chem Eq 4

pH of weak electrolites

Indicators

The Henderson-Hasselbalch equation

Titration

Examples for strong/weak electrolites

pH of weak electrolites

Indicators

The Henderson-Hasselbalch equation

Titration

Examples for strong/weak electrolites

Dr. M. Sasvári: Medical Chemistry LecturesDr. M. Sasvári: Medical Chemistry Lectures 4. 4.

chem Eq 4

pKa = -log Ka

pH of Weak Acids: CalculationspH of Weak Acids: Calculations

pH=pKa-logc

2

Calculation of pH from : [H+]= c

Ka =

1. No common ion:

Assumptions:

[H+] = [A-] cKa =[H+][H+]

[H+]= Kac

Calculation of pH from Ka

Calculation of pH from pKa

The same assumptions as above

2. is small: [HA] c

chem Eq 4

pH of Weak Bases: CalculationspH of Weak Bases: Calculations

pOH = -lg [OH-]

pH = 14-pOH

pH = 14-pOH

Calculation of pH from : [OH-]= c

Kb is an equilibrium constantof basic dissociation

(concentration of H2O included)

NH3 + H2O NH4++ OH-

Kb =

[OH-]

Calculation of pH from Kb

[OH-]= Kbc1. No common ion:Assumptions:

[OH-] = [NH4+]

2. is small: [NH3] c

pOH=pKb-logc2

Calculation of pOH from pKb

The same assumptions as above

chem Eq 4

Acid-Base IndicatorsAcid-Base Indicators

chem Eq 4

Effect of strong acid on the ionization of a weak acidEffect of strong acid on the ionization of a weak acid

HA + A1-H+Strong acid

Weak acid HA H+ + A2-

Common ion: [H+] Mainly from the strong acid

Common ion: [H+] Mainly from the strong acid

Ionization of weak acid is inhibited by the strong acid

[HA] c weak acid

Ionization of weak acid is inhibited by the strong acid

[HA] c weak acid

chem Eq 4

Indicator: a weak acid, e.g. methyl orange (HA)

HA + A-H+HCl

Indicator H+HA + A-

Color in acidic range Color in basic range

chem Eq 4

pH = pKa + lg

The Henderson-Hasselbalch equationThe Henderson-Hasselbalch equation

HA H+ + A-

If a common ion is present:

[H+] = [A-] [H+] =

Ka

Ka =[H+][A-]

[HA]

the ratio of the two formsthe ratio of the two formsfor a compoundfor a compound

If pH = pKa, the two forms are equalIf pH = pKa, the two forms are equal

protonated form

protonated form

deprotonated form

deprotonated form

The pH will determineThe pH will determine

chem Eq 4

pH = pKa + lg

Colorchange

Acid-Base IndicatorsAcid-Base Indicators

HInd Ind-H+ +

Intermediate colorpH = pKa

pH < pKa pH > pKa

pH = pKa=

pH = pKa-1

pH = pKa+1

=

=

chem Eq 4

pH 1 2 3 4 5 6 7 8 9 10 11

Methyl orange

Phenolphtalein

Bromophenol blue

chem Eq 4

Acid-BaseTitrations

Acid-BaseTitrations

chem Eq 4

0

4

6

8

10

12

14

0 20 40 60 80

2

Methyl orange

HCl present

Eqv. Point pH=7

Titration of strong acid with a strong baseTitration of strong acid with a strong base

HCl neutralizedNaOH present

Unknown: (0.1 N) HCl (50 ml) titrated with 0.1 N NaOH:

chem Eq 4

Start:

-lg 0.1 = 15/50 = 0.150 ml 0.1 N HCl 50*0.1 = 5 meqv

pH = -lg cc= meqv/mlc*v= meqv

Calculations-Titration

0.1 N HCl (50ml)

0.1 N NaOH (0 ml)

chem Eq 4

pH = -lg 0.067 = 1.2

+ 10 ml 0.1 N NaOH - 10*0.1 = 1 meqv

60 ml 4 meqv

50 ml 0.1 N HCl 50*0.1 = 5 meqv

4/60 = 0.067 N

HCl excess

HW: +20 ml, +40 ml, +49 ml 0.1 N NaOH pH = ?

Calculations-Titration

0.1 N HCl (50ml)

0.1 N NaOH (10 ml)

c= meqv/mlc*v= meqv

chem Eq 4

100 ml 0 meqv+ 50 ml 0.1 N NaOH - 50*0.1 = 5 meqv

50 ml 0.1 N HCl 50*0.1 = 5 meqv

pH = 7

Neutralization

Calculations-Titration

0.1 N HCl (50ml)

0.1 N NaOH ( 50 ml)

chem Eq 4

101 ml 0.1 meqv+ 51 ml 0.1 N NaOH - 51*0.1 = 5.1 meqv

50 ml 0.1 N HCl 50*0.1 = 5 meqv

0.1/101=0.001 pOH = 3 pH = 11

Base excess

Calculations-Titration

0.1 N HCl (50ml)

0.1 N NaOH ( 51 ml)

chem Eq 4

Unknown: (0.1 N) NaOH (50 ml) titrated with 0.1 N HCl:

0

2

4

6

8

10

12

14

0 20 40 60 80

pH

ml of HCl

Eqv. point: pH=7 neutralization

Start: pH =13 NaOH excess

Titration of strong base with a strong acidTitration of strong base with a strong acid

HCl excessEnd: pH below 2

chem Eq 4

Acidic strenght of HYDRIDESAcidic strenght of HYDRIDES

NH3 < H2O < HFammonia water hydrogenfluoride

FH

High electronnegativity atom → electron withdrawing effectElectron density decreases around H → enhanced acidity

V. VI. VII. VIII.

N O F

EN increases

chem Eq 4

Acidic strengthincreases by atomic size

V. VI. VII. VIII.

BrClF

EN

incr

ease

s

I

OS

Siz

e in

crea

ses

HF < HCl < HBr < HIH2O < H2S

F H

Cl

Br

I

H

H

H

Acidic strength of HYDRIDESAcidic strength of HYDRIDES

chem Eq 4

HClO (hypochlorous acid)

Oxidation No.

HClO2 (chlorous acid)

HClO3 (chloric acid)

HClO4 (perchloric acid)

Acidity increases by oxidation number

Acidic strength of OXYACIDSAcidic strength of OXYACIDS

+1

+3

+5

+7

chem Eq 4

Weak acids are common

acetic acid (vinegar)nicotinic acid (vitamin)ascorbic acid (VitC)citric acid (lemon juice)formic acid(ants)oxalic acid (sorrel)

chem Eq 4

Conjugated bases (salts of weak acids)

antacids (NaHCO3) baking soda (NaHCO3)washing powder (Na2CO3)

All the weak acidsare in their conjugated base forms

in our body

chem Eq 4

Weak bases are rear

ammoniaalkaloids

quininenicotinecocainecaffeine