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The Percentage Purity of an Unknown Acid.

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The Percentage Purity of an Unknown Acid Introduction: A solid monobasic acid with a relative molecular mass of 163g was bought from a leading supplier. On the label the percentage purity by mass of the acid was said to be 99.1?. A student was then asked to check this value by titration against sodium hydroxide. *The student began by placing an empty bottle on a balance reading to one decimal place, zeroed the balance then added the acid to the bottle. When a suitable amount had been added, the student then recorded the balance reading as being 7.1g. The student then transferred the acid to a beaker and dissolved the acid in a small quantity of deionised water. He then poured this solution into a 250cm3 graduated flask and made the liquid level up to the mark with deionised water. The flask was then well shaken to ensure complete mixing. After doing this the student then went on to fill a burette with the acid solution These following results were obtained when 25.00cm3 portions of 0.150M sodium hydroxide solution were titrated with the acid solution. ...read more.


in M) to calculate the acids concentration. * V1C1 = V2C2 * (25 ? 1000) ? (0.15) = (22.325 ? 1000) ? C2 * C2 = (25 ? 0.15) ? (22.325) * C2 = 0.168M The concentration of the acid is therefore 0.168M, which can then be used further to calculate the concentration of the acid in grams per dm3. * Mass = No. of Moles ? Relative Molecular Mass * Mass = (0.168) ? (163) ? 27.384 g/dm3 Though as he was only using 250cm3 then this value must be divided by four. * (27.384) ? (4) ? 6.846 g/dm3 From this I am then able to calculate the true percentage purity of the unknown acid. * (Calculated mass ? Original mass) ? 100 = ......? * ( 6.846 ? 7.1 ) ? 100 = 96.4? By this method the percentage purity was found to be 96.4? and not 99.1?. Apparatus error * Balance ? 0.1g ? (0.1 ? 7.1) ...read more.


The students titration technique was also at fault with him only filling the burette correctly twice out of five titrations. There is nothing mentioned either about him rinsing the burette or the pipette beforehand with the acid and sodium hydroxide respectively. This should always be done before attempting a titration to remove any impurities from the apparatus. After examining the students method I'm not that surprised my calculated percentage purity was a bit off as there were plenty of things that could have caused inaccuracy in the experiment. If the student was to attempt the whole thing again I would urge him to listen to the improvements listed below: * Use a balance that reads to two decimal places instead of one. * Weigh the bottle and acid together, then the bottle by itself and take the difference. * Make sure all washings are transferred to the volumetric flask. * When filling the burette make sure to fill it right to the top (0.00cm3) each time. ...read more.

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