pH values taken from http://en.wikipedia.org/wiki/PH_indicator
As can be seen from the above table, bromothymol blue is the ideal pH specific indicator to be used in this particular investigation as its colour change point lies exactly on the neutral pH of 7.0. As calcium chloride is soluble it is ideal for titration. Insoluble acids like sulphuric acid would not be suitable for this investigation.
Method Prior to beginning the investigation the hydrochloric acid must be diluted to the same concentration as the limewater. This is because if the acid were not diluted, a very small volume of acid would be needed to neutralise the limewater, thus leading to high percentage inaccuracies. The concentration of the limewater was calculated using the following method;
Number of moles (mol) = Mass (grams) / Molar mass
Therefore (1 x 40.1) + (2 x 16.0) + (2 x 1.0) = 74.1
Therefore number of moles present = 1/74.1 = 0.013495 mol
To dilute the 2.0mol dm-3 solution to 0.01mol dm-3 deionised water will be used to eliminate the possibility of impurities being present within the solution, the probability of this increasing would have been increased if tap water had been used. An entire decimetre of solution was not required for this investigation, therefore only 500cm3 was made up. The dilution was carried out accordingly;
10.00cm3: 1000.00cm3 = 0.02mol dm-3.
Therefore 5.00cm3: 1000.00cm3 = 0.01mol dm-3. Therefore 2.50cm3: 500.00cm3 = 0.01mol dm-3.
For this dilution a graduated pipette will be used to accurately measure 2.50cm3 of acid, following this the deionised water will be added until the bottom of the meniscus rests on the 500cm3 mark. The lid will then be firmly placed on the volumetric flask and the solution will be shaken well, at least 20 shakes to ensure that the concentration of acid is equally distributed throughout the solution. Prior to pouring the acid into the burette a small funnel will be placed into the top to aid accurate pouring, ensuring none of the solution is wasted. Also, a small amount of deionised water is passed through the burette with the valve open to remove any impurities remaining on the sides. The burette will then be filled with the diluted acid until the meniscus rests on the 0.00cm3 line. A pipette filler will then be used to obtain 25.00cm3 of the limewater solution, which will then be transferred to a clean conical flask. Two drops of bromothymol blue will then be added to the solution, before placing the conical flask directly below the end of the burette, relatively close to reduce splashing. The hydrochloric acid will then be gradually added at first, at relatively frequent intervals until the drops of hydrochloric acid begin to instantaneously turn a greeny yellow, signifying that the solution is close to being neutralised. The hydrochloric acid will then be added at less frequent intervals and in smaller amounts until the solution turns a specified shade of green. The burette line at which the meniscus of the hydrochloric acid rests will then be taken as a final reading, from which the starting reading will be taken away, in this case being zero. Prior to carrying out three actual tests a preliminary test will be conducted to see at what approximate reading the solution does neutralise.
Apparatus
- Graduated pipette
- Burette
-
500 cm 3 Volumetric flask
- Conical flask (s)
- Pipette filler
- Teat pipette
Safety The only significant hazards in this investigation are the 2.0mol dm-3 hydrochloric acid prior to its dilution, and the mild alkaline limewater solution. Goggles will be worn throughout the investigation due to these hazards to the eyes. If either chemical comes into contact with the skin, the area should be washed thoroughly with cold water.
Variables Controlled
- Concentration of hydrochloric acid
- Concentration of limewater
- Quantity of limewater used
- Atmospheric pressure
- Temperature
Varied
- Quantity of hydrochloric acid used
Results
Average titre = 24.25cm3 ((24.20 + 24.20 + 24.25) / 3)
Analysis The balanced equation:
2HCl (aq) + Ca (OH) 2(aq) → CaCl2 (aq) + 2H2O (l)
Therefore to find the concentration of the limewater I obtained the average titre, and then worked out the number of moles of hydrochloric acid that were used using the equation: No. of moles (mol) = Concentration (mol dm-3) x Volume (dm3
Therefore 0.01 x 0.02425 = 0.0002425mol.
To work out the number of moles of limewater used, the ratio of limewater to hydrochloric acid is 1:2, therefore 0.0002425/2 = 0.00012125. To work out the concentration of the limewater this figure is divided by the volume of limewater used using the equation:
Concentration (mol dm-3) = No. of moles (mol) / Volume (dm3).
Therefore 0.00012125/0.025 = 0.00485. To work out the concentration in g dm-3 we must first obtain the molar mass of calcium hydroxide which is 74.1. Using the equation:
Mass (g) = Number of moles (mol) x Molar mass
Therefore 0.00485 x 74.1 = 0.359385gdm-3. This being only accurate to approximately 0.36gdm3 due to the limited precision of the apparatus used.
