Determination of the concentration Essay

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The aim of my investigation is to determine the concentration of limewater solution in g dm-3 as accurately as possible against a standard solution of HCl acid. BACKGROUND INFORMATION ABOUT LIMEWATER Limewater solution is a clear coloured saturated Ca(OH)2 produced when calcium carbonate (limestone) is decomposed to form calcium oxide CaO. Water is then added to form CaO (quicklime), to produce slaked lime. Again, excess water is added to form Ca(OH)2 , limewater.

Limewater, which is an alkali is used principally in medicine as an antacid as a neutraliser for acidic poisoning or treatment of burns. Limewater as an alkali would have a pH scale of 9-14. I am going to set up my equipment in the form of an acid/base titration since it is a way of measuring quantities of reactants and can be very useful in determining an unknown concentration or following the progress of a reaction which is related to my investigation. The reaction is between an acid (HCl) and alkali, (Ca (OH)2) and therefore its a neutralisation reaction.

In order to determine the concentration of limewater in an experimental way, it is very important to use the most appropriate equipment available. This is why I have chosen to use a pipette, volumetric flask and a burette, as they have an accuracy of +-0. 005 cm3, which should produce a suitable accuracy for my experiment. The apparatus I am going to use in my experiment is;  Burette (50 cm3)  Pipette (5 cm3)  Beaker (250 cm3) Measuring cylinder (100 cm3)  Volumetric flask (250 cm3)  Pipette filler  Funnel  Stand and Clamp  Conical flask (250 cm3).

White tile I will be using a 250 cm3 volumetric flask as it is what is provided for my experiment and also it will ensure that my results is reliable because it is the exact volume needed to dilute the a HCl acid whereas if it was a much bigger volume we would have to measure 250 cm3 from that volume and this brings about errors. WORKING OUT DILUTION OF HYDROCHLORIC ACID The hydrochloric acid provided has a concentration of 2. 00 mol dm-3. This acid is too concentrated and therefore if it were to be used in a titration, a very small amount would need to be added in order to neutralise the calcium hydroxide.

The end point would be reached too quickly and this will make it difficult to measure accurately the amount of acid needed for neutralisation and will therefore need to be diluted. Balanced equation for my neutralisation reaction is; Ca(OH)2 (aq) + 2HCl (aq) CaCl2 (aq) + 2H2O (l) Calcium hydroxide and Hydrochloric acid react in the ratio 1: 2 Molecular mass of Ca(OH)2 (calcium hydroxide) is 40 + (16i?? 2) + (1i?? 2) = 74g n = m Mr n = 1 74 Where n = the concentration Calcium Hydroxide n = 0. 01mol rounded up to 2 decimal places to make volumes more appropriate for measuring.

Therefore if the number of moles of Ca(OH)2 is 0. 01; according to the stoichiometry of the equation the number of moles of HCl used should be 0. 01 i?? 2 = 0. 02mol 2 : 0. 02 100 : 1 1000 : 10 250 : 2. 5cm3 The 2M HCl needs to be diluted 100-fold to 0. 02M. Therefore 2. 5cm3 of Hydrochloric acid is needed for dilution with distilled water since I will be using a calibrated volumetric flask of 250 cm3. SETTING UP THE APPARATUS To avoid any accidents the apparatus should be set up correctly as shown in the diagram below, it should also be set up in a suitable place such as the middle of the lab table.

The solutions should be labelled correctly and the lids of the solutions should be placed on the right bottles and should be secured tightly on. Also, all the apparatus should be securely fastened in place such as the burette attached to the clamp and the clamp to the stand. I am going to use a graduated pipette in my experiment as it will help me to achieve a more accurate result compared to one that is not graduated. I am going to repeat my investigation 3 times to make sure I get reliable results. The titre volumes must be concordant to 0. 1 cm3 for maximum reliability.

HAZARDS OF CHEMICALS The chemicals that we will use are limewater and hydrochloric acid and the hazards are: Hydrochloric acid is very corrosive and irritant. Limewater (alkali) is irritating to eyes, skin and respiratory system. Its reaction with water is vigorous and generates heat. The reaction between these two chemicals will also produce calcium chloride and water both of which have no hazards and is not dangerous. Phenolphthalein, which will be used as well, has no dangers to health and will not react with any other chemical to produce anything dangerous.

