Concentration of a Limewater Solution

To determine the concentration of a Limewater Solution

Aim

To determine the concentration a 250 cm³ limewater solution, this contains approximately 1g dm-3 of calcium hydroxide. Also known is the concentration of HCL at 2.00mol dm-3.

Introduction

A titration method will be used to work out the concentration of the limewater

But before the concentration of the limewater can be worked out, the correct concentration of HCL must be worked out and an ideal indicator used in the titration method must be chosen.

HCL Concentration

The known concentration of limewater is 1g dm-³ and the concentration of the HCL is 2.00 mol dm^-3. The concentration of HCL too high and must be reduced by dilution.

Indicator

Indicators in titrations are used to determine the end point of the neutralization of the reaction of an analyte solution (unknown concentration) by the addition of a titrant solution (known concentration and volume). The end-point is shown by the change in colour of the indicator present in the analyte solution.

An ideal indicator should:

• Have a sharp colour change - so that just 1 drop of acid will cause the analyte solution to change colour instantly, rather than a gradual colour change from as more acid is added.

• A distinct colour - this makes it easier to spot the colour change, when the analyte has completely been neutralized.

• The colour change should happen at the equivalence point, which is the point that the number of moles of titrant solution is equal to number of moles of the analyte solution.

The indicator ‘Phenolphthalein' has these ideal properties and will be suitable for the titration. In this titration, the regent or titrant solution will be HCL and the analyte solution will be the limewater, the phenolphthalein indicator will be placed in the limewater [Ca(OH)_2(aq)]. In a base, phenolphthalein changes colour to ‘pink' and when in a neutral solution, it will become colourless. The reaction:

Ca(OH)_2(aq) + 2HCl_(aq) CaCl_2(aq) + 2H₂O_(l)

Is primarily a base [Ca(OH)_2(aq)] plus an acid [2HCl_(aq)] which produces water and a salt [CaCl_2(aq)] (both together as a solution). Therefore in his titration, as HCL is added to Ca(OH)_2(aq) the indicator should turn from ‘pink' to colourless' upon the complete neutralization of Ca(OH)_2(aq).

Preparation

Before the titration can begin, we must find the correct concentration of HCl worked out from the giving concentration of Ca(OH)_2(aq). However the Ca(OH)_2(aq) is giving in

‘grams dm^-3' and HCL is giving in ‘mol dm^-3', thus one of the concentration has to be converted to the same units as the other.

Limewater [Ca(OH)_2(aq) ] converted from ‘grams dm^-3' to ‘mol dm^-3'

Ca(OH)_2(aq) has a concentration of 1g dm-3, which means that 1 gram of Ca(OH)_2(aq) is dissolved in 1 dm^-3 of water or 1000cm^-3.

Using the equation Moles = Mass/RMM we can convert the units.

Moles = Mass/RMM

Mass Ca(OH)_2(aq) = 1 gram

RMM Ca(OH)_2(aq) = 40 + 2(16 + 1) = 74

Moles = 1 / 74 = 0.0135 moldm^-3 (3 sig)

Concentration of HCL

The Limewater [Ca(OH)_2(aq) ] has now, the same units of concentration as HCL.

Ca(OH)_2(aq) + 2HCl_(aq) CaCl_2(aq) + 2H₂O_(l)

This reaction shows that 2 moles of HCL are reacting with 1 mole

Limewater [Ca(OH)_2(aq) ]. Therefore the concentration of HCL must be double that of Limewater [Ca(OH)_2(aq) ] at approx 0.0200 moldm^-3, as it would be impractical to get it at exactly at 0.270 moldm^-3. This means that the HCL must be diluted by a factor of 100, again this also impractical, so it will be twice by a factor of 10.

Hazards and Safety

Key points on safety and hazards when performing the experiment.

Hazards

• HCL is corrosive and toxic.

• Limewater [Ca(OH)] can have adverse affects with long-term exposure.

Safety

• Equipment must be washed before and after the experiment, to avoid any errors in equipment from chemical traces.

• Safety goggles and chemical resistant- clothing such as a lab coat must be worn at all times around during the procedure due to high concentration of HCL.

• The work area must be kept safe and tidy, to avoid any practical hazards

• Make sure that the burette tap is closed when filling it and use a funnel to avoid any spillages.

• The funnel must be removed from the burette after use to avoid errors in results from the regent (HCL) dripping into it from the funnel and would be a hazard.

