Acids & BasesLesson 13

Weak Base/Strong Acid calculations

Strong acid and strong base

Weak acid and STRONG BASE

Weak base and STRONG ACID

Three types of Titrations

Choosing an IndicatorWhen 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

3. Titration Curve: Strong Acid and Weak BaseHCl + NH3 → NH4

+ + Cl- acid saltIndicator pH = 5 Methyl Red- see ind chart60 mL of 1.0 M NH3 is added to 30 mL of 1.0 M HCl

pH

Volume 1.0 M NH3 added

14

7

0

0 30 60

1.0 M HCl pH = 0

Acid Salt pH 5

1.0 M NH3 pH 10

Weak base and STRONG ACID• We expect to produce An acidic solution.

• Therefore, equivalence point at pH <7

• Indicators around pH = 5

• Rules found through experiments to make your life EASY.

Weak base and STRONG ACID

Rules: pKb = pOH ½ means pH when half Volume is used, NOT HALF pH!!!!

Kb = 10 –pKb or Kb = 10-pOH ½

Kb = [ OH-]1/2 where [OH]1/2 can be found from pOH 1/2

[Base]equilibrium = [ OH-]2 ….. this [OH] is found from pH or pOH initial

Kb Kb found from above equation

[Base]original= [Base]eq + [OH] from pH initial

Weak base and STRONG ACIDThe following data was obtained when titrating ethylamine, a monoprotic weak base, with HCl25.00 mL= volume of ethylamine solution titrated

19.22 mL = volume of HCl required to get to equivalence point

11.855 = initial pH of ethylamine solution

10.807= pH at 9.60 mL point of titration

A) Calculate the Kb value for ethylamine

Weak base and STRONG ACIDThe following data was obtained when titrating ethylamine, a monoprotic weak base, with HCl25.00 mL= volume of ethylamine solution titrated

19.22 mL = volume of HCl required to get to equivalence point

11.855 = initial pH of ethylamine solution

10.807= pH at 9.60 mL point of titration

B) Calculate the initial [ethylamine]

Weak base and STRONG ACIDThe following data was obtained when titrating ethylamine, a monoprotic weak base, with HCl25.00 mL= volume of ethylamine solution titrated

19.22 mL = volume of HCl required to get to equivalence point

11.855 = initial pH of ethylamine solution

10.807= pH at 9.60 mL point of titration

C) Calculate the [HCl] used

Weak base and STRONG ACIDThe following data was obtained when titrating ethylamine, a monoprotic weak base, with HCl25.00 mL= volume of ethylamine solution titrated

19.22 mL = volume of HCl required to get to equivalence point

11.855 = initial pH of ethylamine solution

10.807= pH at 9.60 mL point of titration

D) Suggest a suitable indicator.

Homework

#127 on page 176