Courseworks

Determining the Solubility of Calcium Hydroxide

1. Using a measuring cylinder put 25cm3 of hydrochloric acid into a 100cm3. Using some distilled water rinse out the measuring cylinder and pour the contents into the same beaker containing hydrochloric acid.2. Next transfer the contents of this beaker to a 250cm3 volumetric flask. It may be necessary to use a volumetric flask, and if so rinse this out once it has been used. Again rinse out the beaker using distilled water and pour the contents into the volumetric flask.3. Once the hydrochloric acid / water solution is in the volumetric flask, fill the volumetric flask with distilled water until you are about 2-4cm3 from the graduation mark. At this point add water to the volumetric flask using a pipette until the bottom of the meniscus is just touching the graduation mark. Stopper the flask and invert it several times to mix the contents together.4. Now, using the pipette and pipette filler, withdraw 25cm3 of the hydrochloric acid/ water solution and place this into a conical flask. Once this is done add 3 drops of methyl orange indicator to the contents.5. Next, rinse out the burette using distilled water and then rinse it using calcium hydroxide. After doing so fill up the burette using calcium hydroxide. If an air bubble is present near the jet, run out some liquid until the bubble is removed6. Record the initial burette reading to two decimal places and then start titrating. Keep swirling the contents of the flask as you add the alkali to the acid until there is a colour change of the solution. This is a rough titration and should then be repeated more accurately until the results lay within 0.1cm3 of each other.Quantities of Materials to use2HCl(l) + Ca(OH)2(l) CaCl2(l) + 2H2O(l)From the above equation it is clear that I require 2 moles of hydrochloric acid to react with 1 mole of calcium hydroxide. Therefore I will use a hydrochloric acid solution that is twice as concentrated as the calcium hydroxide.The concentration of the calcium hydroxide is 0.015mol dm-3. Therefore the hydrochloric acid needs to be a concentration of 0.03mol dm-3. Originally the concentration is 0.3mol dm-3 so I need to dilute this by a factor of 10. As the volumetric flask holds 250cm3 I require 1/10 of this to be hydrochloric acid. I will therefore use:1/10 x 250 = 25cm3 hydrochloric acidI will therefore need 225cm3 water to make up this concentration as well as ncm3 of water to wash out the other apparatus.I predict that I will need a volume of about 25cm3 of calcium hydroxide for each titration, but can not say the exact total amount I require as I do not know how accurately I will be able to carry out the titration.As well as this I require 3 drops of methyl orange indicator per titration and once again I can therefore not say the total amount I need as I don’t know how many titrations I will carry out.Justification of MethodDuring this experiment I have decided to use a burette as it has a large resolution- i.e. it can be read very accurately. This will enable me to obtain more reliable results and therefore to calculate a more reliable reading for the concentration of the sulphuric acid.For the same purpose I have used the volumetric flask as I can accurately make up a concentration of hydrochloric acid/ water solution. The instruments I am using are mainly all precision instruments which will allow me to make accurate readings.The fact that I am rinsing things I have placed reactants in also make my results more accurate as I will not be losing any of the hydrochloric acid (e.g. the measuring cylinder and burette in steps 1 and 2 of my practical procedure) and therefore will not be making slightly lower concentrations. Again when I calculate the solubility of the calcium hydroxide solution I will get a more reliable answer.I will also only add a few drops of methyl orange indicator so I can clearly observe and colour changes when the acid alkali solution neutralises.In these ways I believe that when I carry out my experiment I will be ensuring the test is fair and therefore I will obtain more accurate and reliable results.E.g. Below are shown some of the percentage errors for several of the apparatus I will use and demonstrates the precision of the apparatus I shall use.250cm3 Volumetric Flask = 0.08%Burette = 0.2%Risk AssessmentChemicalHazardPrecautionCalcium HydroxideMay evolve toxic fumes in fireDoesn’t apply to my experimentIrritant to skin, eyes and respiratory systemWear protective clothing such as plastic or rubber gloves whilst handling, as well as safety goggles. At higher concentrations it is corrosiveHydrochloric AcidMay evolve toxic fumes in fireDoesn’t apply to my experimentIrritant to eyes and skinWear safety goggles as well as plastic or rubber glovesMethyl OrangeIrritant to eyes and skinWear safety goggles as well as plastic or rubber glovesDetails of Sources1. Salters Advanced Chemistry 2000- Worksheets Elements of Life 2.1 page 4This was used to help me prepare a procedure for the titration2. Salters Advanced Chemistry 2000- Worksheets Elements of Life 2.1 page 5This was used to help me in developing a strategy for the equipment I will use in my titration, as they allow me to see how accurately results can be taken for pipette and volumetric flask.3. http://www.chemistry-react.org/go/Tutorial/Tutorial_4428.htmlThis was used to help me in developing a strategy