lab 8 - 1 EXPERIMENT 8 CHARACTERIZATION OF A WEAK ACID 07F...

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1 EXPERIMENT 8 CHARACTERIZATION OF A WEAK ACID 07F It is apparent from tables in the texts for CHEM 111 and for CHEM 119 that all weak acids do not have identical ionization constants. Therefore, acids may be identified by their ionization constants (or pK A ) and equivalent weights. In this experiment you will determine the equivalent weight and the pK A of a solid sample of an unknown pure weak acid from a Gran plot. The equation for the Gran plot is derived and discussed on p. 211 – 212 in Harris. Each unknown is an acid salt of the form, MHX, in which you are titrating one proton. Do not dry the samples. They may decompose. A. Standard Titration Weigh ~ 0.3 g (± 0.1 mg) of the unknown into an Erlenmeyer flask and dissolve in 50-75 mL of distilled water – enough that the solution doesn’t splash much while being stirred. Add 2 – 4 drops of phenolphthalein and titrate with standard NaOH to the first permanent appearance of the pale pink color. Near the end point wash the sides of the flask with small amounts of distilled water to ensure that no sample is lost on the walls. The equivalent weight of the acid is calculated in the usual fashion from ( ) = = = acid wt eq acid eq 1 g acid, wt acid wt meq acid meq 1 mg) acid, (wt acid meq # base meq # N * V B B in which V B
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