Weak Acid-Strong Base Titration Lab - Yola

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Weak Acid-Strong
Base Titration Lab
Sam White
Pd. 2
Titration
Titrations are a neutralization reaction
between a base and an acid
 The 2 types of titrations you need for this
lab are:

 Strong Acid-Strong
Base
 Weak Acid-Strong Base
Strong Acid-Strong Base

In a proper Strong Acid-Strong Base titration, the H+ and
OH- are used up to create water in the following reaction:
H+ + OHH2O

The ions attached to the H+ ion (like Cl-) and to the OHion (like Na+) form a soluble salt (like NaCl) which splits
into ions in water. These become spectator ions in the
reaction, and do not affect the pH
When done properly, this type of titration creates a
solution with a pH of 7, since the H+ and OHconcentrations are equal, and water is neutral.

Strong Acid-Strong Base Titration
Curve
http://www.chemguide.co.uk/physical/acidbaseeqia/summary.gif
Weak Acid-Strong Base

Weak Acid-Strong Base reactions follow this
reaction:
HA + OHH2O + A-

While the acid is fully neutralized (all H+ ions are
used up to make water), the conjugate base of
the acid (denoted generically as A-) remains.
This means your titrated solution will have a pH
above 7 (usually somewhere between 8-9)

Weak Acid-Strong Base Titration
Curve
http://www.chemguide.co.uk/physical/acidbaseeqia/summary.gif
Diprotic Acids
So far we’ve only seen examples of
monoprotic acids (acids with one H+ ion)
 Weak diprotic acids (acids with two H+
ions) behave a little differently:

H2A + OHHA- + OH-

H2O + HAH2O + A-2
As you can see, weak diprotic acids must
dissociate twice, resulting in unique
titration curves
Weak Diprotic Acid Titration Curve
http://img.sparknotes.com/figures/3/3a5994498f24d59f5d5d762b40844a2a/polycurve.gif
Purpose

To determine the concentration of a
sample of an unknown weak acid through
titration with NaOH
Materials








50 mL Buret
Ring Stand
Buret Clamp
.1 M NaOH
Ehrlenmeyer Flask
Graduated Cylinder
Acetic Acid
Phenolphthalein
Procedures: Step 1
Set up titration
aparatus as shown,
with 50 mL of
NaOH in the buret
Buret Clamp
Buret
Ring Stand
Ehrlenmeyer Flask
http://science.birkenheadschool.co.uk/senior/bleaches/titration.gif
Procedures: Step 2
Measure out 25 mL of acetic acid. Add to
Ehrlenmeyer Flask.
Acid
25 mL Acid
From Clipart
From Clipart
Procedures: Step 3
Add 3 drops of
Phenolphthalein
indicator to the flask
of acid.
Acid
Made From Clipart
Procedures: Step 4
Slowly add drops of NaOH from the buret to the
flask of acid. Make sure to stop when it is a light
pink color.
Before
Stop Here
Way too Far
Procedures: Step 5
Record the amount of NaOH needed to
neutralize the acid.
Sample Data
Trial 1
Trial 2
Trial 3
Amount of
Acid
25 mL
25 mL
25 mL
Amount of
NaOH
14 mL
17 mL
15 mL
?
?
Molarity of
Acid
?
Calculations
Moles of NaOH:
0.1 M = moles NaOH
Liters solution
Calculations
Each trial NaOH:
0.1 M = moles NaOH
.014 L
moles of NaOH = .014 x .1 = .0014
0.1 M = moles NaOH
.017 L
moles of NaOH = .017 x .1 = .0017
0.1M = moles NaOH
.015 L
moles of NaOH = .015 x .1 = .0015
Calculations
Average moles of NaOH:
0.0014 + 0.0017 + 0.0015 = 0.0046
0.0046 / 3 = 0.00153 moles of NaOH
Calculations
This means, on average, 0.00153 moles of
NaOH were used in neutralization. This
means, on average, there were 0.00153
moles of H+ neutralized. This also means,
on average, 0.00153 moles of Acetic acid
were used.
Calculations
Average molarity of Acetic Acid:
M = 0.00153 moles
0.025 L
M = 0.0621
This means the concentration of the acetic acid
solution was 0.0621 M
Bibliography

For 3 images on Procedure 4:



http://images.google.com/imgres?imgurl=http://www.sciencebuddies.org/mentoring/project_id
eas/Chem_img030.jpg&imgrefurl=http://www.sciencebuddies.org/science-fairprojects/project_ideas/Chem_p045.shtml&usg=__PfZM_64irO37nSlQQbvgF2GjGao=&h=40
0&w=300&sz=16&hl=en&start=21&tbnid=LNlcnR6dlBX7zM:&tbnh=124&tbnw=93&prev=/ima
ges%3Fq%3Dtitration%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN%26start%3
D20
http://educ.queensu.ca/~science/main/concept/gen/g09/N.%20Sabet/titration1.gif
http://www.titrations.info/img/phenolphthalein-s.jpg
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