AS- Assessed Practical
Chemistry
Aim: To determine the concentration of lime water
Plan
The aim of this experiment is to find the concentration of a sample of limewater solution in g dm?3. I am required to design my own experiment and choose a range of appropriate equipment and apparatus. The indicator will be supplied to me. I am provided with 250 cm3 of limewater, which has been made such that it contains approximately 1g dm?3 of calcium hydroxide. Also available is hydrochloric acid which has a concentration of 2.00 mol dm?3, this concentration is too much though so it is necessary to dilute it.
The apparatus, equipment and chemicals that I will use are as follows: -
1) Pipette 25cm3
2) Volumetric flask 500cm3
3) Conical flask 250cm3
4) Burette
5) White tile
6) Clamp and stand
7) Indicator (methyl orange)
8) Limewater (250cm3 with 1g dm?3 of calcium hydroxide)
9) Hydrochloric acid- standard solution (conc. 2.00 mol dm?3)
10) Distilled water
Slaked lime dissolved in water is called limewater.
Quicklime + Water ? Slaked lime
CaO(s) + H2O(l) ? Ca(OH)2(s)
Calcium oxide reacts violently with water to produce calcium hydroxide (slaked lime)
Addition of further water to Calcium hydroxide produces the saturated aqueous solution known as Limewater.
In the experiment it is necessary to react the Hydrochloric acid with the Limewater in order to establish the concentration of the limewater.
Acid + Metal Hydroxide ? Salt + Water
The balanced equation that takes place is:
Limewater + Hydrochloric acid ? Calcium chloride + Water + Hydrogen
Ca(OH)2(aq) + 2HCl ? CaCl2 + 2H2O
In order for the experiment to be accurate and reliable it will be necessary to dilute the Hydrochloric acid significantly. It is in my belief that a concentration of 0.1mol dm?3 would enable me to achieve reliable results. This would require a dilution factor of twenty. I will be making 500 cm3 of dilute hydrochloric acid. I will also be using 25cm3 of Limewater.
In order to reduce errors it is necessary to choose accurate and reliable equipment. That is why I have chosen the pipette, volumetric flask and burette. All of these instruments have an accuracy of ±0.05cm3, which is suitable for my experiment and should produce accurate and reliable results. In making my choice of indicator I also had to be very careful. The Limewater used will be a relatively weak base so it will be appropriate to use methyl orange as it has an end point on the ph scale between 8-5. Whereas phenolphthalein indicator has an end point much higher up in the ph scale. So if I were to use the phenolphthalein indicator then the end point (colour change) would be reached before the equivalence point (when the chemicals, acid-base break up).