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.

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Analysis

To calculate the mean titre value I simply added the last four values together (2-5) and divided by four. I decided to exclude the first value as this wasn’t concurrent with the others and was most likely only a trial run (to get a rough estimate to work towards). The rest were all within quite a small range and so using this first value could lead to inaccuracies in calculations carrying on through the investigation from the very start.

*   Mean titre (cm3) = [22.20 + 22.30 + 22.50 + 22.30] ÷ 4

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