Determine the concentration of a limewater solution

As we already know according to the balanced equation 1 mole of Ca(OH)2 reacts with exactly 2 moles of HCl, so you should try and make HCl two times concentrated as the Ca(OH)2.

So in order to make HCl twice as concentrated as Ca(OH)2  you will do the following calculation: 0.0135 x 2 = 0.027 mol dm-3 . This means the 2 mol dm-3 will need to be diluted to 0.027 mol dm-3 however this is to difficult to measure so a better concentration to dilute HCl to would be 0.02 mol dm-3.

The dilution factor of HCl will be 100 times as: 2 / 0.02 = 100.

I have decided to dilute the 2.00 mol dm-3 HCl acid to 0.02 mol dm-3. So I will carry out cross multiplication to work out the amount of acid I will need.

 

? = (250/2) x 0.02

   = 2.5cm3 of HCl needed for 0.02 mol dm-3.

Amount of water needed for dilution = Total Volume – Volume of HCl acid

                                                = 250 – 2.5

                                                = 247.5cm3 of water needed.

In order to complete the titration experiment you will need a suitable indicator.

*1   2HCl           +      Ca(OH)2 →     CaCl2     +     2H2O

 Strong acid            Weak base

CaCl2 is the salt of a strong acid and weak base so the solution of CaCl2 will have a pH of <7. Therefore a suitable indicator would be methyl orange because it has a pH between 3.1 and 4.4.

Now that you know the quantities and concentrations of the reagents, Ca(OH)2 and HCl you will now carry out the dilution process.

DILUTION:

APPARATUS:

• Burette and burette stand.
• 250cm3 of HCl, 2.00 mol dm-3.
• Distilled water.
• 250cm3 volumetric flask and stopper.
• Funnel.
• Eye protection.

METHOD:

1. Set up the burette stand and place the burette inside it.
2. Fill the burette with HCl acid, 2.00 mol dm-3 using a funnel to avoid spillage making sure the bottom of the meniscus touches the line.
3. Make sure the volumetric flask is clean and dry.
4. Place the volumetric flask underneath the tap of the burette.
5. Gently open the tap of the burette to allow the HCl acid to flow into the volumetric flask and close the tap when 2.5cm3 of HCl have been added.
6. Add distilled water to the volumetric flask containing the acid. *2 However you should not add water to HCl acid as the first drops of water will react completely, exothermically with the acid, boiling it and causing spillage. Nevertheless in this case you will add water but a little bit at a time and then put the stopper onto the volumetric flask and shake it to dilute the acid each time. Continue doing this gradually each time adding water until the bottom of the meniscus touches the line measuring 250cm3. At the end thoroughly shake the volumetric flask and the acid is diluted.

TITRATION:

APPARATUS:

• Diluted HCl acid, 0.02 mol dm-3 produced in dilution process.
• 250cm3 limewater- Ca(OH)2.
• Methyl orange indicator.
• Burette.
• Burette stand.
• Funnel.
• Conical flask.
• White tile
• 25cm3 pipette and filler.
• Eye protection.

METHOD:

1. Set up the burette stand and place the burette inside it.
2. Fill the burette using a funnel to avoid spillage with the diluted HCl acid so that the bottom of the meniscus touches the line at 0cm3. Record the initial titre of the volume.
3. *3 Gently insert the pipette into the filler.
4. You will now need to create a vacuum in the filler so with your thumb and index finger squeeze the valve, ‘A’ which means ‘aspirate’ or ‘air’. Whilst doing this use the other hand to squeeze the bulb.
5. Place the pipette into the Ca(OH)2. In order for the Ca(OH)2 to enter you must squeeze on the ‘S’ valve which represents ‘suction’ until the bottom of the meniscus touches the reference line on the pipette measuring 25cm3.
6. Then remove the pipette from the flask containing Ca(OH)2 and place in the conical flask.
7. In order to remove the Ca(OH)2 squeeze the ‘E’ valve this stands for ‘expel’. The Ca(OH)2 will flow out of the pipette into the conical flask.
8. If there are any more drops still remaining, then squeeze the ‘E’ valve with your thumb and forefinger and squeeze the bulb.
9. Place a white tile at the base of the burette stand below the tap and put the conical flask containing 25cm3 of Ca(OH)2 on top of the white tile.
10. Add three drops of methyl orange indicator to the Ca(OH)2 turning the solution pale orange.
11. The acid is then added to the Ca(OH)2 from the burette by opening the tap. A right handed person should use his left hand to open to open the burette tap and the right hand to hold the flask at the neck.
12. Release the solution of the HCl into the limewater solution slowly, observing the volumetric flask. When the solution changes from a light orange to a very light pink colour, this is an indication to stop and record the result. The white tile underneath the volumetric flask helps you with this because it gives you a better view of the colour change.
13. If the solution has turned pink then this is an indication that you have stepped over the end point and the titration is not valid.
14. Read off the side of the burette accurately how much volume is left and take this away from the initial volume to give you the volume of acid required to neutralise the base and record the titre.
15. Repeat the experiment 3 more times to give you more accurate results as anomalies do occur in this experiment.
16. Note all readings and work out the average of the volumes and work out the concentration of the limewater solution using the formula: concentration = no of moles / volume.

BIBLIOGRAPHY:

*1 http://

*2 http://antoine.frostburg.edu/chem/senese/101/safety/faq/always-add-acid.shtml 

*3 http://www.umd.umich.edu/casl/natsci/slc/slconline/PIPET/sld031.htm