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