Acids- Base
Titration and pH
Aqueous Solutions
and the Concept of
pH
When two molecules of water
produce a hydronium and
hydroxide ion by the transfer of
a proton.
H2O + H2O H3O+ + OH-
At 25C the [H3O+] = [OH-]= 10-7 M
So, the products of [H3O+] and [OH-] can be
represented by:
[H3O+] x [OH-]
10-7 M x 10-7 M = 10-14 M2
Where 10-14 M2 Is the Kw ( ionization constant
of water)
So, [H3O+] x [OH-]= Kw at 25C
Neutral solutions:
[H3O+] = [OH-]= 10-7 M
Acidic solutions:
[H3O+] > [OH-]
Basic solutions:
[H3O+] < [OH-]
We
can find the [H3O+]
and[OH-] by using the
formula:
[H3O+] x [OH-]= Kw= 10-14 M2
Practice p: 484
It is the negative logarithm of the
hydronium ion concentration
pH= - log [H3O+]
Likewise,
pOH It is the
negative logarithm of the
hydroxide ion
concentration
pOH= - log [OH ]
So, pH + p OH= 14 at 25 C
[H3O+]=10-pH
Practice
p:
487
1- [H3O+] x [OH-]= Kw
2- pH= - log [H3O+]
3- pOH= - log [OH-]
4- pH + p OH= 14 at 25C
5- [H3O+]=10-pH
Determining pH
and Titrations
1- Acid- Base indicators: are compounds
whose colors are sensitive to pH. Indicators
come in many different colors. The exact pH
range over which an indicator changes color
also varies.
2-Universal indicators: the pH of
solution can be determined by
comparing the color it turns with the
scale of paper.
3- pH meter: determines the pH of a solution by
measuring the voltage between the electrodes
that are placed in the solution. (It’s the most
accurate way to determine the pH)
Is the controlled addition and measurement of
the amount of a solution of known
concentration required to react completely with
a measured amount of a solution of unknown
concentration.
The equation of titration is:
H3O+ + OH- 2H2O
1-
strong acid- strong base
2-
strong acid- weak base
3-
weak acid- strong base
Indicators are specific, each type of titration
needs a specific indicator
For example:
Strong acid- strong base titration:
We use bromothymol blue (6.2- 7.6)
For strong acid- weak base titration:
We use bromophenol blue ( 3-4.6)
For weak acid- strong base titration
We use phenolphtalein(8-10)
The
point at which the two
solutions used in a titration
are present in chemically
equivalent amounts is
called the equivalence
point.
The
endpoint in
titration marks the
point at which the
color of indicator
changes.
The solution that contains precisely known
concentration of solute is known as the
standard solution.
To find the molarity or the volume of an
unknown solution we use the equation:
No of moles of acid= No of moles of base
CxV (acid)= CxV (base)
Practice p: 503 (1 and 2)
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