Chapter 15
Acid-Base Titration and pH
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Solution Concentrations*
Molarity – one mole of solute dissolved in enough
solvent (water) to make exactly one liter of
solution.
Molality – one mole of solute dissolved in exactly
1,000 grams of solvent.
Normality – one gram equivalent weight (gew) of
solute dissolved in enough solvent to make
exactly one liter of solution.
These are on your Ch 14/15 handout titled:
“ph/Acid/Base Equations”
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Molarity (M) Formulae
M = grams of solute given
GFW of solute
liters of solvent
Grams of solute needed= M(GFW of solute)(Liters Solvent)
L of solvent needed = g solute/GFW solute / M
3
Some molarity problems are on
pages 420 and 421
You have 3.50L of solution that contains 90.0g of
NaCl. What is the molarity of the solution?
g NaCl x 1mol NaCl = mol NaCl, the solute
g NaCl
mol of Solute = molarity of solution
L of solution
90.0g NaCl x 1 mol NaCl = 1.54 mol NaCl
58.44 g NaCl
1.54 mol NaCl / 3.50 L = 0.440 M NaCl
4
Another molarity problem. P421
practice.
1. What is the molarity of a solution composed of
5.85g KI, dissolved in enough water to make
0.125 L of solution?
5.85g KI x 1 mol KI = 0.0352 mol KI
166g KI
0.0352 mol KI = 0.282 M KI
0.125 L
Complete #2 and # on P421. Check your answers in
the back of the book.
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molality, m – one mole of solute
dissolved in exactly 1,000 g of solvent.
m = g of solute given X 1000
GFW of solute X g of solvent
g of solute needed = m(GFW)(g solvent)
1000
g solvent needed = g solute(1000)
GFW solute x m
Molality = molarity if water is the solvent
(aqueous solutions)
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Normality, N – one gram equivalent weight
of solute dissolved in enough solvent to
make exactly one liter of solution.
N = _________g solute_____
GEW solute x L solvent
g solute = N X GEW solute X L of solvent
L solvent = ____g of solute___
GEW of solute X N
Normality to Molarity
M = N(valence of cation)(subscript of cation)
GEW = _________GFW of solute_______
charge X subscript of solute cation
GEW – gram equivalent weight
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pH – What is it?
pH is an indication of the hydronium ion
concentration present in a solution.
[H30+] is the symbol for concentration of
hydronium ion in moles per liter or mol/L
pOH is an indication of the hydroxide ion
concentration present in a solution.
[OH-] is the symbol for concentration of
hydroxide ion in mol/L
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Water self ionizes
H20(l) + H20(l)
H30+(aq) + OH-(aq)
In the above reaction, two water molecules
produce a hydronium ion and a hydroxide ion
by transfer of a proton. Water is self Ionizing.
At 25oC, the concentrations of H30+ and OH- are
each only 1.0x10-7 mol/L of water.
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Math product of these ions
is a constant kw, the ionization constant of water.
Kw = [H30+ ] [OH-] = 1.0x10-7(1.0x10-7) =1.0x10-14
This occurs at 25oC. If the temperature changes, the
ion product, Kw changes.
When both [H30+ ] and[OH-] are 1.0x10-7, the
solution is neutral.
If [H30+ ] is greater than 1.0x10-7, the solution is
Acidic. (10-6 or 10-4 would be greater)
If [OH-] is greater than 1.0x10-7, the solution is
Basic.
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Calculating without a calculator
Kw = [H30+ ] [OH-] = 1.0x10-7(1.0x10-7) =1.0x10-14
Let’s say that the [H30+ ] is 1.0x 10-6 and you are
asked to find the [OH-].
-14
Kw = [H30+ ] [OH-] --> [OH-] = kw
1.0x10
=
[H30+ ] 1.0x10-6
-14 – (-6) = -14 + 6 = -8 so: [OH-] = 10-8 mol/Liter
More practice: 10-14/10-2 = 10-12
and 10-14/10-9 = 10-5
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Calculating [H30+ ] and [OH-]
Your own scientific calculator is a MUST here!!!
Find these keys: 2nd, either EE or EXP, and
change sign (-) or (+/-) on your calculator.
Let’s practice putting in numbers in sci. not.
1x10-7: Press keys in this sequence:
1 2nd EE (-) 7 on your display you see
something similar to this: 1E -7
2 x10-4: 2 2nd EE (-) 4 display: 2 E -4
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For concentration, M means moles/L
The [H30+] is 2.34 x 10-5 M in a solution. Calculate
the [OH-] of the solution.
[OH-] = Kw = 1.0x10-14
[H30+ ] 2.34 x 10-5
Key sequence:
1 2nd EE (-) 14
: 2.34 2
nd
EE (-) 5 enter
Display: 4.27 E -10 which means: 4.27 x 10-10 M
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Calculate hydronium and hydroxide ion
concentrations in a solution that is 1x10-4 M HCl.
HCl is a strong acid that ionizes completely. So
the concentration of H30+ is 1x10-4 M.
-14
Find [OH-]: [OH-] = Kw
1.0x10
=
[H30+ ]
1x10-4
Answer: [OH-] = 10-10 M
Asgn: Page 502 in book: Practice 2,3,4
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The pH Scale is used to show how
acidic or basic (alkaline) a solution is.
pH of a solution is the negative of the common logarithm of
the hydronium ion concentration.
pH = - log [H30+ ]
A common logarithm of a number is “the power to which
10 must be raised to equal the number.”
