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Compare the concentrations of some alkalis in saturated solutions.

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Introduction

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. 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. ...read more.

Middle

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. ...read more.

Conclusion

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 Page 1 ...read more.

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