and seeing accuracy of burette and other equipment4. Handout Sheets referring to Health and Safety about:- Calcium Hydroxide- Hydrochloric Acid- Methyl Orange5. Information Sheet on the use of indicators in Acid Alkali TitrationsEnabled me to see what kind of indicator I will need to use for my experiment- Methyl Orange in my caseResultsTitrationRough 1 2Initial Burette Reading(cm3)1.001.000.00Final Burette Reading(cm3)25.4528.2027.10Titre(cm3)24.4527.2027.10Average Titre(cm3)_______27.15AnalysisCa(OH)2+ 2HCl CaCl2 + 2H2OTherefore number of moles of HCl = twice the number of moles of Ca(OH)2Moles of HCl used = Concentration x Volume(mol) = 0.0300 x 0.025= 7.5 x 10-4 molTherefore moles of Ca(OH)2 used = 7.5 x 10-4 / 2= 3.75 x 10-4Therefore concentration of Ca(OH)2 = moles / volume= 3.75 x 10-4 / 0.02715= 0.0138 moldm-3 (3 sig. fig)EvaluationIn my experiment I did not obtain any anomalous results, which demonstrates the precision and accuracy of the experiment. The two real titrations I carried out- i.e. those excluding the rough titration- were within 0.1cm3 of each other and illustrates the reliability of the experiment. The results I obtained were also very accurate as they could be read to 2 decimal places, which demonstrates the precision of the experiment.Below I have calculated the percentage error for some of the main pieces of apparatus I used and once again shows the accuracy of the instruments.Percentage Error = (error x 100) / readingPercentage error for a250cm3 volumetric flask = (0.2 x 100) / 250= 0.08 %Percentage error for a25cm3 pipette = (0.06 x 100) / 25= 0.24 %Percentage error for a50cm3 burette = (0.05 x 100) / 27.15= 0.18% (2 d.p)It is therefore clear that despite the very small percentage error I obtained for each piece of apparatus, the most inaccurate piece of apparatus was the pipette I used. However the combined percentage error for this experiment is only about 0.5%, which is very small. This means that the actual volume of calcium hydroxide required to neutralise 25cm3 of hydrochloric acid could go out either way by 0.5%, which equates to just over 0.1cm3 either way of 27.15 (i.e. 27.05- 27.25). This is still a very small cumulative percentage error for the whole experiment and therefore emphasises the precision of the apparatus.Despite the fact there is an error of approximately 0.5% in my readings I do not believe this is sufficient enough to question the reliability of the experiment. The concentration I obtained for the calcium hydroxide was 0.01381 moldm-3 (4 sig. fig) If I now take into account that the volume of calcium hydroxide needed was 27.05 instead of my actual reading of 27.15, the concentration of the calcium hydroxide would be 0.01386 moldm-3 (4 sig. fig). The difference in concentrations is therefore approximately 0.00005 moldm-3. It is therefore clear that the percentage error is not hugely significant to the reliability of the experiment.Despite this the experiment could be improved as to obtain more reliable results. The main source of error for this experiment was most probably observing the colour change at which the two chemicals had neutralised. The reason for this being such a large source of error is due to the experimenter having to decide at which point the colour change is sufficient to say the two have neutralised. In this way it is very easy to go slightly over or slightly under the required colour change. As a result, the reading you obtain from the experiment can be greater or smaller than what it should be. It is therefore difficult to produce consistent results and therefore has a big impact on the experiment.Another source of error from this experiment may be regarded as reading the burette. If you were to look at the position of the bubble whilst your eyes are not horizontal to it, the value you read will be slightly larger or smaller then what it should be. This is another human error which can give inaccurate results.Based on the data I think the experiment produced very reliable results, as I obtained two titrations within 0.1cm3 of each other. This therefore means the experimental instruments I used were very accurate, which can be seen by the percentage error calculations on the previous page. I obtained no anomalous results also, which demonstrates this.In order to improve the experiment I would use a pH meter to judge when the two solutions have neutralised. Doing this would enable me to clearly observe when the two have neutralised. This would be seen when the pH meter reads 7. I would therefore quickly add the calcium hydroxide until the pH was about 6, and then slowly add drops until the meter reads 7. At this point the solution would be exactly neutralised and I would eliminate the fact of having to observe the correct colour change. For this reason I would not go over or under the end-point of the experiment, and would therefore help me produce much more consistent results. My experiment would therefore be much more reliable, as well as the apparatus I used being more accurate.As well as this I would ensure that whilst reading the burette I would be doing this at eye level. This would ensure that I do not read a slightly higher or lower reading which would cause me to calculate an inaccurate concentration of the calcium hydroxide solution. In this way my experiment would produce more reliable results.

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