Plan. In this experiment I intend to discover the exact concentration of an unknown limewater solution using the method of titration.

* Conical flask- The reaction vessel.

* Measuring beakers- For reactant and distilled water that is used to clean burette, as well as for storing hydrochloric acid solution that is to go in burette.

* 100 cm3 Volumetric Flask- Vessel in which dilution of hydrochloric acid will take place.

The reaction between limewater (calcium hydroxide solution) and hydrochloric acid is as follows:

Calcium Hydroxide + Hydrochloric Acid Calcium Chloride + Water

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

The hydrochloric acid provided for the experiment is a 2 mol dm-3 solution; however this is too concentrated to be used for this experiment. This is because the limewater contains roughly 1g dm-3, the concentration of which can be worked out thus:

Moles = Mass

Mr

= 1

74.1

= 0.0134952766531713

= 0.01 mol dm-3 (to 3 significant figures)

As a result, the hydrochloric acid at 2 mol dm-3 would make the experiment unreliable as it would take too little acid in order to neutralise the limewater (Ca(OH)2). If the values of the results are too small, the burette's accuracy is not sufficient to produce reliable results for the concentration of the limewater when calculated.

As a result of possible loss of accuracy due to a hydrochloric acid solution that is too concentrated, it is necessary to dilute the hydrochloric acid to 0.2 mol dm-3. This has been chosen rather than an exactly equivalent concentration to the limewater (Ca(OH)2), which would be 0.02 mol dm-3. (The reason for having double the concentration is that in the reaction between calcium hydroxide and hydrochloric acid, two molecules of HCl are used up for each molecule of Ca(OH)2. This means that twice the amount of molecules of HCl is needed in the reaction.) The reason for not choosing 0.02 mol dm-3 is that it would take too much hydrochloric acid to reach the end point of the titration at this concentration and it would also be harder to dilute, resulting in increased errors in the experiment and therefore less accurate results, so 0.2 mol dm-3 was chosen as more suitable.

It is necessary to dilute the provided 2.00 mol dm-3 using a 10 cm3 pipette and a 100 cm3 volumetric flask. Prior to diluting the hydrochloric acid it is necessary to rinse the flask with distilled water. 10 cm3 of the HCl is transferred to the volumetric flask, the flask is then filled to the top with distilled water, this solution must then be shaken vigorously to produce a 0.2 mol dm-3 solution. A 0.2 mol dm-3 solution that is suitable for an acid-base titration procedure has been produced:

2.00

10

=0.20 mol dm-3

Before commencing the titration, the burette should be rinsed out with hydrochloric acid to remove any impurities on the inner surface of the glass. The reaction vessel (conical flask) should be rinsed with distilled water for the same reason. Before transferring any limewater solution with the pipette this should also be rinsed out with distilled water to avoid contamination, which may lead to erroneous results.

Once all equipment is ready for the process to begin the hydrochloric acid solution should be poured into the burette from a clean beaker, taking care to shut the tap to avoid spillage. The burette should be placed on a stool for easy access to minimise the risk of knocking it over. Once the burette is full or nearly full, the tap should be opened for a few seconds and some hydrochloric acid poured into an empty beaker to release any trapped air in the tap. Once all bubbles appear to be removed the tap is closed and the volume read from the bottom of the meniscus of the solution. The burette is replaced on the work bench.

In order to transfer the limewater solution a 10cm3 pipette should be used. Using the safety filler and ensuring that the meniscus of the solution is at the correct level, the solution is transferred to the conical flask. Any excess limewater should be removed from the pipette by carefully touching the tip against the side of the flask. (Throughout the procedure safety goggles must be worn to avoid any acidic or alkaline solutions making contact with the soft tissues of the eyes).

Five drops of methyl orange indicator (methyl orange has been chosen as the indicator as it is appropriate for a strong acid-weak base titration such as this one) should be added to the limewater until it is an orange colour. Once all is in place and the reading on the burette has been noted down, the titration can begin. The tap is opened to add hydrochloric acid until a distinct colour change has occurred (the previously orange limewater should turn colourless), stirring constantly throughout. As soon as the colour has changed the tap is closed and the final reading is taken from the burette (ensuring that it is from the bottom of the meniscus).

At least 3 readings should be taken, the first being a rough reading and less care is needed when adding the sodium hydroxide, the other two need more precision so they should be carried out practically drop by drop until the colour changes. The first result tends to be discarded unless it is very similar to the other results. An average is taken of the two (or three) results. My results shall be recorded in the following tabulated form:

Reading

(rough)

2

3

4

Quantity of HCl needed to neutralise the limewater solution (cm3)