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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

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

slide3

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()

slide4

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

slide5

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.

procedure
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
calculations
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

week 2

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

slide9

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

determination of ka
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!!!