Lab Partner: Lab # 28: What is the Molarity of Vinegar? Accelerated Chemistry 1

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1 Name: Lab Partner: Date: Lab # 28: What is the Molarity of Vinegar? Accelerated Chemistry 1 Introduction: When a solution of acid is mixed with a solution of base, a chemical reaction occurs that can be represented by the following net ionic equation: H + (aq) + OH - (aq) H 2 O (l) This is called a neutralization reaction, and chemists use it extensively to change the acidic or basic properties of solutions. Since the reaction is quantitative, it can be used to determine the concentrations of acidic or basic solutions through a process called titration. In a titration, a solution of known concentration is added from a buret to a measured solution of unknown concentration until the number of moles of H + ions equals the number of moles of OH - ions. This is called the equivalence point, or end point of the titration. The end point is determined by using a chemical called an indicator that changes color at the proper point. The indicators used in acid-base titrations are weak organic acids or bases that change color when they are neutralized. One of the most common indicators is phenolphthalein, which is colorless in acid solutions but becomes pink in when the ph of the solution becomes 9 or higher. When a solution of a strong acid is titrated with a solution of strong base, the ph at the endpoint will be about 7. At the end point a drop of acid or base added to the solution can change its ph by several ph units, so that phenolphthalein can be used as an indicator in such titrations. If a weak acid is titrated with a strong base, the ph at the equivalence point is somewhat higher than 7, perhaps 8 or 9, and phenolphthalein is still a satisfactory indicator. In this lab you will be making a 0.50 M NaOH solution. When this is done, you will want to ensure that your concentration is accurate. To do this, you will be titrating 10.0 ml of the NaOH solution you made with a 1.0M solution of HCl. This process is called standardizing. Once you confirm the concentration of the NaOH solution, you will use it to determine the concentration of acetic acid in commercial white vinegar. Goal: To create and verify the concentration of a NaOH solution. To use the standardized solution to determine the molar concentration of acetic acid in commercial distilled white vinegar. Materials Available: NaOH (s) 1.0 M HCl standardizing solution 100-mL volumetric flask Graduated cylinders Phenolphthalein indicator 125 ml Erlenmeyer flasks Commercial white vinegar Buret and buret clamp Ring stand Lab28-VinegarTitration 1

2 Safety Concerns: Students should wear safety goggles and laboratory aprons throughout this experiment. Students should exercise care when working with the chemicals. Clean up all spills as soon as possible. Clean up small spills immediately using lots of water. For large spills, contact the teacher. Students should wash their hands after completing this experiment. Prelab- 1. Explain how you would make ml of a 1.5M NaOH solution if NaOH from NaOH(s). 2. What is the ph of the sodium hydroxide solution that you made in question 1? 3. Calculate the concentration of a 10.0 ml sample of NaOH if the indicator turned clear after adding 9.5 ml of 0.50 M HCl was added. 4. Write a balanced equation for the reaction of NaOH with acetic acid. If ml Vinegar (d = 1.06 g/ml) contains 5% by mass acetic acid a. How many moles of acetic acid are in ml of vinegar? b. How many ml of 0.11 M NaOH are required to neutralize the vinegar? Lab28-VinegarTitration 2

3 Procedure Part 1: Making 0.5 M NaOH 1. Create ml of 0.50 M solution of NaOH as accurately as possible. a. Calculate amount of grams of NaOH necessary to make the solution. b. Add solid NaOH to a volumetric flask c. Fill flask to ½ full with water and swirl until solid has dissolved. d. Finish filling flask to 100mL line using pipette. Bottom of the meniscus should be touching the line. Show Calculation for making NaOH : Part 2: Standardizing the 0.50 M NaOH 1. Set up your buret apparatus as demonstrated by your teacher. 2. Rinse a 50.0 ml buret with a few ml of the stock 1.0M HCl solution for standardization. Thoroughly wet the sides of the buret. Then fill the buret with HCl. DO NOT pour the solution above your head. Bring the buret to eye-level. Use a funnel. Open the stopcock to fill the tip of the buret and to drain any excess HCl from the buret. Fill accurately to the 0.0 ml mark. 3. Measure accurately 10.0 ml of your NaOH in a 125-mL Erlenmeyer flask. 4. Add 2 drops of phenolphthalein. 5. Slowly add the 1.0 M standardized HCl from the buret. Add drop by drop near the end of the titration. 6. Record the volume of HCl needed to neutralize the NaOH. 7. Repeat this procedure for another trial. Equation for the above titration : Procedure Part 3: Determining the Molar Concentration of Acetic Acid in Vinegar 1. Measure accurately 10.0 ml of vinegar. Add to a clean, dry 125-mL or 250-mL Erlenmeyer flask. 2. Add two drops of the phenolphthalein indicator to the vinegar solution. 3. Add the standardized NaOH to the second buret as you did in the procedure above with the HCl. 4. Titrate the vinegar slowly. Make sure the drops are steady. Once the color begins to stay titrate one drop at a time, swirling between drops. Do not titrate significantly past the endpoint. 5. Each group should perform 2 trials. Each person should conduct one trial. Lab28-VinegarTitration 3

4 Data and Calculations: Table 1. Making Solutions of NaOH Grams of NaOH used Your Base Solution (NaOH) Total volume of solution Molarity of NaOH Table 2. Standardization Trials Concentration of HCl (M) Trial 1 Trial 2 Volume of base titrated (ml) Volume of acid used (ml) 1) Calculate the concentration of your NaOH solution based on the two standardizing titrations. 2) Calculate the average value for the concentration of the NaOH. 3) Determine the percent error for the making of your 0.50 M NaOH. Lab28-VinegarTitration 4

5 Table 3. Titration of Vinegar Concentration of Base (M) Trial 1 Trial 2 Volume of vinegar titrated (ml) Volume of base used (ml) 1) Calculate the molar concentration of vinegar in your two trials. 2) Calculate the average concentration of vinegar based on the three titrations. FORMAL LAB REPORT : Use formal lab report format. ( 40 points ) Rubric: Lab Report in Google Docs: jgiammanco@goog.prs.k12.nj.us Results in form of data table 1. Submit a data table which summarizes the following measurements: o Volumes of vinegar, volumes of NaOH, and molarity of the acid ( 10 pts) Calculations 1. Show all calculations for the molarity of the vinegar solution. (8 pts) 2. Show the calculation of your average molarity of acetic acid in vinegar. (5 pts) 3. The industry standard for white distilled vinegar is 5% by volume, which equates to about 0.8 M. Show a calculation for your percent error and give two reasons of error. (6 pts) 4. Using concept of hydrolysis of salt, explain using an equation what the ph of the resulting salt solution will be at the end point of the titration. (5 point) 5. Be sure to follow the rules for significant figures here. Lab28-VinegarTitration 5