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8-0025khp - CHM151LL DETERMINATION OF%KHP IN A MIXTURE | 1...

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CHM151LL: D ETERMINATION OF % KHP IN A M IXTURE | 1 Determination of % KHP in a Mixture OBJECTIVE: further develop the skills needed for accurate acid-base titration DISCUSSION: Your mixture contains the acid KHP (potassium hydrogen phthalate, KHC 8 H 4 O 4 ) and a salt, in unknown proportions. You will titrate three samples of the mixture with your standardized NaOH solution, under the assumption that only the KHP will react. The standardized NaOH solution is used as a secondary standard in this titration. When you have added enough base to react with all of the acid in the mixture, you have reached the equivalence point of the titration. When the phenolphthalein turns pink, you have reached the indicator endpoint . Because you know the molar concentration of the base, measuring its volume allows the calculation of the amount of NaOH needed to reach the endpoint. With the right choice of indicator, the endpoint and equivalence point will fall close to one another, so you can closely approximate the amount of base needed to neutralize the equivalent amount of acid. The example uses typical student data to illustrate the calculation process. Example calculation 34.56 mL NaOH soln 1,012 mg mixture ! " # $ % & ' 0.1234 mol NaOH 1 L NaOH soln ! " # $ % & ' 1 mol KHP 1 mol NaOH ! " # $ % & ' 204.2 g KHP 1 mol KHP ! " # $ % & ' 100% = 86.05% KHP in the mixture, by mass Notice how expressing the mixture mass in milligrams allows the cancellation of the milli- prefix (meaning 10 –3 ) with the volume in milliliters of NaOH solution. This is a useful shortcut. Measurement uncertainty As you did last time, you will perform this titration in triplicate. We take the arithmetic mean, or average value, as the best estimate of the true % KHP in the unknown mixture. This is equivalent to saying that we expect all three samples of the unknown to have the same composition. Does this expectation seem reasonable to you?
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