Weak Acid/Strong Base calculations Acids & Bases Lesson 12 Titration Curves & Weak Acid/Strong Base calculations

Titration Curves  A titration curve is a graph of the pH changes that occur during an acid-base titration versus the volume of acid or base added.   The equivalence point is the end of a titration where the stoichiometry of the reaction is exactly satisfied, or moles H+ = moles OH-. The transition point refers to when an indicator changes color and [HInd] = [Ind-].

Three types of Titrations Strong acid and strong base Weak acid and strong base Weak base and strong acid

Choosing an Indicator When you choose an indicator, you must pick one so that the transition point of the indicator matches the equivalence point of the titration. Look at the salts produced, if   Rule of thumb Salt Equivalence Point SA + SB Neutral 7 WA + SB Basic 9 WB + SA Acidic 5

1. Titration Curve: Strong Acid and Strong Base HCl + KOH → KCl + HOH neutral salt Indicator pH = 7 Bromothymol Blue- see Ind Chart 50 mL of 0.10 M KOH is added to 25 mL of 0.10 M HCl We start here The pH of 0.10 M HCl is 1.0 The pH of 0.10 M KOH is 13.0 pH Volume .10 M KOH added 14 7 25 50 0.10 M KOH 0.10 M HCl Neutral Salt pH = 7.0

2. Titration Curve: Weak Acid and Strong Base HCN + KOH → KCN + H2O basic salt Indicator pH = 9 Phenolphthalein- see ind. chart 20 mL of 1.0 M HCN is added to 10 mL of 1.0 M KOH is added to We end here pH = 3 We start here pH = 14 pH Volume 1.0 M HCN added 14 7 1.0 M KOH Basic Salt pH = 9 1.0 M HCN 10 20

3. Titration Curve: Strong Acid and Weak Base HCl + NH3 → NH4+ + Cl- acid salt Indicator pH = 5 Methyl Red- see ind chart 60 mL of 1.0 M NH3 is added to 30 mL of 1.0 M HCl 1.0 M NH3 pH  10 pH Volume 1.0 M NH3 added 14 7 Acid Salt pH  5 30 60 1.0 M HCl pH = 0

4. Match the Curve with the Reaction pH Volume .10 M KOH added 14 7 25 50 A. 3HCl + Al(OH)3 → AlCl3 + 3HOH B. HCl + KOH → KCl + HOH C. HCN + KOH → KCN + HOH

5. Match the Curve with the Reaction pH Volume 1.0 M NH3 added 14 7 30 60 A. 3HCl + Al(OH)3 → AlCl3 + 3HOH B. HCl + KOH → KCl + HOH C. HCN + KOH → KCN + HOH

6. Match the Curve with the Reaction pH Volume 1.0 M NH3 added 14 7 30 60 A. 3HCl + Al(OH)3 → AlCl3 + 3HOH B. HCl + KOH → KCl + HOH C. HCN + KOH → KCN + HOH

3 Types of Titrations Strong acid and strong base Weak acid and strong base Weak base and strong acid

Strong acid and Strong base They produce neutral salts and water. We have practiced these calculations before.

Weak acid and strong base We expect to produce a basic solution. Therefore, equivalence point at pH > 7 Indicators around pH = 9; between (8-10) Rules found through experiments to make your life EASY.

Weak acid and strong base  

Weak acid and strong base Rules: pKa = pH ½  means pH when half Volume is used, NOT HALF pH!!!! Ka = 10 –pKa or Ka = 10-pH ½ Ka = [ H3O+]1/2 where [ H3O+]1/2 can be found from pH 1/2 [Acid]equilibrium = [ H3O+]2 ….. this [H3O+] is found from pH initial Ka Ka found from above equation [Acid]original= [Acid]eq + [H3O] from pH initial

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of Furoic acid with NaOH. 28.8mL = volume of NaOH required to get to equivalence point 2.021 = initial pH of furoic acid solution 3.170 = pH at 14.4mL point of titration A) Calculate the Ka value for furoic acid

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of Furoic acid with NaOH. 28.8mL = volume of NaOH required to get to equivalence point 2.021 = initial pH of furoic acid solution 3.170 = pH at 14.4mL point of titration B) Calculate the initial concentration of the furoic acid solution

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of Furoic acid with NaOH. 28.8mL = volume of NaOH required to get to equivalence point 2.021 = initial pH of furoic acid solution 3.170 = pH at 14.4mL point of titration C) Calculate the concentration of the NaOH used.

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of Furoic acid with NaOH. 28.8mL = volume of NaOH required to get to equivalence point 2.021 = initial pH of furoic acid solution 3.170 = pH at 14.4mL point of titration D) Is the titration mixture acidic, basic or neutral at the equivalence point?

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of Furoic acid with NaOH. 28.8mL = volume of NaOH required to get to equivalence point 2.021 = initial pH of furoic acid solution 3.170 = pH at 14.4mL point of titration E) Suggest a suitable indicator for the titration

Weak acid and strong base TRY: The following data was obtained during the titration of 25.0 mL of propanoic acid, with 0.100 M NaOH. Initial pH= 2.950 Volume of NaOH used= 23.8 mL pH at 11.9 mL = 4.873 A) What is Ka for propanoic acid?

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of propanoic acid, with 0.100 M NaOH. Initial pH= 2.950 Volume of NaOH used= 23.8 mL pH at 11.9 mL = 4.873 B) What is the concentration for propanoic acid based on the [NaOH] and the volumes of the reactants?

Weak acid and strong base The following data was obtained during the titration of 25.0 mL of propanoic acid, with 0.100 M NaOH. Initial pH= 2.950 Volume of NaOH used= 23.8 mL pH at 11.9 mL = 4.873 C) What is the concentration of propanoic acid as calculated from your value of Ka, found in part,a and the initial pH value?

Homework #129 on page 177