Method to dilute HCL

Equipment and Chemicals

• Standard flask (500cm³)

• Pipette pump

• Graduated pipette (50cm³)

• Beaker (500cm³)

• 2.00 moldm^-3 of HCL (50 cm³)

• Distilled water (900cm³)

Method

• Using the graduated pipette connected to the pipette pump, obtain the 50cm³ of HCl and place into the standard flask.]. (Note the graduated pipette need to be washed after use).

• Then pour 450cm³ of distilled water in to the flask

• Place the stopper onto the standard flask, and shake the solution to mix it thoroughly taking care not to spill it.

• Pour the solution then from the standard flask into the beaker.

• Then using the graduated pipette connected to the pipette pump, obtain 50 cm³ of the solution and place in into the standard flask.

• Wash out the contents of the beaker

• Then pour 450cm³ of the distilled water into the standard flask a

• Repeat step 3 to 4.

• Wash out the standard flask and graduated pipette.

• The 0.02 moldm^-3 solution of HCl (500cm³) is now made.

Method of Titration

Fig.1

Equipment and Chemicals 

Test	Start Value (cm3)	End Value (cm3)	Titration (cm3)
1st (Rough)	0.00
2nd	0.00
3rd	0.00
4th	0.00
5th	0.00
Average	0.00

• Burette stand.

• Burette (50cm³).

• Funnel.

• Graduated pipette (25cm³).

• Pipette pump.

• Conical flask (250cm³).

• Beaker containing the

250cm³ of the 0.002moldm^-3 of

Fig.2

HCL from the dilution method.

• Beaker containing 250cm³

of the unknown limewater [Ca(OH)_2(aq) ]

solution.

• White Tile

• Phenolphthalein Indicator.

• A drawn table, as shown in Fig.1.

Method

• Set up the equipment as shown in the diagram to the right in Fig.2.

• Obtain and transfer using the graduated pipette connected to the pipette pump up to the fill line to the conical flask.

• add 5 drops of the phenolphthalein indicator (keep this number of drops the same for each of the titrations)

• Fill the burette with HCL up to 50cm³ and ensure the tap is closed.

• The First titration

• The first titration is a rough titration, to show an indication at what rough volume of HCL the colour-change will be at. This anticipation of the rough volume allows the next titrations to be more accurate.

• Open the tap slowly and let the HCL drip into the conical flask at a consistent rate and at the same time swill the conical flask Continue this until there is a colour change from the limewater [Ca(OH)_2(aq) ] in the conical flask from ‘pink' to ‘colourless', when this colour change happens, close the tap .

• Record on a table the volume of HCL at the point of the colour change in the End point column in the drawn table.

• Wash out he conical flask and repeat steps 1 to 4

• Further titrations

• Open the tap slowly and let the HCL drip into the conical flask and higher rate than in the rough titration and swill the conical flask.

• Do this until the volume of HCL is 10 cm³ away from the rough end point volume.

• At this point reduce the rate of the drip, to at least 1 drop a second, and swill the conical flask more (be careful not to spill it), this will allow each drop to be neutralized and the limewater to show a colour change before the next one drips, ensuring more accurate results.

• Close the tap at the point of the colour change and record the end point volumes in the drawn table.

• Repeat steps 7 and 8 five times and records the results in the drawn table, ensure that at least 3 of the further titration end point values are consistent within 0.2cm³ of each other.

• Wash out graduated pipette, beaker, burette, funnel and conical flask.

 

Results

The results need to be processed. The titration column is difference between the start point value and the end point value The units of all the results need to be at 2 decimal places and the averages taken the start point value, the end point value and the titration value for excluding the rough titration results.

Test	Start Value (cm3)	End Value (cm3)	Titration (cm3)
1st (Rough)	0.00
2nd	0.00
3rd	0.00
4th	0.00
5th	0.00
Average	0.00

Analysis

The average titration (represented by X in the table below) will be in cm^-3 and will need to be converted into dm³ to make its easier to use in calculations. To convert the value from cm³ to dm³, divide the value by 1000.

Xcm³ / 1000 = Xdm³

	HCl	Ca(OH)2
Moles / moles	?	?
Volume / dm3	X	0.025
Conc. moldm-3	0.02	?

Using the equation for a moles in a solution

Moles = Concentration x Volume

The moles of HCL can now be worked out as the concentration is now known and the volume of HCL is