Tang 06 titrations & buffers

TITRATIONS
Purpose:
To determine the
unknown
concentration of
the acid.

TITRATIONS
Equivalence point: The point (pH) in the titration when an
equal number of moles of acid and base have been added
End point: The point (pH) at which the indicator changes
colour indicating an end to the titration
For a successful titration, choose an indicator that
changes colour at a pH value close to the pH at the
equivalence point.

Titration CurveTitration Curve
TITRATIONS

Indicator RangesIndicator Ranges
TITRATIONS

Indicator RangesIndicator Ranges
TITRATIONS
Strong acid + Strong base titration: resulting solution has a pH = 7, so
bromothymol blue could be used (pH range is 6.0 – 7.6)
Weak acid + Strong base titration: resulting solution has a pH > 7 so
phenolphthalein could be used (pH range is 8.2 – 10.0)
Strong acid + weak base titration: resulting solution has a pH < 7 so
methyl orange could be used (pH range is 3.1 – 4.4)

IndicatorsIndicators
How do indicators work?
Through equilibrium!
+ H+
Phenolphthalein
TITRATIONS

Basic Calculations (Gr. 11)Basic Calculations (Gr. 11)
Example #1
If it takes 54.0 mL of 0.1 M NaOH to neutralize
125.0 mL of an HCl solution. What is the
concentration of the HCl?
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
nNaOH=CV
nNaOH=(0.1M)(0.0540L)
nNaOH=5.4x10-3
mol NaOH = nHCl
CHCl = n
V
CHCl = 5.4x10-3
0.1250L
CHCl = 0.0432M
.: [HCl] = 0.0432M
TITRATIONS

Example #2Example #2
What is the pH of the final solution where 30.0 mL
of 0.1 M NaOH is mixed with 18.0 mL of 0.5 M
HCl?
Do you have a good guess as to whether the final
solution is acidic or basic?
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
nNaOH=CV
nNaOH=(0.1M)(0.0300L)
nNaOH=0.00300mol
nHCl=CV
nHCl=(0.5M)(0.0180L)
nHCl=0.00900mol
nHCl leftover=0.00900mol-0.00300mol
nHCl leftover=0.00600mol HCl leftover
pH = -log [H+
]
pH = -log [0.125M]
pH = 0.9
.: the pH is 0.9
C=nHCl leftover= 0.00600mol = 0.125M
V 0.048L
TITRATIONS

Example #2Example #2
What is happening at the chemical level?
NaOH  Na+ + OH-
HCl  H+ + Cl-
The OH-
and H+
ions come together to
produce H2O. The remaining ions will
determine the final pH of the solution.
TITRATIONS

STRONG-WEAK TITRATIONS
& BUFFERS

Strong-Weak Titration CurvesStrong-Weak Titration Curves
STRONG-WEAK TITRATIONS & BUFFERS

BuffersBuffers
Buffer solutions contain a mixture of an
acid and its conjugate base or a base with
its conjugate acid.
Due to the presence of the conjugate acid-
base pair, the addition of more acid or
base will not cause the pH to drastically
change.

BuffersBuffers
Buffers are important in biological systems
where large changes in pH could be
detrimental to the organism.
Almost all organic substances in our body
act as a buffer.
H2O + CO2 <===> H2CO3 <===> H+
+ HCO3
-
Weak acid
(CO2 controlled by lungs)
Weak base
(Controlled by kidneys)

BuffersBuffers

Buffers in Food, Drink & MedicinesBuffers in Food, Drink & Medicines
Sodium citrate + citric acid : buffer in food that
acts like a preservative
Citric acid + calcium citrate: buffer that deters
bacterial growth
Aspartame: decomposition is pH dependent
Medicines: coating reduces the risk of upset
stomach
BuffersBuffers

Strong-Weak TitrationsStrong-Weak Titrations
Example #3
What happens when you add a strong base
to a weak acid?
CH3COOH <===> CH3COO-
+ H+
NaOH  Na+
+ OH-

Two-step process:
1. All OH-
ions from the strong base will
react with H+
to produce H2O.
2. Equilibrium is shifted to the right.
3. pH is then calculated from the new
equilibrium based on the remaining [H+
].

