Lab Schedule. Pages xix and xx (Introduction) of your laboratory manual. Orange Team (Sections AB1, AB3, AB5, BB1, BB3, BB5, BB7): Three (3) week lab

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1 Lab Schedule Pages xix and xx (Introduction) of your laboratory manual. Orange Team (Sections AB1, AB3, AB5, BB1, BB3, BB5, BB7): Three (3) week lab

2 Lab Schedule Pages xix and xx (Introduction) of your laboratory manual. Blue Team (Sections AB2, AB4, AB6, BB2, BB4, BB6, BB8): No Chromatography lab Three (3) week lab

3 Characterization of a Weak Acid Monoprotic and diprotic acids Acid base Equilibrium Molar Mass Calculation pka determination using Henderson-Hasselbach equation Equivalence point determination Top Ten List

4 Analytical Methods Gravimetry Colorimetry Solubility/Conductivity Acid-base Titration

5 Acids (p65) A monoprotic acid molecule can donate one hydrogen ion to solution. A diprotic acid molecule can donate two hydrogen ions to solution. Goal: identify your unknown acid How: determine molar mass and pka of your unknown acid

6 Acid-base equilibrium (monoprotic acid) HA H + + A - K eq = K a = [H + ][A - ]/[HA] pk a = - logka = - log [H + ] - log ([A - ]/[HA]) pk a = ph - log ([A - ]/[HA]) ph = pk a + log ([A - ]/[HA]) or ph = pk a - log ([HA]/[A - ]) Henderson-Hasselbach equation

7 HA H + + A - Acid-base titration (monoprotic acid) Critical: Equivalence Point (E) At the equivalence point (E): moles of acid = moles of base (NaOH) added volume (25 ml): HA is completely converted into A - At ½ of volume of equivalence point (12.5 ml): Half of HA is converted into A - i.e., [HA] = [A - ]. Then pka = ph - log ([A-]/[HA]) = ph log (1) = ph 1/2

8 Monoprotic Acid: Molar Mass Calculation moles of HA = moles of NaOH (at the equivalence point) mass of HA = [NaOH] x V eq pt molar mass of HA molar mass of HA = mass of HA [NaOH] x Veq pt

9 Monoprotic Acid: pka Calculation Ka = [H + ][A - ]/[HA] pka = ph - log([a - ]/[HA]) pka' = ph 1/4 - log (1/3) (ph 1/4 : ph reading at 1/4 of V eq pt ) pka'' = ph 1/2 - log (1) (ph 1/2 : ph reading at 1/2 of V eq pt ) pka''' = ph 3/4 - log (3) (ph 3/4 : ph reading at 3/4 of V eq pt ) pka = (pka' + pka'' + pka''')/3

10 Two different ways of determining the equivalence point By observing a color change using an indicator (also called end point): e.g. phenolphthalein: Acidic (colorless) Basic (pink) By observing the ph change directly using a ph meter and find the equivalence point by computer

11 Determination of the Equivalence point using computer First Derivative (ΔpH/Vol) Vol. of NaOH (ml) First Derivative (ΔpH/Vol) Vol. of NaOH (ml)

12 Determination of the Equivalence point using computer

13 Acid-base equilibrium (diprotic acid) H 2 A H + + HA - HA - H + + A 2- K a1 = [H + ][HA - ]/[H 2 A] K a2 = [H + ][A 2- ]/[HA - ] Critical: Equivalence Points (E)

14 Diprotic Acid: Molar mass Calculation moles of H 2 A = moles of NaOH (at the 1st equivalence point) mass of H 2 A = [NaOH] x V 1st eq pt molar mass of H A molar mass of H 2 A = 2 mass of H [NaOH] x V 2 A 1st eq pt

15 Diprotic Acid: pka Calculation Ka 1 = [H + ][HA - ]/[H 2 A] pka 1 = ph - log([ha - ]/[H 2 A]) pka 1 ' = ph 1/4 - log (1/3) (ph 1/4 : ph reading at 1/4 of V 1st eq pt ) pka 1 '' = ph 1/2 - log (1) (ph 1/2 : ph reading at 1/2 of V 1st eq pt ) pka 1 ''' = ph 3/4 - log (3) (ph 3/4 : ph reading at 3/4 of V 1st eq pt ) pka 1 = (pka 1 ' + pka 1 '' + pka 1 ''')/3 Ka 2 = [H + ][A 2- ]/[HA - ] pka 2 = ph - log([a 2- ]/[HA - ]) pka 2 ' = ph 1/4 - log (1/3) (ph 1/4 : ph reading at 1/4 of (V 2nd eq pt - V 1st eq pt )) pka 2 '' = ph 1/2 - log (1) (ph 1/2 : ph reading at 1/2 of (V 2nd eq pt - V 1st eq pt )) pka 2 ''' = ph 3/4 - log (3) (ph 3/4 : ph reading at 3/4 of (V 2nd eq pt - V 1st eq pt )) pka 2 = (pka 2 ' + pka 2 '' + pka 2 ''')/3

16 Standardization of NaOH The exact concentration of NaOH is not known (It changes with time) NaOH It can be determined by titration using an acid (KHP) of known concentration Unknown weak acid concentration: can be determined by titration using the NaOH that has been standardized. Acid

17 Known acid: KHP Note: the P in KHP stands for phthalate, not phosphorus.

Total number of titrations 3 NaOH standardizations (Manual titrations using ph indicator) 3 titrations of your unknown (Computer titrations generating ph

18 Total number of titrations 3 NaOH standardizations (Manual titrations using ph indicator) 3 titrations of your unknown (Computer titrations generating ph titration curve) 3 titration of your partner s unknown (Computer titrations generating ph titration curve) NaOH standardizations titrations of your unknown

19 Top ten silly act list 1. My unknown acid does not dissolve in water (because I kept my unknown calcium in my draw and mistaken it as my unknown acid) 2. I dried (and melted) my unknown acid 3. I forgot to dry my known acid (KHP). 4. My manual titration did not turn color no matter how much NaOH I added 5. During manual titrations, I washed the tip of buret with NaOH. 6. I fill my buret with water. 7. I did not align my drop counter correctly, allowing some streaming or double counting) 8. I did not use the same calibrated buret for all my experiments. 9. I did not mix my NaOH THOROUGHLY (20 times). 10. I did not dissolve my KHP or unknown in boiled water COMPLETELY.

Title Experiment 2: Acid-Base Reactions: the Identification of an Unknown Weak Acid

Title Experiment 2: Acid-Base Reactions: the Identification of an Unknown Weak Acid Title Experiment : Acid-Base Reactions: the Identification of an Unknown Weak Acid Name Manraj Gill (Lab section 1) Introduction The end goal of the set of experiments and analysis performed in this lab