Transcript Document
Acids & Bases Lesson 9 pH, pOH, Kb for Weak Bases
Weak Bases calculations
Helpful Hints: - Weak bases do NOT ionize 100% - There is an equilibrium state - NEED….ICE tables! - Kb
can’t
be used directly from table…use - Kb = kw/Ka
You can calculate the Kb using the Ka from the table and the equation below.
Ka x Kb = 1.0
x 10 -14
@ 25 o C
Kb = Kw Ka(conjugate) or Ka = Kw Kb(conjugate)
Weak Bases calculations
Two types of questions -calculate pOH, or [OH ] -calculate Kb
1.
Calculate the pH of 0.50 M NH 3 .
Weak Base Need Kb( NH 3 ) = = + H 2 O =
⇄
I NH 3 0.50 M C -x E 0.50 - x 0 Small Kb Kw Ka( NH 4 + ) 1 x 10 -14 5.6 x 10 -10 1.786 x 10 -5 NH 4 + 0 x x + OH 0 x x
Kb = = [
NH 4 +
][
OH -
]
[ NH 3 ] x 2 0.50
x = 0.002988 M pOH = 2.52
pH + pOH = pKw = 14.000
pH = 11.48
= 1.786 x 10 -5 = [OH ] pOH = -Log[OH ]
2.
I C E Calculate the pH of 0.20 M Na 2 CO 3 . Weak Base Need Kb( CO 3 2 ) = = CO 3 2 0.20 M + H 2 O -x 0.20 - x 0 Small Kb
⇄
HCO 3 0 x x = Kw Ka( HCO 3 ) 1 x 10 -14 5.6 x 10 -11 1.786 x 10 -4 + OH 0 x x
Kb = = [
HCO 3 [ CO 3 2 ] ][ OH -
]
x 2 0.20
= 1.786 x 10 -4 x = pOH = pH = 0.005976 M 2.22
11.78
TRY : calculate [H + ], [OH ], pH, pOH for a 0.20 M solution of NH 3 .
3.
Calculate the pH of 0.20 M NaCl.
NaCl is a neutral salt pH = 7.00
What if asked to calculate K
b
?
K
b
indicates weak bases, then use ICE tables.
need to know [ ]’s to plug in the equation.
EX:
If the pH of
0.40 M NH 3
@ 25 o C is
11.427
, calculate the
Kb
.
pH = 11.427
[OH ] = 10 -2.573
pOH =
[OH ]
2.573
= 0.002673 M at equilibrium line!!!
I C E NH 3 0.40 0.3973
+ H 0.002673
2 O
⇄
NH 4 + 0 0.002673
0.002673
+ OH 0 0.002673
0.002673
Kb = [NH 4 + ][OH ] [NH 3 ]
= (0.002673) 2 0.3973
= 1.8 x 10 -5
TOGETHER:
The pOH of a 0.50 M solution of the weak acid HA is 10.64. what is K b for A ?
TRY
: A 0.600 M solution of the weak base hydroxylamine, NH 2 OH, has a pH of 9.904. What is K a for NH 3 OH + ?
Next question
• • •
Strong acid and a strong base Calculate excess No ICE tables
4.
15.0 mL of 0.20 M HNO 3 reacts with 40.0 mL of 0.20 M KOH, calculate the pH of the resulting solution.
Next question
• • It says saturated solution = equilibrium • This is a
solubility
equilibrium-
no ICE Remember, unit 3!
4.
Calculate the pH of a saturated solution of Mg(OH) 2 .
This is a
solubility
equilibrium-
no ICE Mg(OH) s 2(s)
⇄
Mg s 2+ + 2OH 2s Ksp = [Mg 2+ ]][OH ] 2 = 5.6 x 10 -12 [s][2s] 2 = 5.6 x 10 -12 4s s 3 = = 5.6 x 10 -12 1.119 x 10 -4 M
2s = [OH ] = pOH = pH = 3.65
10.35
2.237 x 10 -4 M
Be careful
Are they strong or weak acids and bases?
calculations differ!
Use ICE tables for only weak
combinations.
Is it a
Ksp question?
Always
understand
the question BEFORE answering it.
Remember
ALL the equations!
pH Equations You must know the following equations, which are all based on the ionization of water at 25 0 C!
H 2 O
⇄
H + + OH Kw = [H + ][ OH ] = 1.00 x 10 -14 pH [H + ] = -Log[H + ] = 10 -pH Or pOH [OH ] = -Log[OH = ] 10 -pOH pH + pOH = pKw = 14.000
p.153
84, 86, 87, 88, p.154
90, 91, 93.