07unkphtitr[1]

07unkphtitr[1] - IDENTIFICATION OF AN UNKNOWN...

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Unformatted text preview: IDENTIFICATION OF AN UNKNOWN IDENTIFICATION OF AN UNKNOWN WEAK ACID WEAK ACID by pH Titration N a O H N a O H 2 4 6 8 10 12 14 pH 10 20 30 40 VO L U M E O F A D D ED N aO H T IT R A T IO N O F S A A N D A S A W IT H N a O H p H v s V O L U M E O F N a O H A D D E D 300 m g SA p K a E M M ? ? [HA] = [A- ] Purpose: To determine the molar mass and pK a and thereby determine the identity of an unknown acid by pH titration Same technique as last exercise! Same technique as last exercise! Concepts: pH, pK a Nernst Equation Calibration and use of the pH meter Effective Molar Mass Titration Curve Stoichiometry Equivalence Point Techniques: Weighing by Difference Using a pH Meter Graphing Apparatus: pH Meter Buret Analytical Balance As usual, we seek to use about 25 ( 25 3) mL of NaOH to reach equivalence point WHY? Optimizes buret precision and produces an easily interpretable graph BUT Molar mass of the acid is unknown STEP 1: DETERMINE TITER of your acid mg of acid that will consume 1 mL of NaOH Everyone uses ~ 200 mg of acid weighed out ACCURATELY into a 150 mL beaker. Titrate to phenolphthalein end-point - No pH METER The TITER is a quantity that relates the NaOH solution to YOUR unknown acid e.g. IF 225.1 mg of your acid react with 18.46 mL of the NaOH, TITER of Your Acid = 225.1 mg / 18.46 mL = 12.19 mg / mL NaOH STEP 2. From TITER , calculate the weight of your acid that would consume 25 3 mL of NaOH e.g. WEIGHT = ( 25 3 mL ) x 12.19 mg / mL = 305 36 mg OF ACID STEP 3. Weigh out an amount of your acid in the above range on the ANALYTICAL BALANCE BY DIFFERENCE No spatulas, no intermediate containers STEP 4. Calculate volume of NaOH required to titrate actual ample . Suppose you weigh out 0.3127 g = 312.7 mg That will require: STEP 5. Check the pH METER with the pH = 4.00 BUFFER. If pH differs from 4.00 by more than 0.10 pH units, ask TA to recalibrate pH METER Titer 312.7 mg 12.19 mg/mL / = 25.65 mL of NaOH The expected end point STEP 6. Titrate using the pH METER Be sure to Stir solution while titrating Get several points along the steepest rise of the pH curve Add NaOH slowly well before the 25.65 mL of added* NaOH calculated in Step 4 Follow titration WELL PAST Equivalence Point at least 10 mL i.e to ~ 36 mL added NaOH in this case Remember that the 25.65 mL is the net added volume , not the buret reading 2 4 6 8...
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07unkphtitr[1] - IDENTIFICATION OF AN UNKNOWN...

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