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The logarithm of 1.0x10-7 is - 7.0
The pH = - log [H30+ ] = - log (1.0x10-7) = 7.0
pOH is the negative log of [OH-].
pOH = -log [OH-]. In a neutral solution where
[OH-] is 1.0x10-7,
the pOH = -log [OH-] = -log 1.0x10-7 = 7.0
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Kw = [H30+ ] [OH-] = 1.0x10-7(1.0x10-7) =1.0x10-14
From above: pH + pOH = 14.0
pOH can also be found by:
pOH = 14.0 – pH = 14.0 – 7.0 = 7.0
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Determine the pH of these solutions:
a. 1x10-3 M HCl. HCl is a strong acid that ionizes
completely. So the concentration of H30+ is 1x10-3 M.
pH = - log [H30+ ] = - log (1.0x10-3) = 3.0
b. 1x10-4 M NaOH. NaOH is a strong base that ionizes
completely. The concentration of OH- is 1x10-4 M
[H30+ ] = Kw = 10-14 = 10-10
[OH-] 10-4
pH = -log(10-10) = 10.0
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More samples…
Find the pH of a solution where [H30+ ] is 2.8 x
10-5 M?
pH = -log [H30+ ] = -log 2.8 x 10-5 = 4.55
Key sequence: (-) log 2.8 2nd EE (-) 5 =
Find the pH of a 4.7 x 10-2 M NaOH solution.
[H30+ ] = Kw = 10-14 = 2.1x10-13
[OH-] 4.7x10-2
pH = -log [H30+ ] = - log 2.1x10-13 = 12.7
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Assignment
• 505/1. b and d
• 506/1-4
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Review
• Acid + Base --> Salt + Water
• The mixture is neutralized (no longer acidic or
basic).
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In this section we are going to look at:
indicators, pH meters, and titrations.
You used 2 indictors in a lab recently to
determine how acidic or basic several
solutions were.
You used litmus paper and pH paper (Hydrion)
Acid-base indicators are sensitive to pH of acids
and bases. They will change color as a result
of the ions present.
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In acidic solutions, an indicator will be one color
(litmus turns red) and in basic solutions, an
indicator will be another color (litmus burns
blue).
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Indicator Samples
Methyl red, Bromthymol blue, Methyl orange,
Phenolphthalein, Phenol red are indicator
samples.
These will ionize in solution and, depending
upon their acid or base strength will change
color over a range of pH values until the end
point is reached.
The range over which an indicator changes color
is called its transition interval.
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Reading Indicator Values
Litmus gives a very broad reading – a solution is
either acidic or basic.
Indicators are more specific in reading the pH of
an acid or base.
But the most accurate method of measuring pH
is with a pH meter. A pH meter determines
the pH of a solution by electrically measuring
the voltage between the two electrodes
placed in a solution.
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Titration
Titration 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.
More simply: it is using a known concentration
of a solution to determine the concentration
of a solution of unknown concentration.
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When a base is
added to an acid the
solution will
become neutral and
this will be shown
by an indicator
changing solor.
This will occur when
equal numbers of
H30+ and OH- are
present.
Picture retrieved from: cikguwong.blogspot.com
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Equivalence point
The point at
which two
solutions used in
a titration are
present in
chemically
equivalent
amounts.
The figure below shows typical pH curves for various acidbase titrations. The equivalence points and end points are
different for the various combinations of strong and weak
acids and bases.
End point
The point in a
titration at which
an indicator
changes color.
Retrieved from: chemguide.co.uk
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Determination of the acidity/alkalinity of salt
solutions produced by neutralization reactions.
• The relative pH of a neutralized solution can be
determined utilizing the following scale showing A/B
strengths.
pH 1-3
strong acid with a weak base
pH 3-5
strong acid with a moderate base
pH 5-7
moderate acid with a weak base
pH 7(neutral)equal strength acid/base reacton
pH 7-9
moderate base with a weak acid
pH 9-12
strong base with a moderate acid
pH 12-14 strong base with a weak acid
See your Ch 14/15 handout for more information.
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Molarity and Titration
• Standard solution – the solution that contains
the precisely known concentration of a solute.
• Primary standard – highly purified solid
compound used to check the concentration of
the known solution in a titration.
• Knowing the molarity and volume of a known
solution used in a titration, the molarity of a
given volume of a solution with unknown
concentration can be found.
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Titration Set up and Procedure
Your book has a more complete explanation.
1. Fill one buret with an acid. Record volume.
2. Fill other buret with standard solution base.
Record volume.
3. Indicator (Phenolphthalein) will be in a flask.
4. Add a given amount of A to the flask.
5. Begin adding B to the flask until the pink
color of the indicator begins to form. Swirl
the contents constantly.
6. As the pink color begins to remain for longer
periods of time, you are nearing the end
point.
7. When the pink color remains after 30
seconds of swirling, the equivalence point is
reached.
8. Record the exact volume of the base put in
the flask.
Retrieved from web.ysu.edu.
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Formula to calculate molarity
in a titration.
Ma x Va = Mb x Vb
Ma – molarity of acid
Va – volume of acid
Mb – molarity of base
Vb – volume of base
See Ch 14/15 handout for more info.
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Molarity & Titation Problem Steps
1. Start with balanced equation for the
neutralization reaction, and determine the
chemically equivalent amount of the acid and
base.
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