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4. A Rolaids tablet contains calcium carbonate for neutralizing stomach acid. If a Rolaids tablet neutralizes 24.65 mL of 0.547 M hydrochloric acid, how many milligrams of calcium carbonate are in a Rolaids tablet? CaCO3(s) + 2 HCI (aq) + CaCl2 (aq) + H2O(l) + CO2() 5. (optional) A student diluted 15.0 mL of 6 M NaOH solution into 485.0 mL of distilled water. Calculate the molarity of the diluted base solution Explain why this diluted NaOH solution cannot be used as a standard solution.
OBJECTIVES • To prepare a standard sodium hydroxide solution To determine the molar concentration and mass/mass percent concentration of acetic acid in an unknown vinegar solution. • To gain proficiency in the laboratory technique of titration. DISCUSSION In this experiment, we will neutralize an acidic solution of vinegar using a basic solution of sodium hydroxide. We determine the amount of sodium hydroxide necessary by performing a titration using a buret. When the acid is completely neutralized by the base, the titration stops. This is called the endpoint in the titration and is signaled when an indicator changes color. At the endpoint in the titration, a single drop of base is sufficient to bring about a permanent color change. Figure 20.1 illustrates a typical titration Copyright 2013 Pearson Education 229 Preparation of a Standard Sodium Hydroxide Solution We begin by diluting 6 M NaOH with water. Since diluting NaOH provides only an approximate concentration and it is necessary to know the concentration of NaOH precisely, we will prepare a standard solution by titration First, we weigh crystals of potassium hydrogen phthalate, KHC,H,O4 (abbreviated KHP), After dissolving the KHP crystals in water, we will titrate the acid solution with NaOH according to the following equation KHP(aq) + NaOH(aq) → KNP(aq) + H-01) Example Exercise 20.1. Molar Concentration of Standard NaOH A 0.515 g sample of KHP (204.23 gmol) is dissolved in water and requires 12.75 mL of NaOH solution to reach a faint pink endpoint. Find the molarity of the NaOH solution. Solution: Referring to the preceding equation for the reaction and applying the rules of stoichiometry, we have 0.00252 mol NaOH 1 mol KHP 1 mol NaOH 0.515 $ KHP 204.23 g KHP * 1 mol KHP The molarity of the NaOH is found as follows (0.00252 mol NaOH 12.75 ml solution 1000 ml IL 0.198 mol NaOH TL solution 0.198 M NaOH In this example, the concentration of the standard NaOH solution is 0.198 M. 0
6 Titration of Acetic Acid in a Vinegar Unknown After preparing a standard NaOH solution, we will determine the concentration of acetic acid in an unknown vinegar solution. A sample of vinegar will be titrated with NaOH to a permanent endpoint. The equation for the reaction is HC H302(aq) + NaOH(aq) + NaC_H302(aq) + H2O(1) The following example exercise illustrates the calculation for the percentage of acetic acid in an unknown vinegar sample. Example Exercise 20.2 . Percentage of Acetic Acid in Vinegar The titration of a 10.0-ml vinegar sample requires 38.05 mL of standard 0.198 M NaOH. Calculate the (a) molarity and (b) mass/mass percent concentration of acetic acid. Solution: We can calculate the moles of acetic acid from the moles of NaOH solution: 38.05 ml-solution 1000 ml solution * 0.198 mol NaOH 1 mol HC2H302 mol NaOH = 0.00753 mol HC,H,02 (a) The molar concentration of HC2H302 is 0.00753 mol HC,H,O 1000 ml. 0.753 mol HC H,02 10.0 mt solution IL IL solution 0.753 M HC,H,O (b) To calculate the m/m% concentration, we must know the density of the vinegar (1.01 g/mL) and the molar mass of acetic acid (60.06 g/mol). 0.753 mol HE HO 1000 ml solution X 60.06 g HC2H,02 1 ml solution 1 mol HEXH3 * 1.01 g solution * 100% = 4.48% HCH:02 EQUIPMENT and CHEMICALS • buret stand & clamp ring stand & utility clamp (optional) 50-ml buret small, plastic funnel (optional) • 150-ml.beaker graduated cylinder • 1000-ml Florence flask w/stopper 125-ml. Erlenmeyer flasks (3) • 10-ml pipet & bulb • 100-ml beaker wash bottle with distilled water . dilute sodium hydroxide, 6 M NaOH "potassium hydrogen phthalate. solid KHCH.O. (KHP) • phenolphthalein indicator (or, cresol red indicator) • unknown vinegar solution, 3.00-5.00% HC2H:02
Enlarged View -10.45 mL Buret with NaOH solution Enlarged View மாயாமாபார் - 40.55 mL. KHP, water, and phenolphthalein (a) Figure 20.1 Apparatus for Titrating an Acid with a Base (a) Read the initial volume of NaOH in the buret (10.45 mL). (b) A flash of color indicates nearing the endpoint. (c) A permanent color signals the endpoint for the titration (40.55 mL). In the example shown, the volume of NaOH is 40.55 mL-10.45 mL = 30.10 mL.
