# Determination of pKa & Molecular Weight with a Titration Curve

Experiment # 11. Titration of a Weak Acid Aim: To determine the molar mass, pka and the identity of an unknown weak acid by titration. Introduction A weak acid undergoes partial dissociation in an aquoeus solution. There exists an equilibrium between the ions and the undissociated acid molecule. The equilibrium constant for this process is Ka is the acid dissociation constant. Consider a weak monoprotic acid HA: HA (aq) + H2O (l)  A—(aq) + H3O+(aq)

Ka = [A—][ H3O+] [HA]

In any acid–base titration, a solution of known concentration (the titrant) is slowly added from a burette to a solution of unknown concentration (the analyte) until the reaction is complete. The equivalence point occurs when stoichiometrically equal amounts of the acid and base are present. A plot of pH vs volume is known as a titration curve.

When the acid HA is titrated with NaOH, the reaction taking place is: OH— (aq) + HA(aq) → A—(aq) + H2O(l) As the titration proceeds, the nature of the solution will change. 1. Initially when no NaOH had been added, the only species in solution are the weak acid and a small amount of H3O+ and A— formed by the slight dissociation of the weak acid. The pH of the solution depends only on the molar concentration or the acid and the Ka of the acid. 2. After some NaOH(aq) has been added, but before the equivalence point, the acid and its conjugate base will both be present in amounts that are roughly comparable. In this region of the titration the solution is a buffer. At the point in the titration where [HA] = [A—], the pH of the solution = pKa. This will occur halfway to the equivalence point in terms of the amount of NaOH added. Thus, at the "half equivalence point" or "halfway point", pH = pKa. 3. After the equivalence point, the pH of the solution depends on the NaOH in excess. 4. If the acid is polyprotic, then there will be more than one equivalence point which can be identified from the titration curve. We can thus calculate pKa for each step for a polyprotic acid. In today’s lab, a weak acid will be titrated with a known concentration (standardized) solution of NaOH(aq) to determine the pKa, molar mass and the identity of the acid.

SUPPLIES: Buret Ring stand Standardized NaOH(aq) Hot plate with stir option (heat off)

buret clamp funnel Venier with pH meter probe teflon coated stir bar

Possible Unknown acids: Potassium hydrogen phthalate, Ascorbic acid or Maleic acid Spill/Disposal Reaction Mixture : A

Spill/Disposal : B1,

Name_________________________

CHM112 Lab – Titration of a Weak Acid – Grading Rubric Criteria

Points possible Lab Performance

Printed lab handout and rubric was brought to lab

3

Safety and proper waste disposal procedures observed

2

Followed procedure correctly without depending too much on instructor or lab partner

3

Work space and glassware was cleaned up

1 Lab Report

Graph attached and follows all guidelines and includes all labels described in 1, 2, 6 Question 4

Question 5

Question 6 & 7

Question 8

Question 9

Question 10

Total

Subject to additional penalties at the discretion of the instructor.

2 1

2

1

1

2

2

20

Points earned

Tritration of a Weak Acid: Data Sheet

Name ________________________

Unknown acid ________ (letter or number on the unknown bottle) Mass of the Unknown acid: _______________ Volume NaOH added

Concentration of NaOH(aq): ______________ pH

Tritration of a Weak Acid: Calculations and PostLab

Name ________________________

Calculated molar mass of the unknown acid= _________________ 6. On your graph:  Label the areas of the curve where a buffer is present.  Locate the point where pH= pKa (halfway to the equivalence point.) Label pKa on the graph. If the acid is polyprotic, label the pKa for each equivalence point on the graph.

pKa1 = _________________

pKa2 = ___________________ (from the graph)

7. Calculate Ka for your unknown acid. If your unknown is polyprotic, calculate Ka at both the equivalence points. (Show work) Ka1 = _________________

Ka2 = __________________

8. For each of the possible unknows, list their formula, if they are mono or diprotic, their molar mass and their pKa values. (Use a reputable source such as the CRC Handbook or NIST Webook)

Source of data _________________________ 9. Based on the data above, what is your unknown? _________________ Explain the reasoning behing your choice. How do your pKa(s) and molar mass data compare to the literature values?

10. Calculate the initial concentration of your acid. Recall that the original volume of the solution was 0.100 L [Acid]initial = ________________________ Using your calculated value of Ka and the initial concentration of the acid, calculate the initial pH of your acid. Set up an ICE table. You may need to use the quadratic formula.

How does this compare to your initial pH from the titration?

## Determination of pKa & Molecular Weight with a Titration Curve

Experiment # 11. Titration of a Weak Acid Aim: To determine the molar mass, pka and the identity of an unknown weak acid by titration. Introduction A ...

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