Titration with Oxalic Acid

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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 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. An appropriate indicator must be used to determine when chemically equivalent amounts of each chemical are combined, that is, when no excess of either...

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Nội dung Text: Titration with Oxalic Acid

Titration with Oxalic Acid Introduction 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 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. An appropriate indicator must be used to determine when chemically equivalent amounts of each chemical are combined, 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 a base solution can be determined. A measured amount of acid is neutralized by reaction with a base solution titrated from a buret. Consider the following example: HCl + NaOH → NaCl + H2O (Known) (Unknown) The example shows that the reaction is a double replacement in which the products are a corresponding salt and water. For reactions of strong acids and bases, the equivalence point pH is 7, but for weak acids it is somewhat higher. If a graph is made of pH vs. volume of base added, the equivalence point is always halfway along the S curve as labeled in the illustration. Note that there is a large pH change around the equivalence point with very small additions of base. In this investigation, you will determine the concentration of a basic solution by titration with oxalic acid, H2C2O4, a moderately weak acid with and equivalence
point pH of approximately 8. Weak acids are used to titrate bases because the equivalence point is reached more slowly, so the results are more accurate. Phenolphthalein will be used to find the endpoint because it is colorless below pH 8, but turns pink just above pH 8. Concepts - Acid and Bases - Neutralization - Titration Problem How can you use titration process to determine the strength of bases? Materials Goggles Buret Erlenmeyer flask, 250-mL Buret clamp Balance Ring stand Oxalic acid dihydrate (H2C2O4 • Sodium hydroxide solution (NaOH) 2H2O) pH unknown Distilled water 2 beakers, 100-mL Phenolphthalein Funnel Wash bottle Graduated cylinder 100-mL Safety Precautions Wear your goggles at all times during the investigation. Place the top of the buret below eye level when filling it to avoid splashing the solutions into the face. Place the buret and buret stand on the floor if necessary. Acids are corrosive and bases are caustic. Wipe up spills and drips immediately with wet towels, and the dry. Wash affected skin areas with cold water after any contact and notify your teacher. Burets are cumbersome and break easily – handle them with care. Procedure Part A. Preparation 1. Put your goggles on. While one lab partner prepares the oxalic acid (Steps 2 and 3), the other will prepare the buret for titration (Steps 4-8). 2. Determine the mass of a 250mL flask to 0.01g, the add 10. -1.5g oxalic acid dihydrate to the flask and determine the mass again. Caution: Acids are corrosive. Avoid contact with skin, eyes, and clothing. Record these masses in Data Table. 3. Dissolve the acid in the flask with approximately 100mL of distilled water. Add two or three drops of phenolphthalein indicator solution to the flask.
4. Make sure the buret is clean. If it is not, wash the buret with detergent and buret brush. Clean the tip by draining some detergent solution through it. Caution: Burets are fragile. Use great care in handling them. 5. Rinse the buret thoroughly with tap water, then rinse once with distilled water, draining final rinses through the tip. Clamp the clean buret to the support stand. 6. Obtain 50-60mL of sodium hydroxide (NaOH) solution, unknown, in a 100 mL beaker. Position the buret so that the top is below your eye level and place and empty beaker under the buret tip to catch drips. Caution: Sodium hydroxide is caustic. Keep the top of the buret below eye level when pouring sodium hydroxide. Avoid contact with skin, eyes, and clothing. Wash spills and splashes with plenty of water. 7. Pour approximately 5 mL of base into the buret. Drain this through the tip to remove water and coat the inside of the buret with base. 8. Now fill the buret to slightly above the zero line. Drain some base through the tip to clear the buret tip of air. Stop between 0.0 and 2.0 mL. Remove the hanging drop at the tip by touching the tip to the inside of the waste beaker.
Read the initial volume in the buret and record it in Data Table. Part B. Titration 9. Place the flask with acid and phenolphthalein under the buret. The buret tip should be down about 1 cm inside the mouth of the flask to avoid and outside loss of base. Place a sheet of white paper under the flask to highlight the pink indicator color. 10. Drip the base into the flask while swirling the flask. You can add base quickly at first, but as the pink color starts to last longer, slow the drip rate. When the whole flask flashes pink before turning clear again, add only one or two drops at time and swirl until the flask is clear before adding more. Occasionally rinse down splashes on the inside of the flask using a little distilled water form a wash bottle. 11. When the faintest pink color persists, stop and record the final volume in the buret. 12. Flush all chemicals down the drain with plenty of water. Wash all beakers and the flask, and clean the buret as described in Step 4.
Name: ______________________________________________________________ Date: ________________ Period: ______________ Titration with Oxalic Acid PreLab Questions 1. Why is it difficult to see whether you have added the pH indicator phenolphthalein to the flask of acid solution? 2. Write and balance the double replacement reaction equation for this investigation. Show how many moles of the base are needed to neutralize one mole of acid. 3. Why must you fill the buret only when its top is below eye level? 4. Look up the term equivalent. Give its root derivation, and tell how this root helps to explain the meaning of equivalence point. 5. Why is it better to use a weak acid rather than strong acid whey you titrate a base? Data Table
Mass (g) Volume of NaOH (mL) Flask Initial Flask and oxalic acid Final Oxalic acid Titrated PostLab Calculations and Analysis (Show Work on Math Problems) 1. Determine the molarity of the unknown sodium hydroxide solution as follow. a. Calculate the number of moles of oxalic acid used for NaOH, using the molar mass of H2C2O4 • 2H2O. b. Use your answer to Question 2 of the pre-lab to determine the number of moles of base needed to neutralize the calculated number of moles of oxalic acid. c. Calculate the molar concentration of based used. 2. Does the amount of water in which the oxalic acid is dissolved affect the outcome of investigation? Why or why not?
3. Titration is capable of yielding highly reproducible results, equivalent to ± 1 drop of titrant. Explain how each of the following parts of the procedure contributes to this precision. a. Removing drops of titrant hanging from the tip. b. Washing down the inside of the reaction flask with water. c. Rinsing the buret with the base to be used. 4. Use the pH curve for titration diagrammed in the introduction to determine the effect on pH form the addition of one or two drops of base when pH = 3; when pH = 6.
5. Carbonate ions, CO32-, are contained in limestone (the major component of which is calcium carbonate, CaCO3), which is used to neutralize acids in soil. Remedies sold to neutralize stomach acid contain carbonate. The equilibrium reaction that carbonate ions undergo with water is called hydrolysis. Write the reaction for this equation, and explain how carbonate neutralizes acids.