chm-102 chm-102 uncw uncw titrations: determination of the molarities of strong and weak acids...

CHM-102CHM-102CHM-102

UNCWUNCWUNCW

Titrations: Determination of the Titrations: Determination of the Molarities of Strong and Weak AcidsMolarities of Strong and Weak Acids

General Chemistry 101/102General Chemistry 101/102Laboratory ManualLaboratory Manual

University of North Carolina at WilmingtonUniversity of North Carolina at Wilmington


UNCWUNCWUNCW

Titrations: Molarities of Strong and Weak AcidsTitrations: Molarities of Strong and Weak Acids

• PurposePurpose To study the differences between strong and weak acids by To study the differences between strong and weak acids by

titration with a strong base.titration with a strong base.

• Safety ConsiderationsSafety Considerations Safety glasses must be worn throughout the entire Safety glasses must be worn throughout the entire

experiment, even when cleaning glassware to prevent experiment, even when cleaning glassware to prevent damage due to strong acids and bases.damage due to strong acids and bases.

Be cautious when using unknown solutions and clean up Be cautious when using unknown solutions and clean up any spills immediately. Contact with strong acids or bases any spills immediately. Contact with strong acids or bases will affect skin and create holes in clothing. Wash the will affect skin and create holes in clothing. Wash the affected area thoroughly if you suspect any skin contact any affected area thoroughly if you suspect any skin contact any of unknown solutions.of unknown solutions.

Flush all wastes down the drain with plenty of water.Flush all wastes down the drain with plenty of water.


UNCWUNCWUNCW


• ProcedureProcedure• ProcedureProcedure

Connect the Tablet Connect the Tablet computer, Microlab computer, Microlab unit, and the pH unit, and the pH probe as shown in probe as shown in the photo.the photo.


UNCWUNCWUNCW


• ProcedureProcedure To begin determining pH and volume measurements on the To begin determining pH and volume measurements on the

computer, click on the Applications folder, and select the computer, click on the Applications folder, and select the Microlab prompt. Then double click the “pH Titrations” Microlab prompt. Then double click the “pH Titrations” prompt as shown below.prompt as shown below.


UNCWUNCWUNCW


• ProcedureProcedure Click on the experiment entitled “pH Acid-Base – Click on the experiment entitled “pH Acid-Base –

Temperature Manual Titration”, as shown below.Temperature Manual Titration”, as shown below.


UNCWUNCWUNCW


• ProcedureProcedure Current pH values Current pH values will be displayed in will be displayed in the lower right hand the lower right hand corner of the screen. corner of the screen.

Values of pH Values of pH corresponding to corresponding to volume of titrant volume of titrant added will be added will be collected and collected and displayed in the table displayed in the table on the computer on the computer screen as shown in screen as shown in the photo.the photo.


UNCWUNCWUNCW


• ProcedureProcedure

To begin the To begin the titration, click on titration, click on the “Start” the “Start” prompt.prompt.


UNCWUNCWUNCW


• ProcedureProcedure

Before you begin your Before you begin your titration, enter the initial titration, enter the initial volume as 0 mL. volume as 0 mL.

Press “Enter and Press “Enter and Continue” to allow the Continue” to allow the computer to record the computer to record the corresponding initial pH corresponding initial pH value of the unknown value of the unknown acid.acid.


UNCWUNCWUNCW


• ProcedureProcedure Add 1 mL of base Add 1 mL of base (titrant) to your (titrant) to your unknown acid solution.unknown acid solution.

For best results, place For best results, place the tip of the burette the tip of the burette inside and along the inside and along the side of the beaker, side of the beaker, allowing the titrant to allowing the titrant to run down the side of run down the side of the beaker and into the the beaker and into the solution (rather than solution (rather than hitting the pH electrode hitting the pH electrode or splattering). or splattering).


UNCWUNCWUNCW

Titrations: Molarities of Strong and Weak AcidsTitrations: Molarities of Strong and Weak Acids After the 1 mL of base (titrant) has After the 1 mL of base (titrant) has

been added, swirl the solution with been added, swirl the solution with the electrode. In the keyboard the electrode. In the keyboard input enter the value of “1” for input enter the value of “1” for volume in mL and press “Enter and volume in mL and press “Enter and Continue” to record the pH value.Continue” to record the pH value.

Continue to add 1 mL of titrant at a Continue to add 1 mL of titrant at a time. Stop after each addition of time. Stop after each addition of titrant to stir the solution and titrant to stir the solution and record the volume value and the record the volume value and the corresponding pH value.corresponding pH value.

Enter the volume values in Enter the volume values in sequence (1 mL, 2 mL, 3 mL, 4 sequence (1 mL, 2 mL, 3 mL, 4 mL, 5 mL, etc.) in the input box on mL, 5 mL, etc.) in the input box on the computer even though only 1 the computer even though only 1 mL of titrant is added at a time.mL of titrant is added at a time.


UNCWUNCWUNCW


Add 1 more mL of titrant when Add 1 more mL of titrant when the solution turns dark pink and the solution turns dark pink and remains dark pink after swirling. remains dark pink after swirling. Click the “Stop” prompt. Remove Click the “Stop” prompt. Remove the pH probe immediately and the pH probe immediately and rinse with DI water. Place the rinse with DI water. Place the electrode in pH=7 buffer solution electrode in pH=7 buffer solution until next use.until next use.


UNCWUNCWUNCW

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

Equivalence pH

Equivalence volume

Equivalence point


UNCWUNCWUNCW

Titrations: Graph of Weak AcidTitrations: Graph of Weak Acid

The graph of a weak acid will display a gentler curve where the The graph of a weak acid will display a gentler curve where the midway point indicates equivalence point. The pKa = pH at half the midway point indicates equivalence point. The pKa = pH at half the equivalence volume as shown below in black lines. (equivalence volume as shown below in black lines. (13 mL/2 = 6.5 mL13 mL/2 = 6.5 mL))

pH = pKa = 4.75

Equivalence volume = 13 mL½ the Equivalence volume = 6.5 mL

Equivalence point


UNCWUNCWUNCW

Titrations: Molarities of Strong and Weak AcidsTitrations: Molarities of Strong and Weak Acids• CalculationsCalculations To determine the concentration of the unknown acid (Molarity), To determine the concentration of the unknown acid (Molarity),

we can use the following formula: we can use the following formula:

Moles of unknown acid in the flask = moles of base (titrant added)Moles of unknown acid in the flask = moles of base (titrant added)

MolarityMolarityacid acid x volumex volumeacid acid = molarity = molaritybase base x volumex volumebase base

MolarityMolarityacid acid = = molaritymolaritybase base x volumex volumebase base

volumevolumeacid acid

Where volumeWhere volumebasebase = = equivalence volumeequivalence volume determined from the graph determined from the graph

((NOTE: VolumeNOTE: Volumebasebase is not the total volume of base added, nor the volume is not the total volume of base added, nor the volume

of base added when the solution turned pinkof base added when the solution turned pink).).

chm-102 chm-102 uncw uncw titrations: determination of the molarities of strong and weak acids...

Documents

molarities of strong

strong base

strong acids

ph titrations prompt

values of ph

ph electrode

weak acidsprocedureto

ph probe