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Questions and Answers
What is the purpose of the potentiometric titration experiment?
What is the purpose of the potentiometric titration experiment?
To perform a potentiometric titration of an acid to calculate the molecular weight and pKa of an unknown weak monoprotic acid.
What is the equilibrium constant for the dissociation of a weak acid represented as HA?
What is the equilibrium constant for the dissociation of a weak acid represented as HA?
At what point during the titration can you determine the pKa of the weak acid?
At what point during the titration can you determine the pKa of the weak acid?
At the midpoint of the titration, where [H+] = Ka.
The relationship between pH and pKa is given by the equation: pH = -log [H+] = -log Ka = ________.
The relationship between pH and pKa is given by the equation: pH = -log [H+] = -log Ka = ________.
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How should you standardize the pH electrode before starting the titration?
How should you standardize the pH electrode before starting the titration?
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When should you start adding smaller aliquots during the titration?
When should you start adding smaller aliquots during the titration?
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Study Notes
Introduction to Potentiometric Titration
- Potentiometric titration involves measuring the potential across a glass electrode, directly related to H+ ion concentration.
- A sample of a weak acid is titrated, allowing for the determination of the acid's molecular weight and pKa through the titration curve.
- The dissociation of a weak acid (HA) is described by the equation: HA ⇌ H+ + A-.
- The dissociation constant, Ka, is defined as: Ka = [H+][A-] / [HA].
- At the midpoint of titration, [HA] equals [A-], leading to [H+] = Ka, and thus pH = pKa.
Procedure Overview
- Weigh out an unknown weak monoprotic acid sample and dissolve it in approximately 50 mL of water or a water/ethanol mixture if insoluble.
- Transfer the solution to a 200.00 mL volumetric flask, dilute, and mix thoroughly.
- Prepare a 150 mL beaker with 25 mL of water and 50 mL of the acid solution before titration.
- Standardize the pH electrode using pH 4.00 and pH 7.00 buffer solutions.
- Use a stir plate during titration to maintain uniform mixing.
Titration Procedure
- Collect data by measuring pH as NaOH is added from a standardized 50.00 mL buret.
- Focus on regions with significant pH changes: collect more data points in these areas for accuracy.
- In the initial titration, use 1-2 mL steps; follow up with smaller aliquots (0.5 mL, then 0.1 mL) as nearing the endpoint indicated by changes in pH.
- Continue titration until pH exceeds 10-11, recording the titrant volume and corresponding pH readings.
Data Analysis and Results
- Plot the titration curve with volume of titrant added as the x-axis and pH as the y-axis.
- Calculate and plot the first derivative of the titration data to identify key changes in pH.
- The shape of the titration curve aids in determining the equivalence point and the nature of the weak acid.
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Description
Explore the fundamentals of potentiometric titration, focusing on the measurement of potential across a glass electrode. This quiz covers the titration of weak acids, determination of molecular weight, and calculation of pKa, alongside the necessary procedures and calculations involved in the process.
