Be certain not to confuse the samples while determining their masses. ● At your lab bench, add 100 mL of water to KHP sample 1. Add 2–3 drops of phenolphthalein indicator solution. Swirl to dissolve the KHP sample completely. ● Record the initial reading of the NaOH solution in the buret to the nearest 0.02 mL, remembering to read across the bottom of the curved solution surface (meniscus). ● Begin adding NaOH solution from the buret to the sample in the flask, swirling the flask constantly during the addition. (See Figure 27-2) If your solution was prepared correctly, and if your KHP samples are of the correct size, the titration should require at least 20 mL of NaOH solution. As the NaOH solution enters the solution in the Erlenmeyer flask, streaks of red or pink will be visible. They will fade as the flask is swirled. ● Eventually the red streaks will persist for a longer and longer period of time. This indicates the approach of the endpoint of the titration. ● Begin adding NaOH one drop at a time, with constant swirling, until one single drop of NaOH causes a permanent pale pink color that does not fade on swirling. Record the reading of the buret to the nearest 0.02 mL. ● Repeat the titration of the remaining KHP samples. Record both initial and final readings of the buret to the nearest 0.02 mL. ● Given that the molecular weight of potassium hydrogen phthalate is 204.2, calculate the number of moles of KHP in samples 1, 2, and 3. ● From the number of moles of KHP present in each sample, and from the volume of NaOH solution used to titrate the sample, calculate the concentration of NaOH in the titrant solution in moles per liter (molarity of NaOH, M). The reaction between NaOH and KHP is of 1:1 stoichiometry. ● If your three values for the concentration differ by more than 1%, weigh out an addition ● al sample of KHP and repeat the titration. Use the average concentration of the NaOH solution for subsequent calculations for the unknown. Analysis of the Unknown Acid Sample ● Two types of unknown acid samples may be provided. Your instructor may ask you to analyze either or both of these, 1. Analysis of a Vinegar Solution ● Vinegar is a dilute solution of acetic acid and can be effectively titrated with NaOH using the phenolphthalein endpoint. ● Clean and dry a small beaker, and obtain 25–30 mL of the unknown vinegar solution. Cover the vinegar solution with a watch glass to prevent evaporation. Record the code number of the sample. If the vinegar is a commercial product, record its brand name. ● Clean three Erlenmeyer flasks, and labeled as samples 1, 2, and 3. Rinse the flasks with small portions of distilled water.
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- Winter '17
- Chemistry, Titration, Sodium hydroxide, KHP, sodium hydroxide solution, Apchemistry, Acidbase, TitrationLab, AcidBaseLab