AP+Chem+Lab+12

AP Chem Lab #12
 * Acid-Base Titration **

__ Introduction: __ A titration is a process used to determine the volume of a solution that is needed to react with a given amount of another substance. In this experiment, your goal is to determine the molar concentration of two acid solutions by conducting titrations with a base of known concentration. You will be testing a strong acid, HCl, solution and a weak acid, HC 2 H 3 O 2, solution. You will use a sodium hydroxide, NaOH, solution as your base of known concentration. The reaction equations are shown below in net ionic form: H+(aq) + OH– (aq) → H2O(l) HC2H3O2(aq) + OH–(aq) → H2O(l) + C2H3O2– (aq) The stoichiometry of the two reactions is identical; thus, your calculations will be straightforward. However, you will observe a significant difference in how the two acid solutions react with NaOH. In this experiment, you will use a computer to monitor pH as you titrate. The region of most rapid pH change will then be used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the HCl solution.

__ Objectives: __ In this experiment, you will
 * Accurately conduct acid-base titrations.
 * Determine the equivalence point of a strong acid-strong base titration.
 * Determine the equivalence point of a weak acid-strong base titration.
 * Calculate the molar concentrations of two acid solutions.

__ Materials: __ Vernier computer interface computer Vernier pH Sensor 0.100 M sodium hydroxide, NaOH solution hydrochloric acid, HCl, solution, unknown molarity acetic acid, CH 3 COOH, solution, unknown molarity magnetic stirrer stirring bar 50 mL buret 10 mL pipet wash bottle distilled water ring stand utility clamp 250 mL beaker 10 mL pipet with pump 50 mL graduated cylinder buret clamp 100 mL beaker 10 mL graduated cylinder

__ Procedure: __
 * 1) Obtain and wear goggles.
 * 2) Obtain about 25 mL of a hydrochloric acid solution of unknown concentration. Add 50 mL of distilled water to a 250 mL beaker. Use a pipet bulb (or pipet pump) to transfer 10 mL of the HCl solution into the beaker. **// CAUTION: //****// Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin //****// . //**
 * 3) Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is available, stir the reaction mixture with a stirring rod during the titration.
 * 4) Connect a pH Sensor to Channel 1 of a Vernier computer interface. Connect the interface to the computer using the proper cable.
 * 5) Set up a ring stand, buret clamp, and 50.0 mL buret to conduct the titration (see Figure 1). Rinse and fill the buret with 0.100 M NaOH solution. ** Note: ** Record the precise concentration of the NaOH solution in your data table. **// CAUTION: //****// Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing //****// . //**
 * 6) Use a utility clamp to suspend the pH Sensor on the ring stand, as shown in Figure 1. Position the pH Sensor so that its tip is immersed in the HCl solution but is not struck by the stirring bar. Gently stir the beaker of acid solution.
 * 7) Run the Logger // Pro // program on your computer. Open the file “07a Acid-Base” from the // Advanced Chemistry with Vernier // folder.
 * 8) You are now ready to begin the titration.
 * 9) Before adding NaOH titrant, click START Once the displayed pH reading has stabilized, click KEEP. In the edit box, type ** 0 ** (for 0 mL added). Press the ENTER key to store the first data pair for this experiment.
 * 10) Add the next increment of NaOH (enough to raise the pH about 0.15 units). When the pH stabilizes, again click KEEP In the edit box, type the current buret reading as accurately as possible. Press ENTER . You have now saved the second data pair for the experiment.
 * 11) Continue adding NaOH solution in increments that raise the pH by about 0.15 units and enter the buret reading after each increment. When a pH value of approximately 3.5 is reached, change to a one-drop increment.
 * 12) After a pH value of approximately 10 is reached, add larger increments that raise the pH by about 0.15 pH units, and enter the buret level after each increment.
 * 13) Continue adding NaOH solution until the pH value remains constant.
 * 14) When you have finished collecting data, click STOP. Dispose of the reaction mixture as directed. Rinse the pH Sensor with distilled water in preparation for the second titration.
 * 15) Repeat the necessary steps to test the acetic acid solution. Conduct a second trial of the acetic acid solution if directed by your instructor. Analyze, print, and save the titration data for your acetic acid solution trial(s).

Data Table:
 * HCl Trial || Volume HCl (ml) || [NaOH] (M) || Equiv. Point (ml) ||
 * 1 ||  ||   ||   ||
 * 2 ||  ||   ||   ||


 * HC2H3O2 Trial || Volume HCl (ml) || [NaOH] (M) || Equiv. Point (ml) ||
 * 1 ||  ||   ||   ||
 * 2 ||  ||   ||   ||

__ Calculations: __
 * 1) Calculate the molar amounts of NaOH used in the reaction with the HCl solution and with the CH 3 COOH solution.
 * 2) Calculate the molar concentration (molarity) of the HCl solution and the CH 3 COOH solution.
 * 3) Compare the actual molar concentrations of your two acid solutions with your calculated molarities. Were the calculated molarities of your acid solutions within a reasonable range (about 5%) of the actual values? If not, suggest reasons for the inaccuracy.
 * 4) The equivalence points of the two titration curves were not in the same pH range. Explain.

__ Post Lab Questions: __
 * 1) Calculate the molar amounts of NaOH used in the reaction with the HCl solution and with the HC2H3O2.
 * 2) Calculate the molar concentration (molarity) of the HCl solution and the HC2H3O2 solution.
 * 3) Compare the actual molar concentrations of your two acid solutions with your calculated molarities. Were the calculated molarities of your acid solutions within a reasonable range (about 5%) of the actual values? If not, suggest reasons for the inaccuracy.
 * 4) The equivalence points of the two titration curves were not in the same pH range. Explain.

__ Extension: __ // Gran Plot // was proposed in the early 1950’s by G. Gran, this method uses the reciprocal of ∆pH of the titration data (where ∆pH = pH value – previous pH value). The graph of volume of 1/∆pH // vs //. titrant volume resembles a V-shaped plot. The inflection point of this plot is the equivalence point volume of the titration. To use this method, you will first need to create a new calculated column, 1/∆pH. You can do this in Logger // Pro // software by choosing New Calculated Column from the Data menu, and entering a formula, 1/∆pH. (You can select // delta // from the list of available functions.) On your resulting plot of 1/∆pH // vs //. volume (see Figure 3), interpolate to find the intersection of two best-fit regression lines. The precise volume where the two linear fits intersect will be the equivalence point volume.