Weak Acid-Strong Base Titration Lab

1
Weak Acid-Strong Base Titration Lab

• Sam White
• Pd. 2

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

3
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.

4
Strong Acid-Strong Base Titration Curve
http//www.chemguide.co.uk/physical/acidbaseeqia/s
ummary.gif
5
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)

6
Weak Acid-Strong Base Titration Curve
http//www.chemguide.co.uk/physical/acidbaseeqia/s
ummary.gif
7
Diprotic Acids

• So far weve 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

8
Weak Diprotic Acid Titration Curve
http//img.sparknotes.com/figures/3/3a5994498f24d5
9f5d5d762b40844a2a/polycurve.gif
9
Purpose

• To determine the concentration of a sample of an
  unknown weak acid through titration with NaOH

10
Materials

• 50 mL Buret
• Ring Stand
• Buret Clamp
• .1 M NaOH
• Ehrlenmeyer Flask
• Graduated Cylinder
• Acetic Acid
• Phenolphthalein

11
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/bleac
hes/titration.gif
12
Procedures Step 2

• Measure out 25 mL of acetic acid. Add to
  Ehrlenmeyer Flask.

Acid
25 mL Acid
From Clipart
From Clipart
13
Procedures Step 3

• Add 3 drops of Phenolphthalein indicator to
  the flask of acid.

Acid
Made From Clipart
14
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
15
Procedures Step 5

• Record the amount of NaOH needed to neutralize
  the acid.

16
Sample Data
Trial 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
? ?
?
17
Calculations

• Moles of NaOH
• 0.1 M moles NaOH
• Liters solution

18
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

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Calculations

• Average moles of NaOH
• 0.0014 0.0017 0.0015 0.0046
• 0.0046 / 3 0.00153 moles of NaOH

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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.

21
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

22
Bibliography

• For 3 images on Procedure 4
• http//images.google.com/imgres?imgurlhttp//www.
  sciencebuddies.org/mentoring/project_ideas/Chem_im
  g030.jpgimgrefurlhttp//www.sciencebuddies.org/s
  cience-fair-projects/project_ideas/Chem_p045.shtml
  usg__PfZM_64irO37nSlQQbvgF2GjGaoh400w300sz
  16hlenstart21tbnidLNlcnR6dlBX7zMtbnh124
  tbnw93prev/images3Fq3Dtitration26gbv3D226n
  dsp3D2026hl3Den26sa3DN26start3D20
• http//educ.queensu.ca/science/main/concept/gen/g
  09/N.20Sabet/titration1.gif
• http//www.titrations.info/img/phenolphthalein-s.j
  pg