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4 - Errors in Chemical Analysis Examples 1 Three...

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1 Errors in Chemical Analysis • Examples: 1. Three measurements were made of the percent by mass of chloride in an impure salt sample: 60.53%, 60.37%, 60.29%. Calculate the mean, the mean deviation and the sample standard deviation. 2 A l i i d t l th t 2. An analysis was carried out on a sample that contained exactly 20.0 ppm iron. The results obtained by the method gave: 19.4, 19.5, 19.6, 19.8, 20.1, 20.3 ppm Find the mean, mean deviation, standard deviation, CV and relative error of the results. 1 Module 1 Introduction to Analytical Methods Basic definitions and explanations Different ways of reporting results: – %(w/w), % purity, concentration 2 SI units Quantity Units Abbreviation Mass kilogram kg Length meter m Time second s 3 Temperature Kelvin K Amount of substance mole mol Electric current ampere A Luminous intensity candela cd

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2 Common units Litre (L) – 10 -3 m 3 Weight – Mass x gravity Molarity (M) – Moles of solute per litre of solution – mol/L Density ( d ) – Mass/volume 4 Common units • Concentration Analytical concentration ( c ) Concentration of substance initially present Mass/volume, moles/volume, etc. Equilibrium concentration ( [ species ] ) Equilibrium concentration ( Concentration of substance in a solution when it has come to equilibrium Units of molarity are indicated by square brackets around name of species If a substance does not dissolve in the solution, what would it’s equilibrium concentration be? 5 Common units Percent concentration Weight percent ( w/w ) Mass solute/mass solution X 100% Volume percent ( v/v ) V l l t / l l ti X 100% Volume solute/volume solution X 100% Weight per volume percent ( w/v ) Mass solute/volume solution X 100% 6