Perform a direct titration of an unknown diprotic acid with a strong base in order to identify and determine the molecular weight of an unknown acid.

Title : Acid-base Titration of a Diprotic acid and back titration of Unknow

Abstract:

The purpose of this laboratory experiment was to perform a direct titration of an unknown diprotic acid with a strong base in order to identify and determine the molecular weight of an unknown acid. Back titration of an unknown compound was performed to determine the weight percent of CaCO₃ . The molecular weight of a diprotic acid was …… g/mol and it was identified as …… acid ; Also the weight percent of CaCO₃ was calculated to be …… % +/- ……. .

Introduction:

The major aspect of acid-base equilibrium was used during this experiment ; The main laboratory technique that was done during this lab is acid-base titration in which a strong base (NaOH) was titrated with a strong acid (HCl) . Two titration methods were used which are direct and back respectively . Originally an unknown diprotic acid (#839) is provided and certain amount of it was diluted and titrated directly with a strong base NaOH ( 0.0214M +/-0.0002) .Calibrated pH electrode was used during the titration procedure to measure the solution’ s associated pH , therefore creating a titration curve. The unknown diprotic acid deprotonates twice with two equivalence points and  acid dissociation values ( k_a1 , k_a2 ) . In order to determine the largest incremental change in pH change by volume , the derivative of the titration curve was used. The equivalence point represents the moles of used NaOH at equivalence point can be traced through stoichiometric relation and basic molecular mass calculation to identify the provided unknown diprotic acid .

H₂A _(aq)  < – – – — – > H⁺_(aq) + HA^–_(aq)              k_a1

HA^– _(aq) < – – – – -> H⁺_(aq)  + A^2-_(aq)                          k_a2

The second part of this laboratory experiment involves the back titration of an unknown mixture (3-3) with strong base NaOH (0.0214M +/-0.0002) to determine the weight percent of present CaCO₃ .During direct titration the volume of titrant and the associated pH are used to find the two k_a values. However ,by using stoichiometry the molecular formula of diprotic acid is identified and therefore concentration and molecular weight were calculated. The back titration happens only in presence of excess base added to an analyte which is CaCO₃ ; therefore an excess titrant can help to determine the concentration of an analyte. Back titration is useful in finding out the concentration of an insoluble diprotic acid salts such as CaCO₃  .

CaCO_3(aq) < – – – – > Ca²⁺_(aq) + CO_3(aq)^2.      Ksp = 2.7 x 10 ^ -33      pH= 7.00   25C

Experimental:

This experiment is divided into three parts which are direct titration of an unknown diprotic acid, back titration of an unknown compound and clean up . The equipment used in this laboratory experiment include 50mL burette, 100mL volumetric flask, spatula , small weighting boat , calibrated pH meter , 25 mL pipette , watch glass and heating plate . The used materials were 0.02+/- 0.0001 M NaOH, 0.1052+/- 0.0002M HCl , diprotic sample , unknown sample and buffer solutions at pH 4 and 7 .

The preparation of an unknown diprotic acid sample (#839) was done by weighting around 0.12g +/- 10% and diluting it to 100 mL volumetric flask using distilled water. Next. , 25 mL of diluted diprotic acid solution was pipetted using 25 mL pipette and transferred to another 100mL beaker and placed on the magnetic stirrer. A magnetic bar was pulled into the beaker ; also , pH meter was placed into the beaker in a way that it was not touching the stir bar and the burette. Burette filled with twice with NaOH solution and direct titration started. The first equivalent point happened around pH of 4 and the next pH equivalent point was determined quantitively. Once both pH values of equivalence point were determined , the titration could have be repeated once more it was necessary.

The back titration was done on an unknown compound number 3-3, 0.2g +/- 10% was quantitively transferred to 100 mL beaker along with approximately 10 mL of distilled water . 25 mL of HCl ( 0.1052M +/- 0.0002 ) was pipetted into the beaker and the beaker was sealed with watch glass and  placed on hot plate for around 10 minutes in order to boil. After the allocated time , the beaker removed from hot plate and left on fume hood to adjust its temperature to the lab area temperature. The beaker’s content was transferred into 100mL volumetric flask using funnel and diluted to the mark using distilled water. After , 25 mL of the dilution was pipetted into a 100mL beaker and placed on the magnetic stirrer. Like the last part pH meter and burette were adjusted toward the beaker and titration started . pH was recorded by adding every 0.5-1 mL of NaOH into the beaker . The increments of titrant would become even smaller as rapid pH was detected. The process was done until the time that both equivalence points were observed.

After finishing both titrations , all waste solutions were poured into waste sink and all glass equipment were washed with soap and tap water and then rinsed with distilled water . Magnetic stir bar and unknowns were handed back to TA and electronic equipment (pH meter ) was plugged off and wrapped properly .

^{Results :}