GCC CHM 130LL: Vinegar Titration page 1 of 7 CHM 130LL: Vinegar Titration Introduction In an acid-base neutralization reaction, an acid reacts with a base to produce water and a salt: HX (aq) + MOH (aq) H2O (l) + MX (aq) (1) acid base salt The protons (H+) from the acid react with the hydroxide ions (OH–) from the base to form the water. The salt forms from the cation from the base and the anion from the acid. Because water is always formed, acids will always react with bases; whether the salt is soluble or insoluble does not determine whether the reaction occurs. In this experiment, you will determine the molarity (or molar concentration) of acetic acid, HC2H3O2 (aq), present in a sample of vinegar using a standard NaOH solution. (A standard solution has been analyzed, so its concentration is known to a certain degree of accuracy. In this experiment, the NaOH solution was standardized in our stockroom to four significant digits.) You will measure out a small volume of vinegar and use a buret to determine the volume of sodium hydroxide required to completely neutralize the vinegar. The process of slowly adding one solution to another until the reaction between the two is complete is called a titration. The reaction between acetic acid, HC2H3O2 (aq), and sodium hydroxide, NaOH (aq), is shown below: HC2H3O2 (aq) + NaOH (aq) H2O (l) + NaC2H3O2 (aq) (3) When carrying out an acid-base neutralization reaction in the laboratory, you observe that most acid solutions and base solutions are colorless, and the resulting water and soluble salt solutions are also colorless. Thus, it is impossible to determine when a reaction has occurred, let alone when it is complete. To monitor the progress of a neutralization reaction, you use an acid-base indicator, a solution that changes color depending on the pH (or acid-content) of the solution. One commonly used indicator is phenolphthalein, which is colorless in acidic and neutral solutions and pink in basic (or alkaline) solution. During a titration, the indicator is added to the sample being analyzed. The titrant is slowly added to the sample until the endpoint, when the indicator changes color, signaling that the reaction between the two is complete. Note that phenolphthalein turns pink only when excess sodium hydroxide, NaOH (aq), has been added. If the appropriate indicator has been chosen, the endpoint of the titration (i.e. the color change) will occur when the reaction is complete: when moles of HCl = moles of NaOH in the example (4) when moles of HC2H3O2 = moles of NaOH in your titration.GCC CHM 130LL: Vinegar Titration page 2 of 7 Sample Calculations Titrating an Acid Consider the following reaction between hydrochloric acid, HCl (aq), and sodium hydroxide, NaOH (aq): HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq) (5) Using a standardized sodium hydroxide solution with a concentration of 1.020 M, a student titrated 25.00 mL of hydrochloric acid. If 27.14 mL of sodium hydroxide was required to completely neutralize the hydrochloric acid to a phenolphthalein endpoint, calculate the molarityof the hydrochloric acid. The first step in this calculation is recognizing that you are solving for the molarity of hydrochloric acid, which has units of moles per liter and which we can represent as [HCl]: HCl LHCl mol = [HCl] =HCl ofmolarity (6) Since 25.00 mL of hydrochloric acid is used, convert that to liters (by moving the decimal point to the left three times), and put it in the denominator: molarity of HCl= [HCl] = mol HCl0.02500 L HCl (7) To determine the number of moles of hydrochloric acid, convert the volume of sodium hydroxide used to liters then multiply that with the molarity of sodium hydroxide (given as 1.020 M and shown below as a unit factor), as shown in the following: 27.14 mL NaOH 1 L1000 mL 1.020 mol NaOHL of NaOH = 0.02768 mol NaOH (8) By showing the molarity explicitly as a fraction, you can see that the volume units (liters of NaOH) cancel. Since you actually need moles of hydrochloric acid, not moles of sodium hydroxide, you need to include one more step, the mole-to-mole ratio between sodium hydroxide and hydrochloric acid. The complete calculation to get moles of hydrochloric acid is shown below: 27.14 mL NaOH 1 L1000 mL 1.020 mol NaOHL of NaOH 1 mol HCl 1 mol NaOH = 0.02768 mol HCl (9) Finally, we put the number of moles in the numerator, and the molarity for hydrochloric acid is as follows: molarity of HCl= [HCl] = 0.02768 mol HCl0.02500 L HCl1.107M HCl (10) Note that there are 4 significant figures in all of the calculations for this experiment.GCC CHM 130LL: Vinegar Titration page 3 of 7 Laboratory Technique Burets The basic metric unit of volume is the liter (L), but the milliliter (mL) is most commonly used in lab. One mL is equal to 0.001 L. Several types of apparatus are used to measure and deliver specific volumes. Burets are used when it is necessary to deliver a liquid to another container and record the amount delivered. A buret is marked in milliliters much like a graduated cylinder, except buret markings indicate the number of milliliters delivered. This means that 0 (none delivered) is at the top, and the numbers get larger as you go down the buret. The stopcock controls the liquid flow. It is open when parallel to the length of the buret and closed when perpendicular to the length of the buret. Rinsing the buret: Obtain some deionized water in a small beaker. With the buret over the sink and the stopcock open, pour the water through the buret, letting it drain out the tip. After the buret is well-drained, close the stopcock and use a small beaker to pour 5-10 mL of the solution to be used into the buret. Tip the buret sideways and rotate to completely rinse the inside of the buret. Run this solution through the buret tip. Filling the buret: Close the stopcock. Use the small beaker to fill the buret 1 mL above the “0” mark. Place a container under the buret tip and open the stopcock slowly. The glass buret tip should fill with solution, leaving no air bubbles. If the tip does not