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 dmˉ³ as accurately as possible.

        I have been provided with 250.00 cm³ of limewater which has been made such that it contains approximately 1g dmˉ³ of calcium hydroxide. Also available is hydrochloric acid which has a concentration of exactly 2.00 mol dmˉ³. 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 30.00cm³ 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 cm³ of limewater solution of approximately 1.00g dmˉ³. Due to this, the mass of Ca(OH)2 in 1g dmˉ³ of the limewater solution is 1.00g. to work out the molarity of the calcium hydroxide in the solution I need to firstly work out what 1 mole of Ca(OH)2is in grams:

Below I have displayed the RMM of calcium, oxygen, and hydrogen.

Ca = 40

O  = 16                    

H  = 1

Due to this I can then work out the weight of 1 mole of calcium hydroxide:

Ca(OH)2 = 40 + (16 + 1) × 2 = 74

        This shows that there are 74g dmˉ³ of Ca(OH)2 in a 1 molar solution of calcium hydroxide. However approximately 1g dmˉ³ is in my solution. To calculate the exact amount of calcium hydroxide in the given solution I must do the following calculations:

     1g    = 0.0135

    74g

However I shall only be working at 1 significant figure due to the equipment at hand, which can only measure to a certain degree of accuracy. The given solution is therefore of an approximately 0.01 molar concentration.

Ca(OH)2 (aq)  + 2HCl (aq)   →  CaCl2 (s)  +  2H2O(l)

The above equation shows that to neutralise the limewater, the ratio of Ca(OH)2 : HCl must be 1:2. This means that to have an equal amount of the limewater solution and dilute hydrochloric acid neutralising each other I will need to dilute the hydrochloric acid so that it is twice the strength of the limewater solution. This means that I need a 0.020 molar concentration of the dilute hydrochloric acid. To obtain this strength, I will need to have a ratio of ,2.00g dmˉ³ HCl : H2O, at

                   

                               2.00 = 100

                               0.02

Due to this I need a ratio of 1:99 of  2g dmˉ³ HCl : H2O.

I want to create enough dilute hydrochloric acid for 8 titrations, which is the maximum amount of titrations I will be able to do. However due to the possibility of the limewater concentration being stronger than 1.00g dmˉ³ I need to create extra solution to account for this. Due to this I have decided to make 300.00cm³ of the dilute hydrochloric acid, this should cover the possible excess needed. For this I will need 3.00cm³ of the 2.00g dmˉ³ hydrochloric acid, and 297.00cm³ of distilled water. This will give me a 1:99 ratio of HCl:H2O and will create 300.00cm³ of a 0.020 molar solution of hydrochloric acid.

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I chose to use 30.00cm³ of limewater for each titre because it is a large enough amount to get a reasonable degree of accuracy and it is also possible to measure out using a 30.00cm³ glass pipette.

I will be using the following apparatus in my procedure:

Dilution Apparatus:

500.00cm³ conical flask

250.00cm³ volumetric flask

1.00cm³ glass pipette

50.00cm³ glass pipette

Pipette filler

2.00 molar hydrochloric acid

Titration Apparatus:

30.00cm³ bulb pipette

Pipette filler

50.00cm³ burette

25.00cm³ conical flask

Funnel

Phenol phthalein indicator solution

250.00cm³ limewater containing approximately 1.00g dmˉ³ ...

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