Experiment 2 (5) - Experiment 2 PREPARATION OF A STANDARD...

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Experiment 2 PREPARATION OF A STANDARD SODIUM HYDROXIDE SOLUTION, DETERMINATION OF PURITY OF IMPURE KHP AND STANDARDIZATION OF HYDROCHLORIC ACID 1 lab period In this experiment, we prepare solutions of NaOH and HCl . The NaOH will be used to determine the purity of an impure solid acid and the HCl will be used later. We will require a knowledge of the exact concentration of the two solutions, but it is not convenient to weigh out solid NaOH (as it is very hygroscopic), or to measure out concentrated HCl. We do not know the composition of these materials well enough to obtain accurate numbers for our solutions. Instead, we take another approach. We will use the primary standard acid, potassium hydrogen phthalate (a solid), to make an acid solution of known concentration. We can titrate this solution with our NaOH solution to determine the NaOH concentration, as discussed below. Then, with our standardized NaOH solution, we can both determine the purity of an impure sample of KHP and titrate the HCl solution and accurately determine the HCl concentration. Thus, the concentrations determined for the NaOH and HCl solutions ultimately depend on the weight of our primary standard taken and the volumes of the solutions needed to react. Water which is in contact with the atmosphere can absorb carbon dioxide. Dissolved carbon dioxide acts as a weak acid in aqueous solutions as shown in the following reactions. CO 2 ( g ) + H 2 O H + + HCO 3 - HCO 3 - H + + CO 3 2- If basic solutions are prepared using water which contains dissolved carbon dioxide, a portion of the base reacts with the carbon dioxide to yield hydrogen carbonate or carbonate. For this reason standard sodium hydroxide solution is prepared using water from which all carbon dioxide has been removed. Standard sodium hydroxide solutions cannot be prepared by weighing the solid, since the reagent often contains significant amounts of impurities and is highly hygroscopic. Standard sodium hydroxide solutions are usually prepared by diluting a 50% aqueous solution of
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Experiment 2 (5) - Experiment 2 PREPARATION OF A STANDARD...

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