Then the sodium hydroxide was put in a burette of 50 cm3 and it was closed tight with making sure that there was no air bubbles inside. The titration begins whereas the beaker of the hydrochloric acid and the calcium hydroxide solution was placed under the burette with a magnetic stirrer inside the beaker to stir the substance while the titration is carried out.
The tap of the burette was opened and the NaOH was being poured into the beaker. The solution started to change color during the titration and the tap of the burette was closed again when the solution became greenish.
The experiment was repeated three times in order to avoid errors made and in each time the volume of NaOH added was recorded.
Raw data table of the titration being made between the NaOH and the solution of HCl and Ca(OH)2
The average volume of NaOH is
±0.1dm3
Now to determine the mass of the calcium hydroxide we need to know the amount of the hydrochloric acid and hence the amount of the calcium hydroxide
The amount of the alkali from the titration is:
NaOH = c x V= 0.25 x 0.0056 =0.0014mol
The equation of the reaction between the hydrochloric acid and sodium hydroxide is:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O
1mol 1mol
0.0014mol
From the equation the mole ratio between the HCl and NaOH is 1:1 therefore the amount of the hydrochloric reacted with NaOH is 0.0014mol
The amount of the acid that used primarily is
HCl = c x V = 0.05 x 0.50= 0.025mol
Hence the amount of the HCl reacted with the calcium hydroxide from the first place can be calculated:
0.025mol – 0.0014mol = 0.0236mol
The reaction between the hydrochloric acid and the calcium hydroxide is shown in the equation:
Ca(OH)2 (s) + 2HCl (aq) → CaCl2 (aq) + 2 H2O (l)
1mol 2mol
0.0236mol
The ratio between the HCl and the Ca(OH)2 is 2:1 therefore the amount of the calcium hydroxide is
Hence the mass of Ca(OH)2 is = n x M = 0.0118mol x 74.10g mol-1=0.874g
Conclusion and Evaluation
The mass of the calcium hydroxide was determined using the back titration and it is 0.874g. Now to compare it with the real mass of the calcium hydroxide sample 0.74g
This indicated that there is a big error occurred during the experiment. The problem for this error might be the high concentration of the NaOH because it was used only around 6ml to get the substance green while if we wanted to get the real mass of the Ca(OH)2 then ml 20ml of NaOH should have been added.
The experiment was not hard to follow and it was quite easy. To improve our measurements the experiment need to be repeated more times.
