Determination of Ka and Molar mass of an

Unknown Acid

Dr. Fred Omega Garces Chemistry 201 Laboratory

Prelab Objective: This experiment is to identify an unknown weak acid by titrimetry. Analysis of the titration curve will yield the molar mass and the pKa of the unknown acid.

Background: A physical property of acid is the Ka (pka) value. This valued can be determine the mass action expression for the acid.

Consider a weak acid (HA) in aqueous solution-

HA (aq) + H2O (l) D H3O+(aq) + A- (aq)

The mass action expression is-

Knowledge of the equilibrium concentration of [H3O+], [A-] and [HA]

would yield the ka value of the acid.

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Ka =H3O+[ ] A−[ ]

HA[ ]

Prelab Background (continue)...Consider however if the conditions can be controlled so that [A-] = [HA], under such condition-

The condition of [A-] = [HA] can be obtain in two ways.

(1) pKa determination The first is in a titration technique. If a strong base titrant (OH-) is added so that half of the weak acid is neutralize.

i.e. HA (aq) + OH- (aq) g H2O (l) + A- (aq)

s 1.0 mol 0.5 mol

R -0.5 mol -0.5 mol + 0.5 mol

f 0.5 mol 0.5 mol

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Ka =H3O+[ ] A−[ ]

HA[ ] =

H3O+[ ] A−[ ]HA[ ]

= H3O+[ ] or pKa = pH

Prelab Background (continue)...In a titration experiment, this condition occurs when the volume of the titrant added reaches half-way to the equivalent volume as shown in the curve below.

Equivalent point yields the equivalent volume!

Equivalent volume (10 ml) can be use to calculate the half equivalent volume (5 ml)!

Half-way to equivalent volume = ½ equivalent volume (5 ml)!

Under this condition: [HA] = [A-] or pka = [H3O+] or pKa = pH ~ 5.2

Prelab Background (continue)... (2) pKa determination A second experimental technique that can be used reach this condition is to titrate the unknown acid to the equivalent point;

i.e. HA (aq) + OH- (aq) g H2O (l) + A- (aq)

s 1.0 mol 1.0 mol

R -1.0 mol -1.0 mol + 1.0 mol

f 0.0 mol 1.0 mol

The titration is stop at exactly the equivalent point of the titration, and then an equal volume of fresh weak acid is added to the analyte that was just titrated to the equivalent point. At this point the new HA added will be equal to the A- that was formed in the flask.

Analyte titrated to equivalent point!

Equal volume of weak acid added to neutralized analyte.!

Solution contains [A-] = [HA], measure pH yields pKa.

pH = pKa.!

Prelab Background (continue).. Molar Mass calculation:

At the eq point HA (aq) + OH- (aq) g H2O (l) + A- (aq)

s 1.0 mol 1.0 mol

R -1.0 mol -1.0 mol + 1.0 mol

f 0.0 mol 1.0 mol

The moles of the titrant added (Molaritytitrant * Voltitrant = moles Titrant)

At the equivalent point, moles titrant = moles Analyte

Molar mass of weak acid = Mass of acid that was titrated / moles of acid

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Molar Mass Unk Acid = Mass Weak AcidMoles of Acid

, but at equivalent point moles AcidAnalyte = moles BaseTitrant

Molar Mass Unk Acid = Mass Weak Acid (g)Moles of AcidAnalyte (moles)

= Mass Weak Acid (g)Moles of BaseTitrant (moles)

= Mass Weak Acid (g)MolarityTitrant * VolTitrant

Prelab Background (continue).. Possible Unknown Weak Acids

Complete the table-

(Partial List, complete list will be provided on day of experiment)

Equipment and Supplies

Equipment and Supplies

Procedure •

Setting up Vernier Interface pH probes!

Procedure Method 1, Method 2: Solution preparation.

Method 1 Solution Prep

Method 2 Stock solution Prep

Procedure Preparation KHP and NaOH solution.

Method 1 Solution Prep

Method 2 Stock solution Prep

Procedure Method 1: Titrating unknown acid with NaOH using Vernier.

Procedural Change: Use experiment 24, (pH of analyte versus volume titrant). Titrate analyte past equivalent point.

Method 1 x 3 Titration of Weak Acid

First Derivative!

Second Derivative!

Export to Excel!

Procedure Method 2: Titrating unknown acid with NaOH using Vernier. Use experiment 24, (pH of analyte versus volume titrant). Titrate analyte past equivalent point.

Method 2 x 2 Titration of Weak Acid

pH = pKa

Important Experimental Notes 1. Clean all glassware and condition buret before adding titrant

2. Method 1: Use first titrate trial to get an idea of the equivalent point.

Remember to titrate past the equivalent point.

3. Method1: Measure pH of analyte at 0% equivalent point then add 3-4 mL titrant

for each pH reading. As the equivalent pt is reach and the pH increases rapidly,

record volume and pH after increase of 1 pH unit during the titration.

4. Method1: Record data simultaneously in your lab notebook and your observations

5. Method 2: Titrate to end point using phenolphthalein indicator. Take the

solution and add equal volume of acid and record pH. The pH is the pKa of the

acid. Be sure you record the volume at the pH and volume at the equivalence point.

6.  Work efficiently, you need to complete all procedure by 4:25 PM. Clean up all

glassware and pour chemical waste in the waste container at the end of lab.

This is a formal Report. Read the grading Rubric and type up the Report.

Data Analysis Present your results in table form with the following information Unknown # ________ For each of the trial you should have: 1. Mass of unknown to nearest milligram with proper units. 2. Volume of titrant vs pH. Volume read to nearest hundredth mL.

3. Volume at equivalence point, pH at equivalence point and pH at ½ eq point. 4. M. Wt. of unknown (show sample calc in calculation section). For your unknown, you should have:

5. Mol. Wt. of unknown (Average for method 1 and 2) 6. pKa & Ka (average) value for unknown acid. 7. Std deviation of M.Wt, Ka, pKa & percent error

(assuming you have identified your unknown) 8. Unknown number and identity of unknown