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Titrations: Determination of the Molarities of Strong and Weak Acids

Titrations: Determination of the Molarities of Strong and Weak Acids. General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington. Safety Considerations. Titrations: Molarities of Strong and Weak Acids. Purpose.

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Titrations: Determination of the Molarities of Strong and Weak Acids

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  1. Titrations: Determination of the Molarities of Strong and Weak Acids General Chemistry 101/102Laboratory ManualUniversity of North Carolina at Wilmington

  2. Safety Considerations Titrations: Molarities of Strong and Weak Acids • Purpose • To study the differences between strong and weak acids by titration with a strong base. • 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.

  3. Titrations: Molarities of Strong and Weak Acids • Procedure • Procedure • Connect the Tablet computer, Microlab unit, and the pH probe as shown in the photo.

  4. Titrations: Molarities of Strong and Weak Acids • 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.

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

  6. Titrations: Molarities of Strong and Weak Acids • 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.

  7. Titrations: Molarities of Strong and Weak Acids • Procedure • To begin the titration, click on the “Start” prompt.

  8. Titrations: Molarities of Strong and Weak Acids • 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.

  9. Titrations: Molarities of Strong and Weak Acids • 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).

  10. 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.

  11. 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.

  12. 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 point Equivalence pH Equivalence volume

  13. 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) Equivalence point pH = pKa = 4.75 ½ the Equivalence volume = 6.5 mL Equivalence volume = 13 mL

  14. Titrations: Molarities of Strong and Weak Acids • 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) Molarityacid x volumeacid = molaritybase x volumebase • Molarityacid = molaritybase x volumebase volumeacid Where volumebase = equivalence volume determined from the graph (NOTE: Volumebase is not the total volume of base added, nor the volume of base added when the solution turned pink).

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