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

CO2(g) + H2O() H2CO3(aq) H2CO3(aq) H+

(aq) + HCO3-(aq) Ka = 4.3 x 10-7

Protons from carbonic acid can neutralize hydroxide ions thus changing the concentration of OH- in the solution. Also forms Na2CO3, slightly insoluable salt.

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

KHP                      K+(aq) + HP–

(aq)

MW = 204.2236

HP–(aq) + OH–

(aq) → P2– (aq) + H2O()

KHP is potassium hydrogen phthalate

Ka = 3.91 x 10-6

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

Selection of Indicator

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.

Phenolphthalein

OH OH

O

O + 2OH -

-O O

O

O-

+ 2 H2O

H2In (colorless) In-2 (pink)

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

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

)(

)(

acidmass

basevolppt

average

lowhigh21000

Week 2Start 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

HA & A-

pH

Inflection Point

Equivalence Point

Volume of NaOH

Strong Base/Weak Acid Titration Curve

Use the equivalence pt & halfway equivalence point for calculations...how?

End point needs to be past the equivalence point

HA only

A-

OH-, A-

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!!!