Determining the concentration of a limewater solution.

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Determining the concentration of a limewater solution

Task

I am provided with 250 cm³ of limewater (calcium hydroxide), which has been made such that it contains approximately 1g/dm³ of calcium hydroxide.

Also available is hydrochloric acid which has a concentration of exactly 2.00-mol/dm³.  However, this acid is too concentrated to be used so will need to be diluted for the experiment.

My aim is to plan an experiment which will allow me to determine the concentration, in

g/dm³, of the limewater as accurately as possible.  Normal laboratory equipment is available including indicator solutions but no other chemicals may be used.

The apparatus I have chosen:

  • Pipette (25cm³) – to gain an accurate amount of hydrochloric acid ready for dilution

  • Volumetric flask (500cm³) – Very accurate equipment used for the dilution of the hydrochloric acid

  • Conical flask (250cm³) – to put the limewater solution into ready for the titration

  • Burette (50cm³) – to contain the diluted hydrochloric acid for the titration

  • White tile – to put underneath the conical flask to clearly see when the colour changes

  • Clamp and stand  - to hold the burette in place

  • Funnel – to pour the hydrochloric acid into the burette to prevent spillage

  • Indicator (phenolphthalein) – changes the colour of the solution when it reaches neutralisation

  • Limewater (250cm³ containing approximately 1gdm‾³ of calcium hydroxide)

  • Hydrochloric acid (standard solution: concentration of exactly 2.00-mol dm‾³)

  • Distilled water – to dilute the hydrochloric acid and also to wash off any of the apparatus to remove impurities

In order for my experiment to be as accurate as possible and to get an accurate value for the unknown concentration of the limewater sample, it is vital that I choose accurate and reliable equipment.  This is why I have chosen a pipette, a volumetric flask, and a burette.  All these apparatus have an accuracy of + or – 0.05cm³ which is suitable for my experiment and should produce accurate and reliable results.

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Limewater consists of distilled water and calcium hydroxide.  In my experiment I will be titrating a sample of limewater with hydrochloric acid in order to find the concentration of the limewater.

The general equation for a neutralisation reaction is:

Acid + Metal Hydroxide                 Metal Salt + Water

The word equation for this particular reaction is:

Limewater + Hydrochloric Acid                  Calcium Chloride + Water + Hydrogen

The balanced symbol equation for this particular reaction is:

Ca(OH)2 (aq)  + 2HCl (aq)                      CaCl2 ...

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