Lab Report 2 – Titration
CHEM1903 – Chemistry 1A (SSP) Michael West (305159240)
1. Experiment 2.2 – Titrimetric determination of the molecular mass of an organic acid
Method An unknown organic acid was supplied in solid form. The acid was known to be diprotic and had the reference number 19. Using an analytical balance, 1.5397 g of the acid were weighed out, and made up with deionised water into 250 mL of solution. 25 mL of the acid solution was added to a conical flask with phenolphthalein indicator and titrated against standardized 0.0983 M NaOH solution. Three titrations were performed and the results averaged. The molar mass of the acid was then calculated and compared to a list of ...view middle of the document...
Volume (mL) Titration 2 Titration 3 0.00 0.00 18.00 47.95 29.95 18.10 48.25 30.15
Point Origin Phenolphthalein endpoint (volume of titre A) Methyl orange endpoint Volume of titre B
Titration 1 0.00 17.85 47.60 29.75
Mean n/a 17.98 n/a 29.95
The titrations were not exactly reproducible, with the volume of both titre 1 and titre 2 increasing from titration 1 to titration 3. This might be because the titrations were not performed simultaneously, with the soda water aliquots of titrations 2 and 3 allowed to stand exposed to the atmosphere while titration 1 was being carried out. As it warmed, this soda water would have undergone degassing, with HCO3- and H2CO3 being converted to gaseous CO2, released to the atmosphere. Thus, less NaOH reacted with the later aliquots, and more was left to react with the HCl, so the titres of HCl were larger.
Equilibria between carbonic acid, hydrogen carbonate and carbon dioxide, and ions in water:
Titration of HCl with NaOH:
If the density of water at 20°C is 0.998 g/mL, then the solubility of CO2 in water at 20°C of 0.145 g/100 g is equivalent to . The molar mass of CO2 is . .
Hence, the concentration of the gas at saturation is
As a check, we can derive a similar result using Henry’s Law for the concentration of gases in solution. Details of this are given in an appendix, with the result of matching the above result to only 1 significant figure, but at least giving confirmation of the order of magnitude. The numbers for saturation point are hence much lower than the experimentally determined concentration of CO2 in the soda water, . Since the experimental concentration was determined after the cap was taken off, we conclude that even with the cap off, the soda water was still super-saturated in CO2 (at least initially; over time, degassing will cause the soda water to become unsaturated). Before the cap was removed and any degassing occurred, the concentration must have been higher, so it would also have been super-saturated then. Two key general principles are operating. The first is related to Henry’s Law, . As mentioned, more details are available in the appendix, but we note that concentration of CO2 in the aqueous phase is directly proportional to the partial pressure of CO2 in the gaseous phase above the liquid, . Hence, when the seal is broken and the pressure drops, the concentration of aqueous CO2 falls as it is released to the...