concentration of limewater

The reaction between calcium hydroxide and acid is an example of an acid - base titration.

Acid + Base Salt + Water

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

This is a neutralisation reaction. I can write an ionic equation for this reaction:

2H+(aq) + 2Cl-(aq) + Ca2+(aq) + 2OH-(aq) Ca2+(aq) + 2Cl-(aq) + 2H2O(l)

By cancelling out the spectator ions we get the following ionic equation:

2H+(aq) + 2OH-(aq) 2H2O(l)

By dividing by 2 we get the simplest ratio:

H+(aq) + 2OH-(aq) H2O(l)

This ionic equation applies to all neutralisation reactions. The H+ ions can from any acid; the OH- ions can come from any alkali. This reaction is exothermic. The enthalpy of neutralisation is approx 58Kjmol-1.

Apparatus

The apparatus for a titration will include:

* 50cm3 Burette

* 50cm3 5cm3 and 25cm3 graduated pipettes

* 100cm3 volumetric flask

* Small funnel

* Indicator (phenolphthalein)

* 95cm3 Distilled water

* 5cm3 Hydrochloric acid

* 75cm3 of Limewater

In the titration I would expect the titre to be about 5cm3.

Dilution Calculations

25cm3 solution of Ca(OH)2 will contain approx (25 ÷ 250) x 1 = 0.1g

Mr of Ca(OH)2 = 40.1 + 16 + 16 + 1 + 1 = 74.1g

Moles of Ca(OH)2 = mass ÷ Mr = 0.1 ÷ 74.1 = 1.35 x 10-3

From the equation, 2 moles of HCl will react with 1 mole of Ca(OH)2.

Therefore moles of HCl = 2 x (1.35 x 10-3)

= 2.7 x 10-3

Concentration of HCl is 2 mol dm-3:

Moles = (volume ÷ 1000) x concentration

Volume = (moles x 1000) ÷ concentration

= (2.7 x 10-3 x 1000) ÷ 2

= 2.7 ÷ 2

= 1.35 cm3

This shows that 1.35 cm3 of 2 moles of HCl is needed. However this acid is too concentrated and dangerous to use therefore I will need to dilute it.

Concentration1 x volume1 = concentration2 x volume2

2 x 1 = 0.1 x volume2

(2 x 1) ÷ 0.1 = volume2

20 cm3 = volume2

Therefore I will dilute it 20 times.

I will measure out 5cm3 of 2 mol dm-3 HCl using a 5cm3 pipette. Then I will add 95cm3 of distilled water. This will be measured by adding the 5cm3 of HCl into the volumetric flask then adding distilled water until the bottom of the meniscus sits on the 100cm3 line. This will give me 100cm3 of 0.1M of HCl. This acid will be suitable for my titration.

At the end point the phenolphthalein will change colour from pink to colourless. I will do three accurate titrations, all within ±0.1 cm3, then if they are close enough I will do an average.

The titration curve for this reaction is shown right:

Where S = end point (25cm3 approx)

Method

* Get a clamp stand and place it on the desk.

* Then obtain all the other apparatus listed on page 2.

* Next it would be best to clean out the burette. Firstly make sure the burette tap is closed, then simply pour a small amount of the substance that will be used in the burette, in this case it is Hydrochloric acid solution. After pouring the HCl into the burette, slowly rotate the burette so that acid goes all around, and then open the burette tap over a the flask with the acid in it.

* Next we must pipette out our required amount of diluted HCl solution using a 25cm3 pipette. Once the correct amount is taken out, release the acid into the burette making sure the burette tap is closed and a funnel is used and that the burette is filled below eye level.

* Next we must also add a specific amount of our Limewater solution it this case 25cm3 would be best, into a conical flask. Place the conical flask under the burette.

* Now add a small amount (of around 3-4 drops) of Phenolphthalein indicator to the Limewater, so that the solution has gone reasonably purple.

* Making sure the funnel is now removed from the top of the burette we can now start the titration. Open the burette tap and with one hand on the tap, use the other hand to constantly stir the conical flask with a gentle motion.

* Once you see the indicator change colour from purple to colourless close the burette tap and note down the final amount of acid left in the burette. And then work out the difference between the start and end point findings.

* Repeat this around three times so that you end up at least 2 results that are within ±0.1cm3 of each other.

Safety

* As this experiment requires the use of Hydrochloric acid, care must be taken when handling this acid. As this acid is corrosive, gloves and protective clothing should be worn.

* Phenolphthalein has been known to have dangers. It may act as a skin, eye or respiratory irritant, and animal experiments indicate a theoretical risk that this material may be carcinogenic in humans if ingested at high levels over a long period.

Results

Titration 1

Titration 2

Titration 3

Initial Reading

.0

6.7

2.3

Final Reading

6.7

2.3

7.9

Titre

5.7

5.6

5.6

Average Titre

(5.7cm3 + 5.6cm3 + 5.6cm3) / 3 = 5.63cm3

Titration Calculations

Number of moles of HCl:

Number of moles = concentration x volume

0.1 mol dm-3 x 5.63cm3 / 1000 = 5.63 x 10-4 moles

Number of moles of Ca(OH)2 (Calcium hydroxide):

Reacting ratio: 1 Ca(OH)2 : 2 HCl

5.63 x 10-4 mol / 2 = 2.82 x 10-4 mol

Concentration of Limewater in mol dm-3:

25cm3 = 25 / 1000 dm3 = 2.5 x 10-2 dm3

2.82 x 10-4 mol / 2.5 x 10-2 dm3 = 1.13 x 10-2 mol dm-3

Concentration of Limewater in g dm-3:

Formula mass (Ca(OH)2) = 40.1 + (16 + 1) x 2 = 74.1g

Concentration of Ca(OH)2 = 74.1 x (1.13 x 10-2 mol dm-3) = 0.83733 g dm-3

= 0.84 g dm-3

Bibliography

* A - Level Chemistry; E.N. Ramsden

* Introduction to chemistry; B EARL and LDR WILFORD

* Chemistry 1

* Chemistry for A-Level

* http://webs.wichita.edu/chemistry/up_web/BLACKBOARD/Chemistry_112/5