CHM-102 CHM-102 UNCW UNCW Titrations: Determination of the Molarities of Strong and Weak Acids General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Purpose Purpose  To study the differences between strong and weak acids by titration with a strong base. Safety Considerations Safety Considerations  Safety glasses must be worn throughout the entire experiment, even when cleaning glassware to prevent damage due to strong acids and bases.  Be cautious when using unknown solutions and clean up any spills immediately. Contact with strong acids or bases will affect skin and create holes in clothing. Wash the affected area thoroughly if you suspect any skin contact any of unknown solutions.  Flush all wastes down the drain with plenty of water.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  Connect the Tablet computer, Microlab unit, and the pH probe as shown in the photo.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  To begin determining pH and volume measurements on the computer, click on the Applications folder, and select the Microlab prompt. Then double click the “pH Titrations” prompt as shown below.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  Click on the experiment entitled “pH Acid-Base – Temperature Manual Titration”, as shown below.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  Current pH values will be displayed in the lower right hand corner of the screen.  Values of pH corresponding to volume of titrant added will be collected and displayed in the table on the computer screen as shown in the photo.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  To begin the titration, click on the “Start” prompt.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  Before you begin your titration, enter the initial volume as 0 mL.  Press “Enter and Continue” to allow the computer to record the corresponding initial pH value of the unknown acid.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Procedure Procedure  Add 1 mL of base (titrant) to your unknown acid solution.  For best results, place the tip of the burette inside and along the side of the beaker, allowing the titrant to run down the side of the beaker and into the solution (rather than hitting the pH electrode or splattering).

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids  After the 1 mL of base (titrant) has been added, swirl the solution with the electrode. In the keyboard input enter the value of “1” for volume in mL and press “Enter and Continue” to record the pH value.  Continue to add 1 mL of titrant at a time. Stop after each addition of titrant to stir the solution and record the volume value and the corresponding pH value.  Enter the volume values in sequence (1 mL, 2 mL, 3 mL, 4 mL, 5 mL, etc.) in the input box on the computer even though only 1 mL of titrant is added at a time.

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids  Add 1 more mL of titrant when the solution turns dark pink and remains dark pink after swirling. Click the “Stop” prompt. Remove the pH probe immediately and rinse with DI water. Place the electrode in pH=7 buffer solution until next use.

CHM-102 CHM-102 UNCW UNCW Titrations: Graph of Strong Acid  The graph of a strong acid will display a sharp curve indicating equivalence. The midway point of the curve is the equivalence point, where the stochiometrically equivalent number of moles of base (titrant) equals the number of moles of acid in the unknown solution. From this point the equivalence volume and equivalence pH can be determined on the respective x and y axes. Equivalence pH Equivalence volume Equivalence point

CHM-102 CHM-102 UNCW UNCW Titrations: Graph of Weak Acid The graph of a weak acid will display a gentler curve where the midway point indicates equivalence point. The pKa = pH at half the equivalence volume as shown below in black lines. ( 13 mL/2 = 6.5 mL ) pH = pKa = 4.75 Equivalence volume = 13 mL½ the Equivalence volume = 6.5 mL Equivalence point

CHM-102 CHM-102 UNCW UNCW Titrations: Molarities of Strong and Weak Acids Calculations Calculations  To determine the concentration of the unknown acid (Molarity), we can use the following formula: Moles of unknown acid in the flask = moles of base (titrant added) Molarity acid x volume acid = molarity base x volume base Molarity acid = molarity base x volume base Molarity acid = molarity base x volume base volume acid volume acid Where volume base = equivalence volume determined from the graph Where volume base = equivalence volume determined from the graph (NOTE: Volume base is not the total volume of base added, nor the volume of base added when the solution turned pink).