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determining the concentration of a limewater solution

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Determining the concentration of a limewater solution The Task: The aim of this experiment is to determine the concentration of a limewater solution in g dm�� as accurately as possible. I have been provided with 250.00 cm� of limewater which has been made such that it contains approximately 1g dm�� of calcium hydroxide. Also available is hydrochloric acid which has a concentration of exactly 2.00 mol dm��. This acid is too concentrated to be used at full strength, so it will need to be diluted to a suitable strength that will allow us to get a good degree of accuracy without diluting it too far or using too much. I must also use an appropriate quantity of limewater to work out the exact concentration of the hydrochloric acid. I have chosen to use 30.00cm� because it is a large enough amount to give a good degree of accuracy when neutralising it, and will also allow me to do 8 titrations if I need to, which will be more than enough. I have decided to dilute the hydrochloric acid to a suitable strength, but before I do that I need to calculate what strength I need it to be so that I need an approximately equal amount of dilute hydrochloric acid to neutralise the limewater solution. It is necessary to dilute the hydrochloric acid because the amount need to neutralise the limewater would be so small that it would not be able to be measured accurately in a burette and would therefore give a large margin of error. To do this investigation it is necessary to do an acid/base titration. Calculations: I shall be using 30.00 cm� of limewater solution of approximately 1.00g dm��. ...read more.


I then placed the measured limewater into a 250.00cm� conical flask, before adding 5 drops of the indicator solution (phenolphthalein). Once that was done I placed the conical flask on a white tile under where the burette was to be placed. I then flushed the burette with hydrochloric acid to get rid of any bubbles, but making sure that I left some hydrochloric acid in the burette so that no more bubbles accumulate. I then made sure that the burette was vertical, placed it in the clamp and then topped up the burette with hydrochloric acid using a funnel so that none was spilt. I then made sure to remove the funnel so that no more hydrochloric acid could drop into the burette and alter the amount in the burette after I read the measurement which could affect the accuracy of the titration. I then placed a piece of filter paper behind the burette to aid in the reading of the meniscus. This allowed me to read it more accurately because it removed the distractions in the background. The meniscus was at 0.45cm�, making sure to read it at eye level to ensure the reading was entirely correct. The apparatus I used is shown how it was set up in the diagram below: I made sure that the tip of the burette was slightly inside the conical flask so that no hydrochloric acid could escape the flask by splashing outside of it, and cause the titration to have inaccuracies. The dilute hydrochloric acid was then added to the limewater using the tap on the side of the burette to allow the dilute hydrochloric acid to be let out. ...read more.


However I realised this whilst doing my pilot experiment and tested the pH of each titre once it had completed to ensure that the solution had been neutralised. I have shown another diagram below of the changing points of phenolphthalein and methyl orange but this time they are also compared to litmus to show how it is useful to use it in conjunction with one of the other indicators to give a more accurate neutralisation: Diagram to show the point where different indicators change colour in relation to pH levels: As you can see from above, the litmus changes over an unusually wide range that spans over pH7, which is why it allows you to get a more accurate neutralisation. Phenolphthalein is the indicator that I used. Below I have included another small diagram to show why it changes colour. Phenolphthalein is another weak acid, as are all indicators. In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 9.3. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy. This is another reason why I used the litmus paper to ensure that my results were accurate. Other than using a more suitable indicator, I am generally pleased with the results I obtained and feel that the exact molarity that I found out for the limewater is very plausible. I am not saying that I was completely accurate. However, I do feel that with the equipment and apparatus provided I could not have obtained significantly better results. ...read more.

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