Acids and Bases - Travellin -...
Transcript of Acids and Bases - Travellin -...
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The pH scale is a way of
expressing the strength of
acids and bases. Instead of
using very small numbers,
we just use the NEGATIVE
power of 10 on the Molarity
of the H+ (or OH-) ion.
Under 7 = acid
7 = neutral
Over 7 = base
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Calculating the pH
pH = - log [H+] (Remember that the [ ] mean Molarity)
Example: If [H+] = 1 X 10-10
pH = - log 1 X 10-10
pH = - (- 10)
pH = 10
Example: If [H+] = 1.8 X 10-5
pH = - log 1.8 X 10-5
pH = - (- 4.74)
pH = 4.74
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Try These!
Find the pH of these:
1) A 0.15 M solution of
Hydrochloric acid
2) A 3.00 X 10-7 M
solution of Nitric
acid
pH = - log [H+]
pH = - log 0.15
pH = - (- 0.82)
pH = 0.82
pH = - log 3 X 10-7
pH = - (- 6.52)
pH = 6.52
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pH calculations – Solving for H+
If the pH of Coke is 3.12, [H+] = ???
Because pH = - log [H+] then
- pH = log [H+]
Take antilog (10x) of both
sides and get
10-pH = [H+]
[H+] = 10-3.12 = 7.6 x 10-4 M
*** to find antilog on your calculator, look for “Shift”
or “2nd function” and then the log button
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More About Water
H2O can function as both an ACID and a BASE.
In pure water there can be AUTOIONIZATION
Equilibrium constant for water = Kw
Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
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More About Water
Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
In a neutral solution [H3O+] = [OH-]
and so [H3O+] = [OH-] = 1.00 x 10-7 M
OH-
H3O+
Autoionization
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pOH
• Since acids and bases are opposites, pH and pOH are opposites!
• pOH does not really exist, but it is useful for changing bases to pH.
• pOH looks at the perspective of a base
pOH = - log [OH-]
Since pH and pOH are on opposite ends,
pH + pOH = 14
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pH [H+] [OH-] pOH
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[H3O+], [OH-] and pH
What is the pH of the
0.0010 M NaOH solution?
[OH-] = 0.0010 (or 1.0 X 10-3 M)
pOH = - log 0.0010
pOH = 3
pH = 14 – 3 = 11
OR Kw = [H3O+] [OH-]
[H3O+] = 1.0 x 10-11 M
pH = - log (1.0 x 10-11) = 11.00
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What is the pH of a 2 x 10-3 M HNO3 solution?
HNO3 is a strong acid – 100% dissociation.
HNO3 (aq) + H2O (l) H3O+ (aq) + NO3
- (aq)
pH = -log [H+] = -log [H3O+] = -log(0.002) = 2.7
Start
End
0.002 M
0.002 M 0.002 M 0.0 M
0.0 M 0.0 M
What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution?
Ba(OH)2 is a strong base – 100% dissociation.
Ba(OH)2 (s) Ba2+ (aq) + 2OH- (aq)
Start
End
0.018 M
0.018 M 0.036 M 0.0 M
0.0 M 0.0 M
pH = 14.00 – pOH = 14.00 + log(0.036) = 12.56 15.4
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HNO3, HCl, HBr, HI, H2SO4 and HClO4 are
the strong acids.
Strong and Weak Acids/Bases
The strength of an acid (or base) is
determined by the amount of
IONIZATION.
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• Generally divide acids and bases into STRONG or
WEAK ones.
STRONG ACID: HNO3 (aq) + H2O (l)
H3O+ (aq) + NO3
- (aq)
HNO3 is about 100% dissociated in water.
Strong and Weak Acids/Bases
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• Weak acids are much less than 100% ionized in
water.
*One of the best known is acetic acid = CH3CO2H
Strong and Weak Acids/Bases
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• Strong Base: 100% dissociated in water.
NaOH (aq) Na+ (aq) + OH- (aq)
Other common strong
bases include KOH and
Ca(OH)2.
CaO (lime) + H2O -->
Ca(OH)2 (slaked lime)
CaO
Strong and Weak Acids/Bases
Strong bases are the group I hydroxides
Calcium, strontium, and barium hydroxides are
strong, but only soluble in water to 0.01 M
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• Weak base: less than 100% ionized in water
One of the best known weak bases is ammonia
NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq)
Strong and Weak Acids/Bases
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Weak Bases
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Ionization Constants for Acids/Bases
Acids Conjugate Bases
Increase
strength
Increase
strength
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Equilibria Involving
Weak Acids and Bases
Consider acetic acid, HC2H3O2 (HOAc)
HC2H3O2 + H2O ↔ H3O+ + C2H3O2
-
Acid Conj. base
Ka [H3O+][OAc- ]
[HOAc] 1.8 x 10-5
(K is designated Ka for ACID)
K gives the ratio of ions (split up) to molecules (don’t split up)
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This would be a quadratic. If the Ka or Kb is less than
.001 or greater than 1000. You can ignore the –x
when solving for x. There is another rule about if the
original concertation is 100 times Ka. No quadratic on
the AP exam.
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