Determining the Concentration of Limewater Solution

Plan

Slaked lime dissolved in water is called limewater.
        

        Quicklime + Water → Slaked lime
                 CaO(s) + H2O(l) → Ca(OH)2(s)

Calcium oxide reacts violently with water to produce calcium hydroxide (slaked lime).
Addition of further water to Calcium hydroxide produces the saturated aqueous solution known as Limewater.
In the experiment it is necessary to react the Hydrochloric acid with the Limewater in order to establish the concentration of the limewater.

        Acid + Metal Hydroxide → Salt + Water

Balanced Equation

The balanced equation that takes place is:
                

        Limewater + Hydrochloric acid → Calcium chloride + Water
                 Ca(OH)2(aq) + 2HCl → CaCl2 + 2H2O

        

Method

Dilution of HCl

        In order for the experiment to be accurate and reliable it will be necessary to dilute the Hydrochloric acid significantly. A concentration of 0.02mol dm-3 would enable me to achieve reliable results. I have reached this conclusion because Calcium Hydroxide has a molar value of: -

n= m/mr

=1/74

0.01 mols dm-3

And the ratio of Ca(OH)2 to HCl is a 1:2 ratio, therefore: -

1. * 2

        

Thus a concentration of 0.02mols dm-3 would be appropriate.  This would require a dilution factor of 100. I will be making 500 cm3 of dilute hydrochloric acid.

        

Accuracy

To reduce any errors it is necessary to choose accurate and reliable equipment. That is why I have chosen the pipette, volumetric flask and burette. All of these instruments have an accuracy of 0.05%cm3, which is suitable for my experiment and should produce accurate and reliable results.

        In making my choice of indicator I also had to be very careful. The Limewater used will be a relatively weak base so it will be appropriate to use methyl orange as it has an end point on the ph scale between 8-5. Whereas phenolphthalein indicator has an end point much higher up in the ph scale. So if I were to use the phenolphthalein indicator then the end point (colour change) would be reached before the equivalence point (when the chemicals, acid-base break up).

        

Implementing

        Safety is very important and it is necessary to follow safety procedures. It is important to wear eye protection when working with any acid. Also long hair should be tied back and as the chemicals used are corrosive it is important to take extra care while working around them.
                In order to start with the experiment it is required that the HCl be diluted. An accurate way of doing this is to use a volumetric flask and pipette. I want to dilute the acid to 0.02 moles. I will be required to carry out this dilution in two separate steps. The first step will bring the concentration of the HCl down to 0.2, but this is still far too concentrated. Step 2 will bring the concentration level down to 0.02 mols dm-3, which is the appropriate concentration.

Step 1

• Use the pipette to measure out 25.0 cm3 of HCl and place it in the 250 cm3 volumetric flask.
• Add 225.0 cm3 of distilled water to the same volumetric flask. Measure the amount of water by filling it up to the 250.0cm3 mark; making sure that the meniscus is at the top of the graduated mark.
• The concentration of the HCl is now 0.2 mols dm-3.

Step 2

• Pipette out 25.0 cm3 of the dilute HCl and place it in another 250.0 cm3 volumetric flask.
• Add 225.0 cm3 of distilled water to the same volumetric flask. Measure the amount of water by filling it up to the 250.0cm3 mark; making sure that the meniscus is at the top of the graduated mark.
• The concentration of the HCl is now 0.02mols dm-3.               

Once I have obtained the correct concentration of Hydrochloric acid I can proceed with the rest of the experiment.

Titration

• Measure out 25.0cm3 limewater and transfer it into the conical flask using a 25.0cm3 pipette and a suction pump. Empty the limewater out into the conical flask.
• Add the indicator (methyl orange) into the conical flask so that the neutralisation point can be obtained, where five drops of methyl orange should be appropriate.
• Set up and position the burette with a clamp-stand making sure that the top end of the burette is below eye level.
• Make sure the tap of the burette is closed (horizontal position).
• Place a funnel at the top of the burette.
• Pour the HCl through the funnel into the burette filling it just above the 00cm3 mark.
• Place the beaker beneath the tap and open it (vertical position), till it fills up and passes through the ‘dead’ space beneath the tap. This process should rid the burette of any air bubbles.
• Measure and record the reading of the meniscus of the HCl in the burette.  
• Remove the beaker and place the conical flask containing the limewater beneath the tap. (Position yourself in a comfortable manner so that your eyes are level with the conical flask).
• Open the tap by turning it to its vertical position, keeping one hand on the tap ready to close it as soon as the colour change occurs, and the other hand continuously swirling the conical flask.
•  Close the tap as soon as the colour changes from light orange to pink, and read and record the volume of dilute hydrochloric acid used to neutralise 25.0 cm3 calcium hydroxide (limewater).

In order to obtain consistent and reliable results it is necessary to have carry out the titration again and again until two concordant results are found.

Bibliography

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