The chemicals and concentrations I have access to are;  Limewater (250 cm3) containing approximately 1g dm-3 of calcium hydroxide.  Hydrochloric acid (exactly 2. 00 mol dm-3 concentration) Distilled water (to dilute acid) Phenolphthalein indicator solution In order to be safe in an experimental environment, I am going to use a list of safety equipments. These will be; Goggles (should be used to protect eyes from HCl gas or HCl solution)  Rubber lined gloves (for handling the acid)  Lab coat (to prevent spillage of solution to skin or clothes).

Spills, leaks and disposal procedures are as follows;  Spilled hydrochloric acid should be removed immediately by flushing the contaminated area with large volumes of water. Lime should be available for the immediate neutralization of spills.  Hydrochloric acid for disposal should be neutralized with lime or powdered sodium carbonate before getting rid of it into sink. The neutralised HCl should be flushed with plenty of water in order to dilute.

METHOD Before starting the experiment you must dilute the hydrochloric acid. 1. Firstly to dilute the hydrochloric acid you measure out exactly 0.02 mol dm-3 of HCl by working out that the 2. 5 cm3 of HCl will give me 0. 02 mol.

Make sure the burette was clean and double checked by rinsing it out with the diluted hydrochloric acid two to three times and got rid of excess by pouring it out into the sink. If washed with water it could affect the results as it would further dilute the hydrochloric acid which was going to be put in there. 3. Then use the 5 cm3 pipette to pipette 2. 5 cm3 of HCl and then transfer it into the volumetric flask. 4. Fill the volumetric flask with distilled water up to the 250 cm3 level and then mix thoroughly.

By doing this you have diluted the HCl. This makes 0. 02 mol dm-3 solution of HCl. 5. Wash all the equipment with distilled water thoroughly this way will remove any left over chemicals and would not affect the results, then make sure excess water is removed. 6. Fill the burette with 50 cm3 of the diluted HCl, making sure that it has been rinsed with the HCl to avoid any contamination, by placing a funnel on top and pouring slowly. This will avoid any spillage. 7. Remove air bubbles from the tip of the burette by draining several millilitres of titrant.

8. Wash the conical flask with the solution of the calcium hydroxide (limewater) to make sure it is clean. Then fill the pipette with 25 cm3 of limewater and transfer this into the clean conical flask. 9. Then add three drops of phenolphthalein indicator to the limewater (calcium hydroxide) solution which is in the conical flask, the colour should be slightly a light pink solution. This indicator will help us to see the end point where the pink solution turns colourless to indicate that the limewater has been neutralised by the acid. 10.

First record the reading of the burette at eye level so that you dont misread when measuring. Then release the solution of HCl into the pink coloured limewater solution whilst observing the conical flask and gently swirling it. When the colour start to change halfway between pink and colourless, add the HCl drop by drop by doing this the end point could be reached and you will not be under it or overshoot it. Stop adding the HCl when the pink colour changes to become colourless and record the result from the burette. The white tile underneath the conical flask helps to give us a better view of the colour change.

By reading off the side of the burette what volume has been used will give you the volume of HCl acid required to neutralise the alkali. However the result is not accurate enough and anomalies do occur therefore the test must be carried out a further 3 times for any accurate results to become apparent or I can keep repeating the titration until successive titres are within about 0. 1 cm3. Each time you make a reading you must be careful to note the starting volume of acid in the burette and the end volume making sure there are no parallax errors.

REFERENCES 1. www. chemistry-react. org 2. www. studyzones. com 3. www. 4. Hazcards 5. Chemistry 1, Brian Ratcliff, Helen Eccles, David Johnson, John Nicholson and John Raffan. Cambridge University Press 2000 6.

Raymond Chang, General Chemistry Fourth Edition. McGraw-Hill International Edition (2006) I used the chemistry-react website for my investigation because it was really useful in that it was very clear and precise. Also the study zones website was essential as it gave provided me with a lot of ideas, some from other students, and a lot information very clear and in detail. Likewise, the coursework info website gave me ideas as it contained samples of other students coursework.

The Hazcards provided me with clear information about the chemicals for example the risks and how it can be minimised so it was very useful. I also obtained information about pipette sizes and how accurate they are and generally, information about the apparatus that I used for the experiment from the Raymond Chang General Chemistry book. The Chemistry 1 book gave me examples of the calculations for titrations and information about limewater and why titration is carried out. It also provided me with the equation that is needed for my experiment. 1

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