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Determining the concentration of a limewater solution by volumetric analysis

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Introduction

Determining the concentration of a limewater solution by volumetric analysis The best way to determine the concentration of a solution is by a titration, and as limewater (Ca(OH)2(aq)) is a base an acid/base titration would be most useful: Ca(OH)2(aq) + 2HCl(aq) 2H2O(l) + CaCl2(l) Precautions to take: working with chemicals, e.g. corrosive hydrochloric acid, means goggles should be worn as well as an apron to protect clothes and care must be taken in handling them, e.g. phenolphthalein is flammable. To get make accurate measurements of fluid bring to eyelevel and measure from the meniscus. To ensure that this test is fair there is only one dependent variable, the amount of titrant used. Using the same volumes of other chemicals throughout in an accurate way and keeping the surrounding conditions the same with a thermostat in the room also maintains a fair test as well as accurate results. The first thing to do is to dilute the hydrochloric acid titrant. ...read more.

Middle

burette * another 25cm3 pipette with filler to collect accurate and measured amounts of limewater for numerous titrations * limewater * and phenolphthalein to provide a visual indicator of the endpoint. I have chosen phenolphthalein as my indicator over methyl orange as the colour change is more prominent (turning from purple to colourless instead of yellow to red) but more importantly because the acid will be diluted and therefore weak and the pH of the final solution will be nearer to seven and this indicator has an end point closer to seven2 than methyl orange. Set up the burette to the stand and making sure the tap is closed, fill the burette to the 0.0cm3 mark. Do this using a funnel2 to prevent spillage but remember to remove this funnel afterwards as excess drips could ruin readings if left in the burette and ensure there is no air bubble at the tip of the burette1 that will make it seem there is more acid in the burette than there actually is. ...read more.

Conclusion

3.4 (overshot) 10.0 13.0 16.5 19.6 22.7 25.7 28.7 Titration(cm3) 3.4 - 3.1 3.0 3.5 3.1 3.0 3.0 3.0 Average titre = 3.1+3.0+3.1+3.0+3.0+3.0 =18.2 � 6 = 3.03cm3. If, on average, 25cm3 of Ca(OH)2(aq) required 3.03cm3 of HCl for the titration then the concentration of the limewater can be found with the formula concentration=moles�volume and the equation: Ca(OH)2(aq) + 2HCl(aq) 2H2O(l) + CaCl2(l) This equation shows that there is a molar ratio of 1:2 between Ca(OH)2 and HCl respectively. By working out the moles of HCl present the number of moles of Ca(OH)2 can be calculated and the formula can be completed: Moles of HCl = 0.20mol dm-3 � 0.00303dm3 = 6.06�10-4 The amount of moles of Ca(OH)2 is half the moles of HCl, so moles of Ca(OH)2= 6.06�10-4 � 2 = 3.03�10-4 moles. Concentration of Ca(OH)2 = moles � volume = 3.03�10-4mol� 0.025dm-3 = 0.01212mol dm-3 To change this in relation to grams per cubic decimetre simply times this by the molar mass of calcium hydroxide which is 74.08g mol-1 (Ca=40.08, O=16�2, H=1�2): Concentration of calcium hydroxide in the limewater = 0.01212 � 74.08 = 0.898g dm-3 (3.s.f.). ...read more.

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