Compare the concentrations of some alkalis in saturated solutions

The Nature of the Chemicals

Calcium hydroxide is a strong alkali with a pH of around 10. Ca(OH) 2 has a low solubility but dissociates into ions in water.

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Ca(OH)2 (s) + H20 Ca�+ (aq) + 2OH� (aq)

Calcium hydroxides and other metal hydroxides react with acids like hydrochloric acid.

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

Hydrochloric acid is a strong acid with a pH of around 2, because it fully dissociates into ions in water

HCl (g) + aq H+ (aq) + Cl� (aq)

Analysis Technique

A titration would be the most suitable method to determine the concentration of 4 metal hydroxides.

Phenolphthalein would be the most suitable indicator for a reaction between a strong acid and alkali. Phenolphthalein is an organic compound (C20H14O4). The compound is colourless in acidic solution and pinkish in basic solution, with the transition occurring around pH 9. I am supplied with 1 mol dm-3 HCl which I would dilute to 0.1 mol dm-3 HCl.

Chemical Equations

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

The phenolphthalein indicator will change from pink to clear when acid added turns the solution to neutral. If too much acid is added then the solution will stay clear so the change between pink and clear must be caught at the moment of change to get an accurate result.

Safety Considerations

1.0 mol dm-3 HCl is corrosive so goggles must be worn to protect your eyes from splashes. Any splashes must be rinsed off immediately.

Concentrated metal hydroxides are very corrosive so goggles must be worn and any splashes rinsed off immediately.

Phenolphthalein is toxic so care must be taken when in use. Don’t swallow any phenolphthalein and rinse your hands after use.

Glass equipment is delicate so care must be taken when handling is so as not to break anything. Use a stand to safely hold the burette and pipette and make sure it away from the edge of the bench.

Preliminary work

From my preliminary work I found I needed to dilute the 1.0 mol dm-3 HCl to 0.1 mol dm-3 HCl to get a more accurate result around the middle of the burette.

H2 SO4 is a strong acid and fully dissociates in water. It was too strong and produced a result that wasn’t in the midrange of the burette.

H2 SO4 H+ + SO4-

Ethanoic acid (CH3COOH) is a weak acid, it only partially dissociates in water. Approximately 5% of hydrogen in the acid dissociates into hydrogen ions.

CH3COOH H+ + CH3COO-

Equipment

We were supplied with:

* 1.0 mol dm-3 of an acid of our choice

I chose to use HCl because in my preliminary work this was the acid that got the best results within the midrange of the burette

* Ca(OH)2 of an unknown concentration

* 3 other metal hydroxides of our choice

I chose Mg(OH)2 from group 2 in the periodic table, and NaOH and KOH from group 1 in the periodic table, because I can then compare the concentrations of two group 2 metal hydroxides and two group 1 metal hydroxides.

* An indicator of our choice.

I chose phenolphthalein because my preliminary work shows that it is the best indicator to use for this type of titration (see Analysis Technique)

Apparatus

50.0 cm3 burette with 0.1 cm3 division so can be accurate to 0.05 cm3. 25.0 cm3 pipette for accurate measurement of chemicals. Pipette filler because it is the safest way to fill a pipette. A Stand to safely hold the pipette and the burette, 250.0 cm3 graduated flask for accurate measurement of chemicals, a beaker, 3 conical flasks, distilled water and a funnel.

Method

Using a 25.0 cm3 pipette and a pipette filler for accuracy put 25.0 cm3 of 1.0 mol dm-3 HCl into a 250.0 cm3 graduated flask. Fill the graduated flask to 250.0 cm3 with distilled water.

C1V1=C2V2

1.0 mol dm-3 x 25.0 cm3 = C2 x 250.0 cm3

1.0 mol dm-3 x 25.0 cm3 = 0.1 mol dm-3 x 250.0 cm3

C2 = 0.1 mol dm-3

This is an accurate way of making 1.0 mol dm-3 HCl into 0.1 mol dm-3 HCl.

Thoroughly clean out the pipette with distilled water. Fill up the burette with 0.1 mol dm-3 HCl.

Using the pipette and the pipette filler put 25.0 cm3 of metal hydroxide e.g.

Ca(OH)2 into a conical flask and add a few drops of phenolphthalein. This will turn the solution in the conical flask pink.

Put a beaker under the burette and open the tap to release any air bubbles from the bottom. Refill the burette to 0.0 cm3.

Put the conical flask containing the metal hydroxide that is being tested Ca(OH)2 and indicator under the burette and carefully, and slowly add the acid to the conical flask. Swirl the conical flask to mix the acid and alkali together. As soon as the phenolphthalein turns clear, quickly stop adding acid and record the volume of 0.1mol dm-3 HCl that was used.

Refill the burette with 0.1 mol dm-3 HCl to 0.0 cm3 and repeat the procedure 3 times with each metal hydroxide and record the results to obtain accurate and similar results and so that any anomalous results can be detected.

Results table

(containing example data from preliminary work)

Metal hydroxide

First volume used of 0.1 mol dm-3 HCl in cm3

Second volume used of 0.1 mol dm-3 HCl in cm3

Third volume used of 0.1 mol dm-3 HCl in cm3

Average volume used of 0.1 mol dm-3 HCl in cm3

Ca(OH)2

11

11.05

11

11.016

Mg(OH)2

NaOH

KOH

Calculations to determine the concentrations of the metal hydroxide using Ca(OH)2 as an example

(containing example data from preliminary work)

Number of moles of HCl

Number of moles (mol) (n) = Concentration (mol dm-3) (C) x Volume used (dm3) (V)

n = 0.1 x 11.016

1000

n = 0.0011016

Ratio of moles

2HCl : Ca(OH)2

2 moles : 1 mole

0.001106 : 0.001106 = 0.00055083

2

Concentration of Ca(OH)2

Concentration of Ca(OH)2 (mol dm-3) (C) = number of moles (mol) (n)

Volume (dm3)

C = 0.00055083

25

1000

C = 0.00055083

0.025

C = 0.022032