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Determination of the relative atomic mass of magnesium by back titration

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Determination of the relative atomic mass of magnesium by back titration Introduction / Aim In this following experiment, we are aiming to determine the relative atomic mass of magnesium, using a process known as 'back titration'. The basic outlines of this experiment include a strip of magnesium being allowed to reach with excess hydrochloric acid. The excess acid is then determined by titration with a standard alkali (Sodium Hydroxide) and hence the moles of acid that reacted with the magnesium is found by difference. This can be used to determine the number of mole of magnesium used (explained more thoroughly in analysis section). The relevant equation is: 2H+ (aq) + Mg (s) --> Mg2+ (aq) + H2 (g) Safety Precautions Chemical / Apparatus Hazard Precaution Hydrochloric acid Irritant Avoid contact with skin, handle with care, wash off any spileges, wear eye protection. Sodium Hydroxide Irritant See above Methyl orange indicator Stains Handle with care, avoid contact with skin Apparatus * 250cm3 volumetric flask * 25cm3 pipette * Pipette filler * Burette * Burette clamp with stand * 250cm3 conical flash * White tile Reagents * Clean magnesium ribbon * Standard 1.0M hydrochloric acid solution * Indicator (screened methyl orange) ...read more.


This results in more frequent, successful collisions. ****BRIGGS*** Never had time to finish this part off. Varied The volume of standard alkali (sodium hydroxide): The aim of the investigation is to determine the relative atomic mass of magnesium by back titration, and therefore, this is the only varied factor in this experiment. For our experiments to be accurate, we expect all results to be concordant, i.e. very similar, within a +- 0.1cm3 range. Results beyond or below this would be considered anomalous, and therefore will be ignored. Sources of Error The magnesium may not be clean. It may in parts, potentially be magnesium oxide, and also, magnesium does have a tendency of tarnishing. To prevent this, we could use emery cloth, and wipe the magnesium thoroughly. The required mass of magnesium needed is 0.3 grams. Weighing this out by trial and error would take us too much time, not to mention how inaccurate it is. So, to overcome this problem, simple algebra is used. We would take 1m of magnesium, weigh this out (assume this value is K), and substitute this into a simple equation. ...read more.


sodium hydroxide solution Pipette - unknown hydrochloric acid solution Burette Rough 1 2 3 Final vol. (cm3) 22.1 46.3 24.3 48.5 Initial vol. (cm3) 0 22.1 0 24.3 Titre (cm3) 22.1 24.2 24.3 24.2 Average tire (cm3) 24.2 Hence 24.2cm3 of standard 0.1M sodium hydroxide solution reacted completely with 25.cm3 of unknown hydrochloric acid solution. Analysis The reactions that were undergone during the reaction were between magnesium with the hydrochloric acid, and the sodium hydroxide with the magnesium chloride solution. The equations (both word and symbol) are shown below: Magnesium + Hydrochloric acid --> Magnesium Chloride + Hydrogen Mg + 2HCl --> MgCl2 + H2 From our results, we are now able to calculate the number of moles of excess acid in 250cm3 of the solution. To do this, we need to be able to calculate the number of moles in sodium hydroxide, using the following equation: Number of moles = volume (in dm3) x molarity Substituting the values gives us: Number of moles = (25 / 1000) x 0.1 = 2.5 x 10-3 moles Therefore, for calculating the number of moles in excess hydrochloric acid, we multiply our answer (number of moles in NaOH) by 10. This gives us 0.025 moles. ...read more.

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