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Volumetric Acid Determination

Volumetric Acid Determination. Week 1 - Standardize NaOH w/ pure KHP Determine Unknown %KHP Week 2 – Continue Unknown % KHP pH meter titration of an unknown acid. From last week…why Boil Water for Preparation of NaOH Solution?.

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Volumetric Acid Determination

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  1. Volumetric Acid Determination Week 1 - Standardize NaOH w/ pure KHP Determine Unknown %KHP Week 2 – Continue Unknown % KHP pH meter titration of an unknown acid

  2. From last week…why Boil Water for Preparation of NaOH Solution?

  3. KHP                      K+(aq) + HP–(aq) KHP is potassium hydrogen phthalate Ka = 3.91 x 10-6 MW = 204.2236 HP–(aq) + OH– (aq)→ P2–(aq) + H2O()

  4. Selection of Indicator pH? Kb = [HP-][OH-] ─────── = 2.56 x 10-9 [P2-] P2- + H2O → HP- + OH- [P2-]-x x x at equilibrium [P2-] = n(P2-)/Vol n(P2-) = 0.5 g/204.2236 g/mol = .00245 mol V(OH-) = .00245 mol/(.08 mol/L) = 30 mL = .030 L approximately Vtotal = 50 + 30 mL = 0.080 L [P2-] = 0.00245 mol/ (.080 L) = .030 M x2 ───── = 2.56 x 10-9 ; x = 8.76 x 10-6 = [OH-] .030 – x pOH = 5.063 ; pH = 14 - 5.063 = 8.937 HP- + OH- → P2- + H2O complete at endpoint P2- + H2O → HP- + OH- K = ? = Kb = Kw/Ka Let’s derive it … H+ + P2- → HP- 1/Ka H2O → H+ + OH- Kw ________________________ P2- + H2O → HP- + OH- Kb = Kw/Ka = (1.00 x 10-14)/(3.91 x 10-6) = 2.56 x 10-9

  5. Phenolphthalein The color change in phenolphthalein is due to a change in structure of the molecule.  In acid, the molecule is in its H2In form containing a central 5-membered ring, which is somewhat strained. In base the In-2 structure opens up and becomes flatter.

  6. Procedure • Dissolve KHP in ~50 mL DI water, warm if necessary • Add 2-3 drops of indicator • Titrate until faint pink persists • For the unknown %KHP, adjust mass appropriately for ~35mL

  7. Calculations Quick Check of Precision Use to calculate To determine % KHP vol NaOH  moles NaOH  moles KHP  mass KHP mass% KHP (concentration of NaOH)(stoichiometry)(molar mass)mass KHP x 100% mass sample

  8. Week 2 Start unknown %KHP Then go back to pure KHP if necessary pH meter- half the class starts first, then teach the second half by 3:15 PM Continue titrations with time left Clean up after yourselves

  9. Strong Base/Weak Acid Titration Curve OH-, A- A- pH Inflection Point HA & A- Equivalence Point HA only Volume of NaOH Use the equivalence pt & halfway equivalence point for calculations...how? End point needs to be past the equivalence point

  10. Determination of Ka • Use pH meter data to create titration curve • Dissolve 0.4 g of acid (not KHP, record to .1mg) in 250 mL beaker with ~75 mL of water • Record pH every .2-.3 unit change or every 5mL • Graph paper from me (better than printer) Then clean up after yourselves! Wash vials and leave on rack near storage dessicators and ovens. Otherwise, points will be deducted!!!

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