Strong Electrolytes

1
Strong Electrolytes Dissociate
in solution to form solvated ions
Conduct electricity
1) Ionic compounds (salts) that are highly
soluble
All common group I/NH4 cpds (e.g. NaCl) Most
nitrates (NO3-), acetates (CH3COO-), perchlorates
(ClO4-) Chlorides (Cl-) (except AgCl, Hg2Cl2, and
PbCl2)
2) Strong Acids or Bases that dissociate in water
HCl, HNO3, H2SO4, HBr, HClO4, etc. NaOH, Ca(OH)2,
KOH etc.
2
Acid Base Review
2H2O ? H3O OH-
Kw 1 x 10-14 (at 25C) H3OOH-
pH - log H3O
pOH - log OH-
pH pOH 14
3
Weak Monoprotic acids
Generic HA H2O ? H3O A-
pKa -log Ka
Acid (HA) Conjugate base (A-) Ka
HF F- 7.2 x 10-4 HNO2
NO2- 4.5 x 10-4 CH3COOH CH3COO-
1.8 x 10-5 HOCl OCl- 3.5 x 10-8
HCN CN- 4.0 x 10-10
pH and s of all species in 0.23M HOCl?
4
Types of problems ..
1) Ka/Kb and acid/base to pH and other s.
2) Ka/Kb and pH to acid/base other s.
3) pH and acid/base to Ka and other s.
4) pH of solution containing salts of weak acids
(e.g. sodium acetate) or salts of weak bases
(e.g. ammonium chloride). Or vice versa.
5
Weak bases
B H2O ? HB OH-
Base (B) Conjugate acid (HB) Kb
NH3 NH4 1.8 x 10-5
CH3NH2 CH3NH3 5.0 x 10-4 C5H5N
C5H5NH 1.5 x 10-9
C6H5NH2 C6H5NH3 4.2 x 10-10
methylamine if pH 9.7?
6
What is the pH of a 0.280 M solution of HF?
HF H2O ? H3O F- Ka 7.2 x
10-4 (H3O F-)/HF
Let x H3O . 7.2 x 10-4 x2/(0.280 x)
x (0.280 7.2 x 10-4)1/2 (2.016 x 10-4 M)1/2
1.42 x 10-2 M 0.0142 M
Is this acceptable error? 0.0142/0.280 100
5.07
Using quadratic equation
x2 (2.016 x 10-4 7.2 x 10-4 x
x2 7.2 x 10-4x - 2.016 x 10-4 0
x 0.0138 M
pH 1.860 (1.848)
7
Salts of weak acids are bases referred to
as the conjugate base
0.15 M NaCH3COO (sodium acetate)
Na(CH3COO-) ? Na CH3COO-
CH3COO- H2O ? CH3COOH OH-
Kb OH-CH3COOH ?
CH3COO-
Ka 1.8 x 10-5 Kw KaKb so Kb Kw/Ka
5.6 x 10-10
5.6 x 10-10 x2/(0.15 x) where x OH-
x 9.16 x 10-6 M (0.6 error) pOH 5.04
pH 8.96
8
Polyprotic acids An acid with 2 or more acidic
protons
e.g. Phosphoric acid (H3PO4)
H3PO4 H2O ? H2PO4- H3O Ka1 7.5
x 10-3
H2PO4- H2O ? HPO42- H3O Ka2
6.2 x 10-8
HPO42- H2O ? PO43- H3O Ka3 3.6
x 10-13
What is pH and s of all species in 0.270 M
H3PO4?
Use Equilibrium 1 Ka expression to find H2PO4-
H3O.
Use Equilibrium 2 Ka expression H2PO4- from
1 to find additional H3O and HPO4-.
Use Equilibrium 3 Ka expression HPO4 to
find PO43- additional H3O.
9
0.270M Phosphoric acid (H3PO4)
H3PO4 H2O ? H2PO4- H3O Ka1 7.5
x 10-3
H2PO4- H2O ? HPO42- H3O Ka2
6.2 x 10-8
HPO42- H2O ? PO43- H3O Ka3 3.6
x 10-13
Use Equilibrium 1 Ka expression to find H2PO4-
H3O. x12 (.270 7.5 x 10-3 -
7.5 x 10-3x) H2PO4- x1 -7.5
x 10-3 (7.5 x 10-3)2 4 -2.025 x 10-31/2
2 0.0414 M H3PO4 0.23M
Use Equilibrium 2 Ka expression H2PO4- from
1 to find additional H3O and HPO4-.
6.2 x 10-8 (0.0414 x2) x2 (0.0414
x2) x2 6.2 x 10-8 M HPO42-
H3O 0.0414 0.0000062 0.0414 M
Use Equilibrium 3 Ka expression HPO4 to
find PO43- additional H3O. 3.6 x
10-13 (0.0414 x3) x3 (6.2 x 10-8 x3)
x3 PO43- 5.39 x 10-19 M
pH - log H3O - log 0.0414 1.38
10
H3PO4 H2O ? H2PO4- H3O Ka1 7.5
x 10-3
H2PO4- H2O ? HPO42- H3O Ka2
6.2 x 10-8
HPO42- H2O ? PO43- H3O Ka3 3.6
x 10-13
Colas contain 0.106 g L-1 of H3PO4 (98.0 g
mol-1). (for flavor?) Estimate its pH and s
of all species if this is the only acid
present. (note that carbonated beverages contain
CO2/H2CO3 that is also acidic)
7.5 x 10-3 H2PO4-H3O/H3PO4 x2/(0.0011
x)
x is not small compared to 0.00110 M x
0.000974M
H3O H2PO4- 9. 74 x 10-4 M pH
3.01 HPO42- 6.2 x 10-8 M
PO43- 2.3 x 10-17 M