TITRATION OF SODIUM HYDROXIDE SOLUTION AGAINST UNKNOWN ACID
TITRATION OF SODIUM HYDROXIDE SOLUTION AGAINST UNKNOWN ACID.
Observations made in this experiment include:
NaOH pellets were white hard solids with different shapes and sizes.
NaOH pellets required vigorous shaking and swirling to dissolve in the volumetric flask.
Unknown acid was white and solid.
Unknown acid required less shaking and swirling to dissolve in comparison to the NaOH pellets.
Milky substance was observed on initial addition of the phenolphthalein indicator before solution became colourless.
On addition of the titrant (sodium hydroxide), solution in conical flask was slightly pink before full neutralization occurred.
Slightly pink colour was observed in the conical flask once the end point was reached.
/cm3 Rough Accurate
±0.1/cm3 18.55 36.95 18.55
±0.2/cm3 18.45 18.40 18.50
Table 1: Titration results.
Mass of NaOH pellets
±0.02/g Mass of unknown acid
Table 2: Mass of NaOH pellets and Mass of unknown acid.
Concentration of NaOH
number of moles=massMolar Massn=0.89(15.999+1.008+22.990)n= 0.8939.997n=0.022 moles of NaOH number of moles=concentration x volumeconcentration=number of molesvolumevolume= 250.001000=0.2500 dm3concentration= 0.0220.2500concentration=0.089 moldm-3of NaOH NaOH moles in average titre
Average titre= 1st titre volume+2nd titre volume 2Average titre=18.40+18.502Average titre=18.45 cm3number of moles=concentration x volumen=0.089 x 18.451000n=0.016 moles of NaOH in the average titreNumber of moles of acid in volumetric flask
2NaOH+ H2 X?Na2X+2H2OWhere “X” represents the acid
2NaOH+ H2 X2 moles : 1 mole0.0016 moles : x x=0.00082 moles of unknown acid in 25.00 cm325.00cm3 ?0.00082 250.00 cm3? xx=0.0082 moles of unknown acid in 250.00 cm3 Molar mass of the unknown acid
number of moles= Mass Molar MassMolar mass= massnumber of molesMolar mass=0.960.0082Molar mass=117 gmol-1Molar mass=120 gmol-1 (2 significant figures)Name of the acid Molecular Formula Calculated Molar Mass
Octanoic Acid CH3(CH2)6COOH 144.21
Hexanoic Acid CH3(CH2)4COOH 116.15
Oxalic Acid HOOCCOOH 90.03
Succinic Acid HOOC(CH2)2COOH 118.09
Maleic Acid HOOC(CH)2COOH 116.10
Tartronic Acid HOOCCH(OH)COOH 120.06
Glutaconic Acid HO2CC(CH2)3CO2H 130.10
Table 3: Showing the supplied list of acids and their respective calculated molar mass.
Uncertainty in the concentration of NaOH
Acid-base titration is a neutralization reaction This neutraliation reaction occurs when an acid and a base react together to give, in this case, 2 products a salt and a base. Due to both the titrant (NaOH) and the acid being colourless, and indicator was required for this titration. For this experiment, phenolphthalein indicator was used to know when full neutralization had occurred, and the end point reached. A few drops of this indicator is added because it is an acid and excess of phenolphthalein will alter the concentration and volume of the unknown acid in the conical flask therefore altering the titre value and the colour change. The colour change was from colourless to pale pink.
0.89g of NaOH pellets were used to make a 250 cm3 standard solution which was the titrant in this experiment and 0.96g of the unknown acid was used to make a 250 cm3 standard solution. The NaOH solution was titrated against the acid.
In this experiment a standard solution of the titrant which was NaOH was made and was titrated against an unknown acid. The NaOH standard solution was made using a 250 cm3 volumetric flask. A standard solution of the unknown acid was made using a 250 cm3 volumetric flask as well.
The NaOH was
Multiple titrations were carried out till concordant values that were within 0.1cm3 of each other were achieved. This was to ensure that the titration results were accurate and that the average titre volume calculated was not incorrect.
Sources of error in this experiment
Presence of NaOH droplet at the tip of the burette which entered the conical flask containing the salt and water solution after the titration was complete. This affected the titre volume reading and affected the average titre volume calculated as the end point would have been exceeded and the colour change not as expected. Possible solutions to this error could be to turn the tap of completely and swirl the conical flask for a while before the titrant is run down again. Also digital burette which seals the tip shot once the tap is closed to prevent any droplet of NaOH from leaking out could be used.
As a result of the instantaneous colour change in the conical flask due to the phenolphthalein indicator. It was difficult to determine when the end point had been reached and when the titration was complete. This resulted in excess titrant and the wrong volume of NaOH read, affecting the average titre volume as well as the concentration of NaOH calculated. A possible solution to this error is to use a digital colorimeter which would detect the colour change immediately.
From the table of acids provided, the name and molecular formula of the acid provided could be Succinic acid: HOOC(CH2)2COOH, Maleic Acid: HOOC(CH)2COOH, Tartronic Acid: HOOCCH(OH)COOH due to the similarity in the molar mass calculated.
A possible equation for the acid and base reaction using Maleic acid as the chosen acid:
Acid-Base titration was carried out to determine the Molar Mass of the unknown acid and to suggest a possible name and molecular formula for the acid from a supplied list of acids. Multiple titrations of sodium hydroxide against an unknown acid were carried out. The concentration in the standard solution and number of moles in the average titre of NaOH were calculated to be 0.089 ± 0.1 moldm-3 and 0.0016 ± 0.4 moles respectively. The number of moles of the unknown acid was calculated to be 0.0082 ± 0.01 and the Molar Mass of the acid was calculated to be 117 gmol-1. From the supplied list of acids, Maleic acid was chosen as the suggested acid.