Question: Using the provided materials, determine the molar mass and pKa of an unknown monoprotic we — Laboratory Practical Chemistry Question
Problem Context
Question: Using the provided materials, determine the molar mass and pKa of an unknown monoprotic weak acid.
Give a brief description of your experimental plan.
Model Answer
Students need to provide an indication of number of drops in a milliliter. Starting with a known mass of the unknown weak acid, determine the molar mass of the weak acid by an acid-base titration. One will note how many drops of known concentration of base it takes to reach a phenolphthalein end point. Using the volume of base and known concentration, determine the moles of NaOH used to reach the end point. As the acid is monoprotic, the moles of base equal the moles of acid. Using the moles of acid and mass of weak acid used, determine the molar mass of the unknown acid. To determine the pKa, an experiment should be conducted to reach the half equivalence point. After determining the number of drops of NaOH it takes to reach a phenolphthalein end point, perform a titration with the same volume of acid and half the number of drops of base. Approximate the pH of this solution using the provided indicators.
Record your data/observations.
Model Answer
Sample data from Fall 2022 UMW Undergraduate Research Student Iaroslavna Kovalenko
Show all calculations.
The molar mass of the unknown weak acid is ____ g/mol. The pKa of the weak acid is _______.
Model Answer
The pKa portion was a little more challenging. Color of solution after adding 8 drops of NaOH to the acid:
Based on these observations, one could approximate the pKa in the color change region of methyl orange or bromocresol green – around pKa of 4. Alternatively, students could titrate the unknown acid with the different indicators and note the color and the midway point of the titration.