Exp. 18: Laboratory Questions Flashcards
Exp. 18: The pH meter was not properly calibrated. How does this experimental error affect the precision of your data?
Unaffected. The precision is unaffected; the reproducibility of the data can still be obtained.
Exp. 18: The pH meter was not properly calibrated. How does this experimental error affect the accuracy of your data? Explain.
Possibly poor. The accuracy may be poor even though the data could be reproducible. The data may be far from the accepted value for the measurement.
Exp. 18: The pH meter was mistakenly calibrated to be 1.0 pH unit higher than the buffer. Part A. Will this miscalibration result in a reported molar concentration of the weak acid being too high, too low, or unaffected? Explain.
Unaffected. The molar concentration of the weak acid is determined by measuring the volume of base titrated to reach the stoichiometric point. While the measured pH at the stoichiometric point will be high, the moles of weak acid neutralized will be unaffected.
Exp. 18: The pH meter was mistakenly calibrated to be 1.0 pH unit higher than the buffer. Part B. Is the determined pKa of the weak acid too high, too low, or unaffected by the miscalibration? Explain.
Too high. The determined pKa of the weak acid would be reported too high. All of the pH values would be recorded one pH unit higher, resulting in an interpreted half equivalence point being too high.
Exp. 18: Part A.4. The pH reading is taken before the pH meter stabilizes. As a result, the pH reading may be too low. Explain.
The NaOH may not have completely reacted with the acid in solution and therefore the pH meter may not have had time to adjust to the higher pH. The time for a homogeneous solution to form has not been reached. The use of a magnetic stir bar aids in keeping the solution homogeneous in our experiment.
Exp. 18: Part A.4. Explain why it is good technique to slow the addition of NaOH titrant near the stoichiometric point.
Slow addition of the NaOH titrant near the stoichiometric point permits the pH meter to equilibrate, thus measuring a more accurate pH as a function of NaOH volume.
Exp. 18: Part A.5. While not absolutely necessary, why is it good technique to add NaOH titrant beyond the stoichiometric point?
Addition of NaOH beyond the stoichiometric point better defines the inflection point in the pH vs NaOH volume curve. The inflection point is the stoichiometric point of the titration.
Exp. 18: Part B.1. The solid acid is dissolved in 100 mL of deionized water, followed by 10 mL of ethanol. How does this added volume affect the reported molar mass of the weak acid—too high, too low, or unaffected? Explain.
Unaffected. The molar mass of the weak acid is dependent upon mass and moles. Since the moles of weak acid are being neutralized by the standardized NaOH solution, extra water or ethanol will not affect the reported molar mass.
Exp. 18: Part B.4. As a result of adding the NaOH titrant too rapidly and an unwillingness to allow the pH meter to equilibrate before reading its pH, the stoichiometric point is ill defined. As a result of this technique error, will the molar mass of the weak acid be reported as too high or too low? Explain.
Too low. If the NaOH solution is added too quickly, the rapid pH change at the stoichiometric point won’t occur until too much NaOH has been added, thereby surpassing the true stoichiometric point. Too much NaOH infers more moles of acid than actually present, resulting in a calculated lower molar mass.
Exp. 18: Part B.4. As a result of adding the NaOH titrant too rapidly and an unwillingness to allow the pH meter to equilibrate before reading its pH, the stoichiometric point is ill defined. As a result of this technique error, will the pKa of the weak acid be reported as too high or too low? Explain.
Too low. The entire pH curve prior to the stoichiometric point will be low if the NaOH is added to quickly (insufficient time allowed for the solution to become homogeneous and the pH meter to equilibrate). As a result of this, the pKa will be lower. Even though the “true” stoichiometric point is surpassed, and therefore, the halfway point is also surpassed, resulting in a pKa that is slightly larger, the overall pH curve will be at lower pH values prior to the stoichiometric point.
Exp. 18: Ideally, how many stoichiometric points would be observed on a pH titration curve for a diprotic acid? Sketch the appearance of its titration curve and explain.
Two stoichiometric points should appear in the titration curve of a diprotic acid, one for each proton that is neutralized by the base.
