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

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1 Errors in Chemical Analysis •Ex am p l e s : 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. 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 T Kli K 3 Temperature Kelvin 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 ( specie Equilibrium concentration ( [ species ] ) • 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
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3 Common units •P a r t s p e r Parts per million (ppm) • Replace 100% with 1,000,000 • For a dilute, aqueous solution, we can assume that the concentration in ppm is equal to the concentration in mg/L – Assume the density of the solution is the same as the density for water – 1L = 1,000 mL = 1,000 g = 1,000,000 mg – (mass of solute(mg) / 10 6 mg solution) x 10 6 = ppm Parts per thousand (ppt) • Replace 100% with 1,000 7 Introduction to Analytical Methods • Basic definitions and explanations of lab protocols Taring = process of setting a balance to read zero (0.000) in the presence of a tare Tare = the mass of an empty sample containe = the mass of an empty sample container 8 Introduction to Analytical Methods • Correction for buoyancy Buoyancy error – due to density of object
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This note was uploaded on 04/12/2010 for the course ENGINEERIN 1ac3 taught by Professor Xxx during the Spring '10 term at McMaster University.

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

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