Exp. 9: Laboratory Questions Flashcards
Exp. 9: Part A.1. Pure potassium hydrogen phthalate is used for the standardization of the sodium hydroxide solution. Suppose that the potassium hydrogen phthalate is not completely dry. Will the reported molar concentration of the sodium hydroxide solution be too high, too low, or unaffected because of the moistness of the potassium hydrogen phthalate? Explain.
Too high. The reported mass and moles of KHC8H4O4 (KHP) will be too high if wet and therefore the moles of NaOH will be reported too high and the molar concentration of NaOH will be reported too high.
Exp. 9: The student forgot to prepare any boiled, deionized water for the preparation of the NaOH solution and then forgot to cap the bottle. Will the concentration of the NaOH solution be greater than, less than, or unaffected by this carelessness? Explain.
Less than. Carbon dioxide, being a weak acid when dissolved in water, will react with the NaOH, lowering its molar concentration.
Exp. 9: Part A.6. A drop of the NaOH titrant adheres to the side of the buret (because of a dirty buret) between the initial and final readings for the titration. As a result of the “clean glass” error, will the molar concentration of the NaOH solution be reported as too high or too low? Explain.
Too low. The volume of the drop is included in the difference between the initial and final buret readings, but not dispensed. An erred larger volume of NaOH results in a molar concentration of NaOH being too low.
Exp. 9: Part A. The mass of KHC8H4O4 is measured to the nearest milligram; however, the volume of water in which it is dissolved is never of concern—water is even added to the wall of the Erlenmeyer flask during the titration. Explain why water added to the KHC8H4O4 has no effect on the data, whereas water added to the NaOH solution may drastically affect the data.
Water added to the NaOH solution dilutes the solution and thus lowers its molar concentration; we will need the exact NaOH concentration for the calculations in this experiment. While the moles of NaOH neutralized by the KHP remain constant, the added water lowers the NaOH concentration. Conversely, the moles of KHP are unaffected by added water because its moles were determined by a mass measurement and we are not concerned with the KHP concentration.
Exp. 9: Part B.2. The wall of the Erlenmeyer flask is occasionally rinsed with water from the wash bottle (see Part A.6) during the analysis of the acid solution. Will this technique result in the molar concentration of the acid solution being reported as too high, too low, or unaffected? Explain.
No effect. The moles and volume of the acid were measured with the solution dispenser. Added de- ionized water does not affect either of these values. The moles of acid are neutralized with the NaOH titrant.
Exp. 9: Parts A.6 and B.2. For the standardization of the NaOH solution in Part A.6, the endpoint was consistently reproduced to a faint pink color. However, the endpoint for the titration of the acid solution in Part B.2 was consistently reproduced to a dark pink color. Will the reported molar concentration of the acid solution be too high, too low, or unaffected by the differences in the colors
of the endpoints. Explain.
Too high. If the unknown acid concentration is titrated beyond the “correct” (faint pink) endpoint, the dispensed moles of NaOH is too high and therefore, the calculated moles of acid in the measured volume would also be too high, resulting in a reported molar concentration of the acid that is too high.
