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. Therefore there exists an equilibrium between the ions and the undissociated acid molecules.

Consider a weak monoprotic acid HA

HA(aq) + H2O (l) ) ? A—(aq) + H3O+(aq) Ka = [A—][ H3O+]/[ HA]

Ka is the acid dissociation constant.

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

When the acid HA is titrated with NaOH, the reaction taking place is

OH— (aq) + HA(aq) ? A—(aq) + H2O(l).

A polyprotic acid has more than one ionizable proton and each proton results in an equivalence point.

Titration of a weak Acid: Calculations and Post lab questions

Name:

Unknown acid (letter or number on unknown bottle)

Mass of Unknown acid:

Concentration of NaOH(aq):

Plot the titration curve as pH vs. mL of NaOH added using a computer graphing program. Make sure you have chosen the correct X and Y axes. Please include gridlines and print out a 8 ½ x 11 plot. You must attach the graph to the Lab Report.

Locate the equivalence point(s) on the titration curve. It is very unlikely that the equivalence point(s) will be actual data points. You will need to either manually draw a smooth curve connecting your data points, or select the option in your graphing program that will draw the best-fit curve for you. Then find the steepest point(s) on the curve. This is/these are the equivalence point(s).

Label the equivalence point(s) on the curve. (For polyprotic acids, label all the equivaence points.)

How many equivalence points does your graph show?

Recall that at the first equivalence point, moles of acid = moles of base added.

Volume of NaOH added at the first equivalence point:

Moles of NaOH added at the first equivalence point:

Volume of NaOH added at the Second equivalence point (if your unknown is polyprotic)

Use the mass of the acid, the concentration of NaOH and the volume of NaOH added at the first equivalence point to calculate the molar mass of the acid: (Show your work clearly and include proper units.)

Calculated Molar mass of the unknown acid=

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)

Calculate Ka for your unknown acid. f your unknown is polyprotic, calculate Ka at both the equivalence points. (Show work)

Ka1 =

Ka2 =

What is the identity of your unknown? Explain your reasoning behing your choice of unknown.

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 the ICE table. You may need to use the quadratic formula.

Initial pH from the titration:

Calculated initial pH:

Was the calculated initial pH higher or lower than initial pH from the curve ? Calculate the % error.

Look up the actual literature value of the pKa of your unknown in an appropriate source. (CRC handbook or the NIST webbook)

Literature value of pKa =

Calculated pKa =

(List both pK­a’s if your acid is polyprotic)

Source of the data :

Was the actual value higher or lower than the pKa from your graph ? Calculate the % error.