HCl + NaOH → NaCl + H2O
Calculations:
Moles of HCl= n=0.02dm3 x 1.00moldm3=0.02 moles of HCl
Moles of NaOH n=0.0107dm3 x 0.10moldm3= … as 1:1 relation with HCl then n=0.00107 moles of HCl
Total number of moles used=0.02-(0.00107*10)=0.00930 moles
As 2:1 relation with CaCO3 then 0.00932=0.00465 moles of CaCO3
Conversion to grams= 0.00465 moles x 100.08 grams1.00 moles=0.46537 grams of CaCO3
Percentage by mass= 0.465g0.60 g x 100=77.50%
Analysis:
From the graph obtained, the point of equivalence can´t be easily identify by just observing the graph because in this case we titrate a weak acid with a strong base, so as it appears in the graph, at the beginning, big amounts of sodium hydroxide is needed to produce a small change in the pH, when is closer to seven the pH level suddenly jumps fast to then have a constant pH much higher than 7. The pH of the final solution is now alkaline because after the dissociation of the hydrogen ion of the acid which was dissolved in water, it then forms hydroxide ions, which makes the pH to be higher than seven.
In the initial part of the curve, this is between 0-9ml of the base solution used, it is called the buffering region, in this region, the amount of strong base added is still not enough to neutralize completely the weak acid. The buffer solution achieves their resistance to pH change because of the presence of equilibrium between the acid (HCl) and its conjugate base (Cl-).
The use of the phenolphthalein advertises when it reach an equivalence point, but it doesn´t give an exact quantity results, because when it turns into a purple color, the pH has already pass 7.
For this procedure it cannot be used directly to titrate the CaCO3 because it is very slow when the reaction is close to reach the point of equivalence, so instead, the determination is achieved by adding an excess of acid, which was the HCl, to dissolve all of the CaCO3 and then titrating the remaining solution with NaOH solution to determine the amount of acid that did not react with the calcium carbonate. The difference between amounts of the acid (HCl) initially added and the amount left over after the reaction is equal to the amount that is used by the CaCO3. From this, the amount of CaCO3 in the sample was calculated.
Conclusion:
The final results obtained shows that 77.50% of the total mass of the sample eggshell, was calcium carbonate, so this tells that eggshell contains a big amount of CaCO3 , being a good source of it. In respect to the limitations, one of them is in the procedure, where the smash eggshell was mixed with the hydrochloric acid and then it was used directly for the titration. The problem with this was that in the solution, it formed some foam and there was some precipitation of some big pieces of the eggshell, so this it may cause some differences in the volume and concentration of the final solution after the titration.
A way to solve this limitation is by making sure that the sample of the eggshell is smash completely, if necessary the use of a colander could be very useful, which will also help to have a more precise mass of the sample. The second part would be to filtrate the solution of the hydrochloric acid and the sample, to eliminate the foam and precipitation left, and then start the titration.
by
gveliz (student)
