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?

KHP K + (aq) + HP – (aq) MW = HP – (aq) + OH – (aq) → P 2– (aq) + H 2 O ( ) KHP is potassium hydrogen phthalate K a = 3.91 x 10 -6

pH? K b = [HP - ][OH - ] ─────── = 2.56 x [P 2- ] P 2- + H 2 O → HP - + OH - [P 2- ]-x x x at equilibrium [P 2- ] = n(P 2- )/Vol n(P 2- ) = 0.5 g/ g/mol = mol V(OH - ) = mol/(.08 mol/L) = 30 mL =.030 L approximately V total = mL = L [P 2- ] = mol/ (.080 L) =.030 M x 2 ───── = 2.56 x ; x = 8.76 x = [OH - ].030 – x pOH = ; pH = = HP - + OH - → P 2- + H 2 O complete at endpoint P 2- + H 2 O → HP - + OH - K = ? = K b = K w /K a Let’s derive it … H + + P 2- → HP - 1/K a H 2 O → H + + OH - K w ________________________ P 2- + H 2 O → HP - + OH - K b = K w /K a = (1.00 x )/(3.91 x ) = 2.56 x 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 H 2 In form containing a central 5-membered ring, which is somewhat strained. In base the In -2 structure opens up and becomes flatter. Phenolphthalein

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

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

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!!!