Acid – Base Titration

Experiment (6) Acid – Base Titration Objectives: 1- To standardize sodium hydroxide solution. 2- To determine the molarity of strong acid solution. Introduction: Titration is a term used to describe an experiment in which one solution is added to another using a buret. The two solutions contain to compounds which react rapidly with each other. The solution in the buret is added until there is evidence that the reaction between the two compounds is complete. Acid-base reactions are one of the most common types of chemical reactions. The reaction between an acid and a base can be represented by the equation: This reaction is called a neutralization reaction, and proceeds sufficiently to the right to be considered quantitative. Therefore, the titration can be used to determine the concentrations of acidic and basic solutions. The titration is performed by adding a basic solution from a buret to a measured quantity of an acid solution until the number of moles of OH- is just equal to the number of moles of H+. This is called the equivalence point of the titration, and is detected by an indicator which changes color as close to the equivalence point. The change in color is called the end point. As possible, the change in color is called the end point of the titration. In this experiment, you will first determine the molarity of an NaOH solution by titrating against a primary standard acid (potassium hydrogen phthalate which has precisely known concentration). Then you will use the standardized NaOH solution to determine the molarity of a sulfuric acid (H2SO4) solution of an unknown concentration. A- Standardization of NaOH solution: The primary standard "potassium hydrogen phthalate"; KHC8H4O4(KHP); is used to standardize the NaOH solution (to determine exactly the molarity of the NaOH solution). A known weight of KHP is dissolved in distilled water and titrated with NaOH solution in the presence of phenolphthalein as indicator. At the end point (the color of phenolphthalein indicator is changed from colorless in an acidic solution to pink in a basic solution): Moles of KHP = moles of NaOH or Example (1): In an acid-base titration, 20.15 mL of NaOH solution was required to neutralize 0.491 g of KHP (molar mass = 204.23 g/mol). What is the molarity of the NaOH solution? B- Titrating an unknown solution of sulfuric acid: The standardized NaOH solution is used to determine the molarity of the sulfuric acid solution. A known measured volume of H2SO4 is titrated against the standardized NaOH solution using phenolphthalein as indicator. The molarity of H2SO4 solution is determined from measuring the volume of the standard NaOH solution needed to neutralizer the H2SO4 solution. At the end point: Moles of OH- = moles of H+ Moles of NaOH = 2 moles of H2SO4 (MNaOH) (VNaOH) = 2(MH2SO4) (VH2SO4) Example (2): The standardized NaOH solution (0.119 M) was titrated against 28.10 mL of H2SO4. The volume of the standardized NaOH solution needed to neutralize the H2SO4 solution was 24.28 mL. What is the molarity of the H2SO4 solution? Procedure: A- Preparing an NaOH solution by dilution: Pour about 50 mL of approximately 1 M NaOH solution into a 500 mL volumetric flask that contains approximately 50 mL of distilled water. Close the flask with the stopper and shake. Then add distilled water to bring the final volume up to the mark etched around the nick of the flask. Close the flask again and shake thoroughly to make solution uniform. B- Standardizing the NaOH solution prepared by dilution: 1- Weigh, as accurate as possible, about 0.4 g KHP on a weighing paper and transfer it to a clean 125-mL Erlenmeyer flask. Record the mass of KHP on your report sheet. 2- Add about 50 mL of distilled water and swirl to disolove the KHP and then add a few droops of phenolphthalein indicator. 3- Clean a 50 mL buret with soap and water using a buret brush and rinse it thoroughly first with tap water then with distilled water. 4- Add about 3 mL of NaOH solution to the buret. Turn the buret to it is nearly horizontal but do not let NaOH run out of the open end. While you hold the buret nearly horizontally, roll it in your fingers so the base solution touches every part of the inside of the buret. Hold it upright and drain the base through the tip. Repeat this rinsing twice to completely wet the buret with NaOH solution. 5- Remove air bubbles from the buret tip. There should not be any air bubbles in the buret or in the buret tip. 6- Titrate. Read the level of the solution in the buret and record it in your report sheet before you start. Add NaOH solution with continuous swirling of the Erlenmeyer flask. Continue the titration until you reach the end point (the color of the solution is changed to pink which lasts for about 30 seconds). Read the level of the base solution in the buret and record it in your report sheet. The difference between the initial and the final readings is the volume of NaOH required to titrate (neutralize) the acid. 7- Repeat the titration with a new 0.4 sample of KHP. C- Titrating a sulfuric acid solution of an unknown concentration: 1- Pick up a sample of the unknown sulfuric acid solution from the instructor. 2- Clean a 10-mL pipette and rinse it with distilled water. Then rinse the pipette a couple of times with about 2 mL of the acid. 3- Transfer 10.0 mL of the acid solution using the pipette into a clean 125-mL Erlenmeyer flask. Add about 10 mL of distilled water and one drop of phenolphthalein indicator and titrate with the NaOH solution. Record the volume for the NaOH solution used to reach the end point. 4- Repeat the titration with a new 25.0 mL sample if H2SO4 solution. Data and Results: Trail 1 Trial 2 Mass of KHP (g) ……………. ……………. Moles of KHP ……………. ……………. Initial reading of NaOH buret (mL) ……………. ……………. Final reading of NaOH buret (mL) ……………. ……………. Volume of NaOH used (mL) ……………. ……………. Moles of NaOH used ……………. ……………. Molarity of NaOH (mol/L) ……………. ……………. Average molarity of NaOH (mol/L) ……………. ……………. A- Standardization of NaOH solution: (molar mass of KHP = 204.23g.mol) B- Titrating an unknown solution of Unknown sulfuric acid No:…………… . Trail 1 Trial 2 Volume of H2SO4 solution used (mL) ……………. ……………. Initial reading of NaOH buret (mL) ……………. ……………. Volume of NaOH used (mL) ……………. ……………. Average molarity of NaOH (mol/L) ……………. ……………. Moles of NaOH used ……………. ……………. Moles of H2SO4 ……………. ……………. Molarity of H2SO4 (mol/L) ……………. ……………. Average molarity of H2SO4 (mol.L) ……………. ……………. * Show your calculation for trial 1 in part A and part B.