle (this water will not change your results).

8 12. When the faintest pink colour persists for 30 seconds, stop and record the final volume in the buret. 13. Repeat the Titration for trial 2. 14. Flush all chemicals down the sink and clean out the buret as described in step 5. 15. Have your instructor sign your data. 16. Clean up your work area and wash your hands. Observations: Mass of Oxalic acid used: Trial 1: _____g Trial 2: _____g Data Table: Titration of H2C2O4 Oxalic acid with unknown concentration of NaOH Volume of NaOH (mL): Trial 1 Trial 2 Initial Final Used Questions and Calculations: 1. Why is it difficult to see whether you have added the phenolphthalein to the flask solution before you have titrated it?

9 2. Why did you need to rinse the buret with base before you completely fill it with NaOH? 3. Write and balance the neutralization equation for the reaction performed in this experiment. 4. Does the amount of water in which you dissolved the Oxalic acid affect the outcome of the experiment? Explain. 5. Why is it important to wash down the sides of the reaction flask with water as you near the equivalence point? 6. How many moles of base (sodium hydroxide) are needed to neutralize one mole of acid ( Oxalic acid )? 7. Determine the molarity of the unknown NaOH. (Do all these calculations for each trial!) a. Calculate the number of moles of Oxalic acid used. b. Determine the number of moles of base needed to neutralize the moles of Oxalic acid .

10 C. Knowing the volume of NaOH you used in each trial, calculate the molarity of the NaOH. Conclusions: What is the [NaOH] that was used in this experiment? What are some sources of error for this experiment? Why might your result not be accurate? How could you improve your results?