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To determine the concentration of limewater

Free essay example:

To determine the concentration of limewater

The aim of my experiment is to find the concentration of limewater solution provided.

To do this I am provided with the following chemicals.

Limewater: 250cm (1g dm )

Hydrochloric acid at concentration of 2 mol dm

As you can see the hydrochloric acid is too concentrated for titration (i.e. one drop could change the colour of the solution.) therefore this acid will need to be diluted.

Background knowledge:

Limewater can be made by dissolved slacked lime in water to get a solution of calcium hydroxide (Ca (OH) 2 )

Quicklime + water --------> slaked lime

CaO (s) + H2O (l) --------> CaCl2 (aq)  + 2H2O (l)

In this reaction it shows the calcium oxide reacts with water to produce calcium hydroxide.  After adding more water to calcium hydroxide, it produces the saturated aqueous solution known as limewater.

My experiment will be based on this neutralisation reaction between the limewater and hydrochloric acid.

Ca(OH) 2(aq) + HCl (aq) --------> CaCl2 (aq)  + 2H2O (l)

(back ground knowledge of limewater provided by AS chemistry 1 page…)

Diluting hydrochloric acid:

My first task for this experiment is to dilute the hydrochloric acid.  To do this I first needed to work out how many moles I wanted for hydrochloric acid. For this part of my calculation I choose to use text book Calculations for chemistry to help me. Page…..

Ca(OH) 2(aq) + HCl (aq) --------> CaCl2 (aq)  + 2H2O (l)  

1gdm of Ca(OH) 2

Number of moles of limewater

Mr of lime water= 40.1 + (16 x 2) + (1 x 2) = 74.1

Mass of lime water=1g

Moles of lime water =mass(g)/Mr

So 1/74.1=0.13 mol dm-3

Molarities of limewater solution:

Morality = no. moles of limewater / volume

 = 0.013 / 1 = 0.013 mol dm

So there is 0.013 mole of Ca(OH) 2 per every 1000cm3 of  solution.

Therefore 25cm should contain 3.25x10-4 moles of Ca(OH) 2.

 0.013/40=3.25x10-4 mole

Ratio of limewater: Hydrochloric acid = 1: 2

                                                               = 3.25 x 10 : 6.5 x 10

The hydrochloric acid is 2M which is 2 moles HCl in 1dm of solution.

I need 6.5 x 10 moles of acid.  Now I need to work out the volume of hydrochloric acid like this:

Volume = no. moles x 1000 / concentration = 6.5 x 10 x 1000 / 2 = 0.325cm

I want a 25cm titre that contains 6.5 x 10 mol dm

Now I need to work out the concentration of hydrochloric acid.

Concentration = no. moles x 1000 / volume = 6.5 x 10 x 1000 / 25 = 0.026 mol dm

I now know that I want 0.026-mol dm of hydrochloric acid for the reaction to work.  But I have been supplied with 2.00-mol solution.  I want 250cm of dilute hydrochloric acid at 0.026-mol dm.  The total no. moles in 250cm is unknown.  250 cm of the solution should be enough to repeat the experiment.

Total number of moles of HCL:

No. moles = concentration x volume / 1000

This equation is rearranged to make:

Volume = no. moles x 1000 / concentration

So 1000cm contains 0.026 moles therefore 250cm contains 0.0065 moles

Volume of hydrochloric acid I need :

Volume = no. moles x 1000 / concentration = 0.0065 x 1000 / 2 =3.25cm

Apparatus

Pipette25ml

Burette50ml

Conical flask 250cm

Volumetric flask 250cm

Graduated pipette

Clamp + stand

White paper

Distilled water

Squeezing pump

Safety:

Concentrated HCL must be handled with extreme caution due to its corrosive property.

Goggle and lab coat must be worn all the way through the experiment.

Rubber gloves must be worn when handling concentrated HCL.

Hazard sheet must be filled before the experiment.

