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To determine the concentration of limewater solution

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Introduction

Chemistry -AS - Assessed Practical (Skills P & A) To determine the concentration of a limewater solution Planning Limewater is calcium hydroxide in solution. As it's a metal hydroxide, it is slightly alkaline and sparingly soluble in water, which is why it is weakly alkaline and has a pH of 9 to 10. Limewater is used as a test for carbon dioxide gas - carbon dioxide turns limewater cloudy. The cloudiness is due to a precipitate of calcium carbonate being formed. To determine the concentration of the limewater solution (in g dm-3) we can carry out an experiment. This experiment is a simple acid-base titration, in which we are given the concentration of hydrochloric acid which has a concentration of exactly 2.00 mol dm-3. Also we are provided with 250cm3 of limewater which has been made such that it contains approximately 1g dm-3. A titration is basically a neutralization reaction where we record the volume needed to completely react the unknown chemical with the known one. For titration, the concentration of the limewater is to be ascertained by reacting it with hydrochloric acid. The equation for this reaction is shown below: Ca(OH)2 (aq) + 2HCl (aq) � CaCl2 (aq) + 2H2O (l) As you can see, it takes two moles of hydrochloric acid to completely neutralize one mole of calcium hydroxide. ...read more.

Middle

5. Repeat step 2. 6. Empty out the beaker and wash out and then transfer then solution from the flask to the beaker. The concentration of limewater is Mr of Ca(OH)2 = 40 + 2(16 + 1) = 74 Moles = Mass / Mr Moles = 1 / 74 Conc. approx = 1/74 mol dm-3 = 0.0135 mol. Therefore for the equation the acid is 2 � 0.0135 = 0.027mol. The 0.027 mol dm-3 solution of HCl is now prepared and ready to use. Titration Before doing the actual experiment, it is important to know about the systematic errors that could occur during the titration and hence can lead to biased results. I will carry out the following tasks before titrating to avoid any systematic errors: * Close the stopcock and with the aid of a funnel pour the acid solution upto the highest volumetric graduation. * Check the bottom of the burette for any air bubbles as this can cause systematic errors. * Before titration wash out the burette 2-3 times with the reagent to avoid concentration changes. * When reading the measurement off the burette, I will read it from the bottom of the meniscus to the top of a line. * Before reaching the end-point I will slow down the rate of flow and dispense partial drops for precise drops. ...read more.

Conclusion

of the limewater solution I will use the following calculations: The average volume used from the results is required and must be in dm3. To convert from cm3 to dm3, divide by 1000. For the calculations V will be used to represent this volume in dm3. Moles can be worked out using the equation: Concentration = Moles / Volume Moles = Concentration x Volume = 0.02 x V The reaction shows that two moles of HCl react with one mole of Ca(OH)2 so in the neutralisation reaction there must have been twice as many moles of HCl than Ca(OH)2 . Therefore moles of Ca(OH)2 must be (0.02 x V) / 2 = 0.01 x V Again using Concentration = Moles / Volume: Concentration of Ca(OH)2 = (0.01 x V) / 0.025 = 0.4 x V The number of moles in dm3 will be concentration x1 which is 0.4 x V x 1 = 0.4 x V. The mass of the Calcium Hydroxide in the solution can now be worked out using: Moles = Mass / Mr --> Mass = Moles x Mr Mr of Ca(OH)2 = 40 + 2(16 + 1) = 74 Mass of Calcium Hydroxide in dm3 = 0.4 x V x 74 Therefore the concentration of the Limewater in g dm-3 will be: 0.4 x V x 74 = ________ gdm-3 Bibliography * http://en.wikipedia.org/wiki/Bromothymol_blue * http://en.wikipedia.org/wiki/Titration * http://www.ausetute.com.au/dilucalc. ...read more.

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