A. Preparation of Sodium Hydroxide Solution 1. Half-fill a 1000-ml Florence flask with -500 mL of distilled water. Measure-15 mL of 6 M NaOH into a graduated cylinder and pour the NaOH into the Florence flask. Stopper the flask, and carefully swirl to mix the solution. 2. Condition a buret with NaOH solution from the Florence flask. Use a small funnel and half-fill the buret with NaOH. Allow some solution to pass through the buret tip into a 150-ml beaker, and empty the remainder into the sink. 3. Close the stopcock, and fill the buret with NaOH solution from the Florence flask. Note: Carefully add NaOH solution to the funnel so as to not overfill the buret. 4. Label the 125-ml Erlenmeyer flasks #1, #2, and #3. Precisely weigh-0.5 g of KHP into each of the flasks. Add -25 mL of distilled water to each flask, and heat as necessary to dissolve the KHP crystals. Note: If a digital electronic balance is available, the Instructor may direct students to tare weigh the KHP samples. 5. Titrate three KHP samples as follows: Drain NaOH through the tip of the buret to clear any air bubbles. • Position Erlenmeyer flask #1 under the buret as shown in Figure 20.1. • Record the initial buret reading (+ 0.05 mL). • Add a drop of indicator to the flask. Titrate with NaOH to a permanent endpoint while slowly swirling the flask. • Record the final buret reading (+ 0.05 mL). 6. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #2, titrate the KHP sample, and record the final buret reading. 7. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #3, titrate the KHP sample, and record the final buret reading. 8. Calculate the molarity of the NaOH solution for each trial. Record the average molarity of NaOH in the Data Table of Procedure B. RECYCLE Chemical Waste Note: Save the NaOH in the Florence flask for Procedure B.
B. Titration of Acetic Acid in Vinegar 1. Obtain -50 mL of vinegar solution in a dry 100-ml beaker. Record the unknown number in the Data Table. 2. Condition a pipet with unknown vinegar solution, and transfer a 10.0-mL sample into each 125-ml flask (see Appendix E). Add -25 mL of distilled water into each flask. Note: It is not necessary to use dry flasks. 3. Titrate three vinegar samples as follows: • Position Erlenmeyer flask #1 under the buret. • Record the initial buret reading (+ 0,05 ml.). • Add a drop of indicator to the flask. • Titrate with NaOH to a permanent endpoint while slowly swirling the flask. • Record the final buret reading (+ 0.05 mL). 4. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #2, titrate the vinegar sample, and record the final buret reading. 5. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #3, titrate the vinegar sample, and record the final buret reading. 6. Calculate the molarity of acetic acid, HC2H302, in the unknown vinegar solution. 7. Convert the molarity of HC2H2O2 (60.06 g/mol) to mass/mass percent concentration. Assume the density is 1.01 g/mL for the unknown vinegar solution. Note: When the titrations are complete, rinse the buret and glassware with distilled water to remove all traces of NaOH solution.