Chemicals used for my titration:

Chemical in burette: hydrochloric acid( 0.026mol dm )

Chemical in conical flask: lime water

Indicator: methyl orange

Method:

Here is how I am going to carry out my experiment:

Part 1: diluting acid.

  • Add the 3.25cm of hydrochloric acid to the volumetric flask using the graduated pipette to get the exact amount of hydrochloric acid.
  • Fill the same volumetric flask with distilled water until is reaches 250ml.  This should now make my dilute hydrochloric acid with 0.0065 -mol dm.  Now the hydrochloric acid is ready.

Part 2: the titration.

  • Wash burette with tap water, then distilled water and finally with hydrochloric acid.
  • Wash the conical flask with tap water, then distilled water and finally with limewater
  • Set up titration apparatus.
  • Fill the burette with 50cm of the dilute hydrochloric acid making sure the bottom of the meniscus is reading at 50 on the burette.  Using a funnel to put the dilute hydrochloric acid in the burette increases safety against spillage.
  • Fill the conical flask with 25cm of limewater using the squeezing pump and pipette.
  • Add three drops of methyl orange to the limewater and mix until it turns a light yellow depending on the concentration of the methyl orange.
  • Place the conical flask containing limewater onto a white paper.
  • Start to run the dilute hydrochloric acid steadily, starting of constantly when a you see a localised colour change, then start to add small drops, finally into drop by drop. shake the conical flask after every drop of HCL is added.  This will make sure you will use the exact amount of hydrochloric acid when the colour changes.  This improves accuracy and reliability.
  • Repeat all the tests at least three times (Time dependent) so that an average can be obtained. Repeating the experiments several times will help to produce better and more accurate results, as any inaccuracies in one experiment should be compensated for by the other experiments. Note all the results in a table.

One thing I have to decide is what indicator I want to use.  The indicators methyl orange and phenolphthalein are pretty much one-three drops change and a clear change. The pink/colourless change for phenolphthalein is very definite, the yellow/pink change for methyl orange is clear too.   Because these are clear it means I can tell when the reaction has finished easier than using other indicators.  Limewater, which is calcium hydroxide, is a strong alkali, even though it is only sparing soluble. Its pH is 14. Hydrochloric acid is a strong acid. This means that even if you have quite a weak concentration it completely dissociates in water into H3O+ ions and OH- ions giving a pH of 1 (The pH is a measure of how many hydrogen ions are present in solution).

For the reaction of Calcium hydroxide and Hydrochloric acid I would use methyl orange. It changes colour at around pH 4.4 and will change from yellow in acid to pink in alkali by the addition of limewater.  To make this experiment as accurate as possible, the pipette, burette and volumetric flask only have a percentage error of       + or – 0.1%.  This is only a small error.  Also making sure the hydrochloric acid is poured into the burette carefully using a funnel will prevent the hydrochloric acid pouring down the side of the burette.

This student written piece of work is one of many that can be found in our GCSE Aqueous Chemistry section.

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Response to the question

Response to the question is done very well apart from the missing conclusion and results. The candidate sets out the experiment and why they use certain factors well. In their experiments they should consider anything which may bias or ...

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Response to the question

Response to the question is done very well apart from the missing conclusion and results. The candidate sets out the experiment and why they use certain factors well. In their experiments they should consider anything which may bias or confound the experiment to show a higher understanding of what may go wrong or affect the experiment and account for it.

Level of analysis

The candidate shows good understanding of titrations by knowing that the 2m concentration needs to be diluted. The background information shows good research into what will happen during the experiment but should include observations that will be made in the experiment so that the candidate knows that to expect. The first set of calculations are done to a very high level of accuracy. The method and safety precautions taken are explained very well. The candidate explains their choice of indicator well, but the rest of the experiment e.g. results, and conclusion and evaluation are not included.

Quality of writing

Spelling, grammar, and punctuation all to a very good level.


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