TITRATION OF SULFURIC ACID WITH SODIUM HYDROXIDE Purpose To determine the molarity of two unknown solutions of sulfuric acid, H2SO4. Pre-Lab Discussion: Titration consists of the gradual addition of a standard solution (of known concentration) to a measured quantity of a solution of unknown concentration until the number of equivalents (moles in our titration) of each solute is the same. We will titrate a strong acid, H2SO4, with a strong base, NaOH according to the equation: H2SO4(aq) + 2NaOH(aq) → 2H2O(l) + Na2SO4(aq) Since the stoichiometry ratio is 1:2, the number of moles of base added at the end of the process will be equal to twicethe number of moles of acid present. In our titration, we will use a solution of sodium hydroxide whose concentration is exactly 0.200M = MB. The volume of HCl titrated will be measured carefully to determine the VA. The sodium hydroxide will be added to the HCl until the same number of moles of base has been added as are present for the acid. At that point the titration will stop, and the volume of base will be read from the burette to determine VB. At this, the equivalence point, the following equations holds true: 2 moles NaOH = moles H2SO4 The end of the reaction, when the correct number of moles of base have been added to just consume the acid is known as the equivalence point. We will attempt to determine the equivalence point. This can be done with a pH indicator. When a pH indicator is used, a color change will occur at some point in the titration. The point at which the pH indicator changes color is known as the end point. If the proper indicator is chosen, the color change will occur when the number of moles of acid equals the number of moles of base. In this case the end point (where the indicator changes color) will equal the equivalence point. The indicator used for this titration will be phenolphthalein. It occurs in two forms, an acidic form, HIn, which is colorless, and a basic form, In - , which is red. The color change that you are looking for is from colorless to pink. You want to stop adding base when a single drop of base causes the solution to turn a light pink. Prior to the end of the reaction the indicator is in the presence of excess acid in the titration vessel and will exist in the colorless form, HIn. Once all the acid has been consumed by the base, the following reaction will occur: HIn + NaOH → H2O + NaIn If during the titration the color changes from colorless to deep red with the addition of several drops of NaOH, you have over titrated, and must begin again. A single drop of NaOH must change the solution from colorless to light pink. You want to stop the titration when the slightest bit of excess NaOH has been added. QUESTIONS: 1. What would be the problem if we forget to add indicator, phenolphthalein, to the titration vessel? 2. What would be the problem if we add base to the titration vessel until a deep red color results? 3. What is the function of the indicator in a titration? 4. A student titrates 37.6ml of the 0.200M solution into 25 ml of an unknown molarity solution of sulfuric acid. a. Will the number of moles of NaOH equal the number of moles of H2SO4? Explain. b. Calculate the number of moles of NaOH used in this experiment. c. Calculate the number of moles of H2SO4 used in this experiment. d. Find the molarity of the original H2SO4 solution. 5. A student titrates 27.8ml of the 0.200M solution into 25 ml of an unknown molarity solution of phosphoric acid, H3PO4. a. Write the balanced equation for the complete neutralization of phosphoric acid. b. Will the number of moles of NaOH equal the number of moles of H3PO4? Explain. c. Calculate the number of moles of NaOH used in this experiment. d. Calculate the number of moles of H3PO4 used in this experiment. e. Find the molarity of the original H3PO4 solution. 6. Write a procedure for that would fulfill the purpose of this lab using the material s provided. 7. Produce a data table that would be appropriate for this lab. Chemicals: Materials: 2 Unknown M H2SO4 solutions 0.200M NaOH Phenolpthalien Beaker 125ml Erlenmeyer Flask 50ml Burette Ring stand and Burette clamp Questions: 1. A student has a solution of sulfuric acid with a molarity of 0.250M. The student finds that to reach the equivalence point, they must add 47.6ml of the acid to a 12.8 ml solution of sodium hydroxide. What is the molarity of the sodium hydroxide solution? 2. A student has a solution of hydrochloric acid (HCl) with a molarity of 0.375M. The student finds that to reach the equivalence point, they must add 34.2.6ml of the acid to a 12.8 ml solution of calcium hydroxide, Ca(OH)2. What is the molarity of the calcium hydroxide solution? 3. A student does a titration with 5.0ml of 0.10M sulfuric acid with 0.10M sodium hydroxide and measures the pH every 0.10ml of NaOH added. The student graphed the data (left graph) and derived the following: a) The student was expecting 2 different humps like a similar experiment with carbonic acid H2CO3, in the right graph. Explain the difference in the two graphs. b) For sulfuric acid, why did it take 10ml of the NaOH to neutralize the acid? c) Explain why the carbonic acid graph has an equivalence point around 4 and not 7 like the sulfuric acid. d) Would a titration graph for hydrochloric acid look more like the graph for sulfuric acid or carbonic acid? Explain.
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