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Questions and Answers
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?
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 = ________.
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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?
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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.
