LISGAR COLLEGIATE INSTITUTE
SCH4U/G: PERFORMANCE TASK
DETERMINATION OF THE MOLAR MASS
AND THE KA OF A WEAK ACID
Pre-lab Theory
Acids and bases are classified as being “weak” or “strong”. A quantitative measure of the strength of
an acid is known as the “ionization constant” (or Ka) which describes how well the acid ionizes in
solution. This value is characteristic of the acid and can be used to help identify an unknown acid. A
similar system exists for bases (Kb).
One method for determining Ka values is called the “half volume” method. A solution of the acid is
prepared and divided in half as accurately as possible. One portion is titrated to its endpoint with
phenolphthalein. The two portions are then recombined, and the pH of the resulting solution is
measured.
Purpose:
The purpose of this experiment is to determine the molar mass and the ionization constant (Ka) for
a weak monoprotic acid using titration and the “half volume” method. You will carry out the
experimental portion of this investigation in class, your results and analysis will be presented as a
formal lab report.
Equipment/Materials:
Equipment
250 mL volumetric flask
100 mL volumetric flask
buret, clamp and retort stand
pipette and pumps
Erlenmeyer flask
Florence flasks
pH meter
Materials
unknown acid sample
primary standard acid (oxalic acid)
standard base solution (NaOH)
phenolphthalein
distilled water
Procedure:
A)
1)
2)
3)
Preparation and Standardization of NaOH solution
Prepare a 250 mL of 0.100 M NaOH in a volumetric flask.
You must standardize your NaOH solution by titrating it against a primary standard
acid, in this case oxalic acid dihydrate. To do this, you must prepare 100.0 mL of
oxalic acid of precisely known concentration (I suggest between 0.0500M – 0.100M)
Titrate your NaOH solution with your standard oxalic acid solution to determine the
precise molarity of your base solution. (3 consistent results)
-2B)
Determining the Molar Mass of a Weak Acid
1) Obtain a numbered vial containing a sample of an unknown weak acid. Record your vial
number and then accurately determine the mass of your acid sample.
2) Use a clean 250 mL volumetric flask to prepare a solution of your acid sample.
3) Titrate a 10.0 mL sample of your acid solution using your standard NaOH solution. You
may us up to 50.0 mL of your acid solution to obtain at least 3 consistent results.
4) Use the collected data to determine the molarity of your acid sample and the molar
mass of the acid.
C)
1)
2)
3)
4)
5)
Determining Ka by the “Half Volume” Method
Use a 100.0 mL volumetric flask to divide the remaining acid solution into two equal
100.0 mL portions.
One of your 100.0 mL portions should be titrated to a phenolphthalein endpoint with
your standard NaOH solution.
Once the endpoint is reached, mix the titrated solution with the other half of the
acid solution.
Use a pH meter to measure the pH of your final solution.
Use the collected data to determine the Ka of the unknown acid.
Results:
All data should be collected and presented in well organized data tables. Don’t forget to
record your vial number.
Discussion:
Use equilibrium concepts and general equations to describe and define an acid. Briefly, explain the
concept of the equilibrium constant (Ka) and how it relates to acid strength. Briefly, describe the
theory behind acid/base titration, include the balanced chemical equations for the reaction between
oxalic acid and NaOH and using general formula, the reaction between a typical monoprotic acid and
NaOH. Explain our choice of indicator for this titration. Explain what is meant by a primary standard
acid and why it was necessary to standardize our NaOH solution. Research and explain the theory
behind the “half volume” method of determining the Ka of an acid. Include all references, resources
used in a bibliography at the end of report.
Data Analysis and Calculations:
Show complete and well organized calculations for the following:
Part A)
Preparation of 0.100 M NaOH solution
Preparation and [oxalic acid] solution
Determination of concentration of standard NaOH solution
Part B)
Determination of concentration of unknown acid solution
Determination of molar mass of unknown acid
Part C)
Determination of Ka of unknown acid
Conclusions: Summarize your results and suggest sources of error in each part of this
experiment.
SCH4U: Summative Evaluation: Molar Mass and Ka of a Weak Acid Formal Report
Purpose: concise, accurate, complete
/2
Materials + Methods
/5
Results: complete, well organized in data tables, proper use of sig. digs, units
/5
Discussion:
/2
a) Equilibrium + Acid concept
/2
b) Ka and acid strength
/6
c) Acid-Base titration + Equations + Indicator choice
/3
d) Primary Standard and Standardization process
/2
c) Half-volume method theory
Analysis + Calculations: (presented in organized fashion, units + sig. digs)
/4
a) Preparation of NaOH and Oxalic acid solutions
/3
b) Standardization of NaOH
/3
c) [acid solution]
/3
d) Molar mass of unknown acid
/2
e) Ka of unknown acid
/4
Conclusions: concise summary of findings + well thought out error analysis
Presentation: neatly presented, well organized format, complete sentences,
grammar + spelling.
Accuracy of Results: mark based on %error of:
Molar Mass of Unknown Acid %error: _____________________
Total Mark: (worth 5% of year’s mark)
/3
/10
/59