Strong Weak TitrationsStrong Weak Titrations
Example #4
What happens when you add a strong acid
to a weak base?
NH3 + H2O <===> NH4
+
+ OH-
HCl  H+
+ Cl-

Two-step process:
1. All H+
ions from the strong acid will
react with OH-
to produce H2O.
2. Equilibrium is shifted to the right.
3. pH is then calculated from the new
equilibrium based on the remaining [H+
].

BuffersBuffers
Too much acid = acidosis
H2O + CO2 <===> H2CO3 <===> H+
+ HCO3
-
Respiratory (Lungs) Acidosis: Metabolic (Kidneys) Acidosis:

BuffersBuffers
Too much base = alkalosis
H2O + CO2 <===> H2CO3 <===> H+
+ HCO3
-
Respiratory (Lungs) Alkalosis: Metabolic (Kidneys) Alkalosis:

Buffer EquationsBuffer Equations
The following can be used to determine ion
concentrations in buffers:
Acid Buffer:
[H+
] = Ka x [HA] HA = acid
[A-
] A-
= conjugate base
Base Buffer:
[OH-
] = Kb x [B] B = base
[HB+
] HB+
= conjugate acid

Alternatively, you can use moles instead of
concentration to get the same answer:
Acid Buffer:
[H+
] = Ka x moles of HA HA = acid
moles of A-
A-
= conjugate base
Base Buffer:
[OH-
] = Kb x moles of B B = base
moles of HB+
HB+
= conjugate acid

A 1.00 L sample of an aqueous solution contains 0.200 mol of
acetic acid and 0.100 mol of acetate. Calculate:
a) The pH of the solution
b) The pH of the solution after adding 1.00 mL of 12.0 M HCl

A 1.00 L sample of an aqueous solution contains 0.200 mol of acetic acid and 0.100 mol of acetate. Calculate:
[H+
] = Ka x [moles HA]
[moles A-
]
a)
[H+
] = (1.8 x 10-5
) x (0.200 mol)
(0.100 mol)
[H+
] = 0.000036 mol/L
pH = -log [H+
] 0.000036 mol/L
pH = -log (0.000036 mol/L)
pH = 4.44
.: pH = 4.44
HC2H3O2(aq) <===> H+
(aq) + C2H3O2
-
(aq)

A 1.00 L sample of an aqueous solution contains 0.200 mol of acetic acid and 0.100 mol of acetate. Calculate:
b) moles HCl: n = C x V
n = (12 M)(0.001L)
n = 0.012 mol
Assume this produces 0.012 mol of acetic acid and uses up 0.012 mol of acetate
moles acetic acid: 0.200 + 0.012 = 0.212 mol
moles acetate: 0.100 - 0.012 = 0.088 mol
[H+
] = Ka x mols HC2H3O2
mols C2H3O2
-
[H+
] = (1.8x10-5
) x (0.212 mol)
(0.088 mol)
[H+
] = 4.3363 x 10-5
pH = -log [H+
]
pH = -log (4.3363 x 10-5
)
pH = 4.36
.: pH = 4.36

The Effect of Adding a Strong Acid or Base to a BufferThe Effect of Adding a Strong Acid or Base to a Buffer

The Capacity of a BufferThe Capacity of a Buffer
The amount of added H3O+
or OH-
that a buffer
can absorb without a significant change in pH

Tang 06 titrations & buffers

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (20)

Similaire à Tang 06 titrations & buffers

Similaire à Tang 06 titrations & buffers (20)

Plus de mrtangextrahelp

Plus de mrtangextrahelp (20)

Dernier

Dernier (20)