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# The Concentration of Limewater Solution.

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Introduction

The Concentration of Limewater Solution. Aim: The aim of this experiment is to find out the concentration of limewater solution. I am provided with 250.00 cm3 of limewater, which has been made such that it contains approximately 1g dm3 of calcium hydroxide. Also available is hydrochloric acid, which has a concentration of 2.00mol dm3. To determine the exact concentration of the limewater, the hydrochloric acid must be titrated against it. The hydrochloric acid is very concentrated, and a very little amount will be needed to neutralise a flask full of limewater. Therefore, it will be necessary to dilute the hydrochloric acid. I can use an appropriate value of limewater concentration to calculate the approximate value of 2.00mol dm-3 hydrochloric acid that will neutralise 25.00cm3 of the limewater. I can then calculate how much to dilute the hydrochloric acid by, and make up the volume of hydrochloric acid that will neutralise the limewater into a measurable amount. Calculation: Using 25.00cm3 of limewater solution of approximately 1.00g dm-3. Since the concentration of limewater is 1.00g dm-3 Therefore the mass of Ca(OH)2(aq) in 1g dm-3 of the solution = 1.00g Therefore Mass Ca(OH)2 in 25.00cm3 of solution = 1x25 = 1g = 0.025g 1000 40 Hence no. of moles of Ca(OH)2 in 25.00cm3 of solution = Mass = 0.025 = 3.373 x 10-4 moles RMM 74.1 Since Mole Ratio of Ca(OH)2 : HCl = 1: 2 Therefore No. of moles of HCl = 3.373 x 10-4 x 2 = 6.746 x 10-4 moles Volume of HCl = No. ...read more.

Middle

Acid base indicators such as methyl orange, phenolphthalein and bromothymol blue change colour according to the hydrogen ion concentration of solution to which they are added. Consequently, they are used to test acidity and alkalinity. They are used to detect the end point in acid-base titration. In an acid base titration, the base solution is added to the acid until the acid has been neutralised and this is known as the equivalence point of the titration. I chose to use Methyl Orange as the indicator suitable for my experiment as this is suitable for strong acid with weak alkali titration. Bromothymol blue may also be used and so may phenolphthalein but it's not really reliable because it does not begin to change colour until about pH 8. Methyl Orange is a reddish orange liquid within the range 3.0<pH<7.0. This means that it will be reddish orange in the limewater and become yellow at the end point of the reaction between hydrochloric acid and calcium hydroxide i.e. when the solution becomes neutralised. A white tile is needed to place under the conical flask, to make it easier to tell exactly when the indicator has become transparent. The white tile can also be used to help see the liquid meniscus in the burette when reading the volume from it. Safety Precautions Substance Hazard Precautions Calcium hydroxide May cause burns and is corrosive. It is irritating to eyes, skin and respiratory system. ...read more.

Conclusion

Percentage Error I believe that the instruments I have used have a margin of error. % Error of pipette = 0.05 x 2 = 0.1 % 10.00 % Error of Volumetric Flask = 0.6 x 100 = 0.06 % 1000 Total % error = 0.16 It is best to do a rough titre before doing the actual ones, to get an approximate result, so that you know approximately when the end point is, so the following titres can be done more rapidly. Whilst carrying out this procedure, eye protection should be worn as hydrochloric acid is corrosive and irritant and can cause burns. Once the two consistent results have been obtained, e.g. 35.00cm3 and 35.10cm3, the exact concentration of the limewater solution should be calculated as in the sample calculation, which follows: Titration Number 1 2 Start Reading 0.00cm3 0.00cm3 Finish Reading 35.00cm3 35.10cm3 Titre (cm3) 35.00cm3 35.10cm3 (All results taken within an error reading of +/- 0.05) Average titre = 35.00 + 35.10 = 35.05cm3 of HCI 2 No. Moles HCl = concentration x volume = 0.020 x 0.3505 = 7.01x 10-3 Therefore molar ratio HCl: Ca(OH)2 = 2:1 Therefore No of moles of Ca(OH)2 = 7.01 x 10-3 / 2 = 3.505 x 10-3 Mass of Ca(OH)2 = no. Moles x RMM = 3.505 x 10-3 x 74 = 0.259g Concentration of limewater = mass = 0.259 = 1.037 g dm-3 Volume 0.25 Therefore, if the two titres gave the results 35.00cm3 and 35.10cm3of hydrochloric acid, then the actual concentration of limewater would be 1.037 g dm-3. Ibrar Razaq 12PWR 1 ...read more.

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