Weak Acid-Strong Base Titration Lab Sam White Pd. 2.
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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 BaseWeak 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+ + OH- H2O
The ions attached to the H+ ion (like Cl-) and to the OH- ion (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 OH- concentrations 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 + OH- H2O + 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 + OH- H2O + HA-
HA- + OH- H2O + 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
Ehrlenmeyer Flask
Buret
Buret Clamp
Ring Stand
Set up titration aparatus as shown, with 50 mL of NaOH in the buret
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 DataTrial 1 Trial 2 Trial 3
Amount of Acid
Amount of NaOH
Molarity of Acid
25 mL 25 mL 25 mL
14 mL 17 mL 15 mL
? ? ?
Calculations
Moles of NaOH:
0.1 M = moles NaOH
Liters solution
Calculations
Each trial NaOH:0.1 M = moles NaOH moles of NaOH = .014 x .1 = .0014 .014 L
0.1 M = moles NaOH moles of NaOH = .017 x .1 = .0017 .017 L
0.1M = moles NaOH moles of NaOH = .015 x .1 = .0015 .015 L
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 M = 0.0621
0.025 L
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_ideas/Chem_img030.jpg&imgrefurl=http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p045.shtml&usg=__PfZM_64irO37nSlQQbvgF2GjGao=&h=400&w=300&sz=16&hl=en&start=21&tbnid=LNlcnR6dlBX7zM:&tbnh=124&tbnw=93&prev=/images%3Fq%3Dtitration%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN%26start%3D20
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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