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Comparing the Concentration of Some Alkalis in Saturated Solutions.

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Introduction

Chemistry Planning Exercise: Comparing the Concentration of Some Alkalis in Saturated Solutions: Introduction: When metals from groups one and two of the periodic table combine with an OH molecule, the type of intramolecular bonding is ionic bonding. The metals lose one or more electrons and so become positively charged. The non-metal ions/ molecules gain electrons and so become negatively charged. The oppositely charged ions attract each other to form a rigid 3-D lattice a. Each ion in the lattice is surrounded by others of opposite charge e.g. NaOH: Due to their polar nature, ionic compounds are usually soluble in polar solvents, for example water: Water is polar due the difference in electronegativity between the oxygen and hydrogen atoms. This is represented by ?+ and ?-. When an ionic substance dissolves in a solvent, the lattice must be broken and the separated species must then be surrounded by individual solvent molecules b. The solubility of a solute in water, at a given temperature is the maximum amount of it that will dissolve in 100 grams of water at that temperature. When this amount is reached, it gives a saturated solution c. In a saturated solution, e.g. Ca(OH)2 (aq), Ca2+ and OH- ions are in equilibrium with the solid calcium hydroxide: Ca(OH)2 (aq) ...read more.

Middle

From the table on page one, it is possible to calculate the concentrations of the hydroxides that are to be used in this experiment; purely by multiplying by ten (mol/1000g = mol/dm-3). These were chosen because of their suitable concentrations. The chosen hydroxides are Ca(OH)2, LiOH, NaOH and KOH: Hydroxide Moles sat (mol/100g) Concentration (mol/1000g) Ca(OH)2 1.53x10-3 1.53x10-2 LiOH 5.16x10-1 5.16 NaOH 1.05 10.5 KOH 1.71 17.1 To Calculate Concentration of Ca(OH)2 solution: 1. 0.0153 moldm-3 is suitable for concentration. Pipette 50 cm3 of Ca(OH)2 (aq) into a 250 cm3 conical flask as this volume will ensure that spillages are avoided. Add four drops of phenolphthalein. 2. Calculate mole of Ca(OH)2 = conc. x vol = 0.0152 x 0.06 = 7.65x10-4 3. Calculate suitable conc. of HCl to titrate with( approx 15 - 30 cm3 is a suitable titre) Ca(OH)2 + 2HCl CaCl2 + 2H2O 1 : 2 Moles = 7.65x10-4 : 1.53x10-3 Conc. = Moles / Volume = 1.53x10-3 / (30/1000) = 0.051 Thus a suitable concentration of HCl to utilise is 0.05 moldm-3. 4. Using a burette, titrate 0.05 moldm-3 HCl against the Ca(OH)2. Titrate until solution reaches the neutralisation point, i.e. just when the pink colour disappears, and the solution becomes colourless. ...read more.

Conclusion

Use a fine pipette to add distilled water drop-wise for accuracy if necessary. * Do not use measuring cylinders as they are inaccurate compared to pipettes. * Ensure contents of volumetric flask are mixed to form uniform solutions. * Readings must be made to 0.05cm3 when using a burette. * Utilise a white tile to ensure that the end point is easily recognised. * Make sure all experiments are repeated until two recordings are within 0.1 cm3 of each other. This increases the reliability of the results. * Finally, it is vital that the room temperature is maintained at a constant level. If temperature is raised, then energy is used to aid the breaking of the ionic lattice and subsequently the substance becomes more soluble. Similarly, if temperature is lower, the solubility will decrease. Safety and Risk Assessment: According to Cleapss Hazcards, solutions of KOH, NaOH and LiOH ? 0.5 molar are to be labelled corrosive. Eye protection should be worn and gloves also. Ca(OH)2 is not given a hazard classification, but it can be irritating to the eyes ? eye protection should be worn. HCl is not dangerous with the concentrations utilised in this experiment, but eye protection should be worn. The final protection is to use a pipette filler rather than oral suction for pipetting, as the solutions utilised are very corrosive and unsafe. ...read more.

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