CP

Acid-Base Titration Analysis

Determination of the Acid Content in Vinegar

Introduction

Acids and bases react in aqueous solution to form a salt and water. This reaction, known as a neutralization reaction, is actually one type of a double displacement reaction. Two examples are given below.

HCl (aq) + NaOH (aq)  NaCl (aq) + HOH (l)

hydrochloric sodium hydroxide sodium chloride water

acid (a base) (a salt)

H2SO4(aq) + 2 KOH (aq)  K2SO4(aq) + 2 HOH (l)

sulfuric potassium hydroxide potassium sulfate water

acid (a base) (a salt)

Because there are many different acids and bases, neutralization reactions are very common. If a neutralization reaction is run in the laboratory under controlled conditions, it can be used to determine the concentration of the acid or the base in the reaction. This procedure, known as an acid-base titration analysis, involves accurately measuring the volume of a base that is required to neutralize a known volume of acid. In order to calculate the concentration of the acid from the laboratory data, we must also know the concentration of the base used in the titration. Thus to determine the concentration of an acid, we must have a base of known concentration and the equipment to accurately measure the volume of the acid and base reacting. The concentration of the base is given in moles of acid or base per liter of solution or molarity (M).

In this experiment we will determine the concentration of acetic acid (HC2H3O2) in household vinegar. A known volume of vinegar will be measured out with a volumetric pipette and titrated with a solution of sodium hydroxide (NaOH) of known molarity. The NaOH solution will be added from a buret until all of the acid in the vinegar has been neutralized. Since we need to accurately measure the volume of NaOH used in the titration, we must have a means of knowing when just enough of the base (NaOH) has been added to react with all of the acid (HC2H3O2), in the vinegar. This point is called the end point. To detect the end point, an acid-base indicator is added to the vinegar. The indicator, phenolphthalein, is colorless in acidic solutions but turns pink in basic solution. The end point is when the vinegar turns very light pink and the color persists even when you swirl the solution.

Equipment and Materials

Equipment:Materials:

goggles distilled water

buret standacetic acid (unknown concentration)

50 mL burets (2)sodium hydroxide (1.00 M)

125 mL Erlenmeyer flaskphenolphthalein indicator

150 mL beakers (4)

100 mL graduated cylinder

wash bottle

Purpose:

To determine the unknown concentration (in units of mass %) of acetic acid in household vinegar.

Procedure

  1. Obtain approximately 65 mL of 1.00 M NaOH in a clean, dry 150 mL beaker.
  1. Close the stopcock on the base buret and fill it to a level slightly above the 0.00 mL line. Open the stopcock and allow a little of the NaOH to run out into a small waste beaker. This helps to eliminate air bubbles in the tip of the buret.
  1. Adjust the level of the NaOH in your buret so that the bottom of the meniscus is at or below the 0.00 line. (If you need to remove some NaOH from the buret, let it flow out into the waste beaker.)
  1. RECORD THE INITIAL BASE BURET READINGIN YOUR DATA TABLE(record volume to 2 decimal places)
  1. Obtain approximately 65 mL of unknown concentration of vinegar in a clean, dry 150 mL beaker.
  1. Repeat steps 2 and 3 with the acetic acid (vinegar).
  1. RECORD THE INITIAL ACID BURET READINGIN YOUR DATA TABLE(record volume to 2 decimal places)
  1. Transfer 20.00 mL of vinegar from the buret to a clean, dry 125 mL Erlenmeyer flask
  1. Add 3 to 4 drops of phenolphthalein to this flask. The solution will be colorless (colorless in ACID, bright pink in BASE).
  1. Place your flask containing the 20.00 mL vinegar sample and the phenolphthalein under the tip of the buret containing base.

a)Carefully add the NaOH by turning the stopcock almost one half turn. This allows one drop at a time to leave the buret. With experience, you may open the stopcock and let more out at one time.

b)Try to achieve an end point with the palest pink you can see that persists for at least twenty seconds.

  1. If you pass the endpoint, then use some of the acid from the second buret to “back-titrate”. This volume must be added to your recorded volume of “acid used”.
  1. Once you have reached the end point, read and record the volume of NaOH as THE FINAL BASE BURET READING IN YOUR DATA TABLE(record volume to 2 decimal places)
  1. RECORD THE FINAL ACID BURET READINGIN YOUR DATA TABLE(record volume to 2 decimal places)
  1. Subtract the initial volumes from the final volumes to calculate the titration volumes of acid and base, the total volumes required for neutralization.
  1. Perform a second and third titration with a fresh 20.00 mL sample of vinegar and phenolphthalein.
  2. Calculate the average titration volumes (relative volumes of acid and base required for neutralization) and record in the data table. The average titration volume will be used in the calculation of the concentration of the acid.

CP Name:

Titration of Household Vinegar LAB

Observations and Data:

Table 1: Titration Data
****RECORD VOLUME TO 2 DECIMAL PLACES / TRIAL 1 / TRIAL 2 / TRIAL 3
Initial base buret reading
Final base buret reading
Volume of sodium hydroxide used (VB) = Vf - Vi
Initial acid buret reading
Final acid buret reading
Volume of acetic acid used (VA) = Vf - Vi
Average volume of ACIDused (VA)
Average volume of NaOH BASE required for titration (VB)
Molarity of NaOH / 1.00 M

Calculations

  1. Calculate the moles of the vinegar HC2H3O2.

HC2H3O2 + NaOH  NaC2H3O2 + H2O

****must solve for moles of HC2H3O2 = (VB) x (Molarity of NaOH) = moles NaOH x ______=______mol HC2H3O2

  1. Calculate the molarity of the vinegar, HC2H3O2. Put this number into your data table above.

Molarity = moles HC2H3O2

L of acid solution

  • Divide the moles of HC2H3O2 (from calc. #1) by the volume of acid used (VAin your data table)
  1. How many milliliters of .45 M HCl will neutralize 25.0 mL of 1.00 M NaOH?