B When the sodium hydroxide pellets have dissolved, add additional distilled water to the bottle until the water level is approximately 1 inch from the top. Stopper and shake thoroughly to mix. This sodium hydroxide solution is the titrant for the analysis to follow. Keep the bottle tightly stoppered when not in use. Set up a buret in a buret clamp. Rinse the buret and fill it with the sodium hydroxide solution just prepared. Part III- Standardization of the Sodium Hydroxide Solution A Clean and dry a small beaker. Take the beaker to the oven that contains the primary standard grade potassium hydrogen phthalate (KHP). Using tongs or an oven mit to protect your hands, remove the bottle of KHP from the oven, and pour a few grams into the beaker. If you pour too much, do not return the KHP to the bottle. Return the bottle of KHP to the oven, and take the beaker containing KHP back to bench. Cover the beaker of KHP with a watch glass. B Allow the KHP to cool to room temperature. While the KHP is cooling, clean three 250-mL Erlenmeyer flasks with soap and water. Rinse the Erlenmeyer flasks with 5- 10 mL portions of distilled water. Label the Erlenmeyer flasks as 1, 2, and 3. C When the KHP is completely cool, weigh three samples of KHP between 0.6 and 0.8 g, one for each of the flasks. Record the exact weight of each KHP sample at least to the nearest milligram, preferably to the nearest 0.1 mg. be certain not to confuse the samples while determining their masses. D 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 0.02mL. E Begin adding NaOH solution from the buret to the sample in the flask, swirling the flask constantly during the addition. 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 flask streaks of red or pink will be visible. They will fade as the flask is swirled. Eventually the red streaks will persist for longer and longer periods, this indicates that approach of the endpoint of the titration. F Begin adding NaOH 1 drop at a time, with constant swirling, until a 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 0.02mL. Repeat the titration of the remaining KHP samples. Record both the initial and final readings of the buret to nearest 0.02mL. Given the molecular weight of potassium hydrogen phthalate is 204.2, calculate the number of moles of KHP in samples 1, 2, and 3. G 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. 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 additional sample of KHP and repeat the titration. Use the average concentration of the NaOH solution for subsequent calculations for the unknown.
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- Winter '16
- Samuel Dillender
- Chemistry, Sodium hydroxide, KHP