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UNITS OF CONCENTRATION
There are a number of different ways of expressing solute concentration that are commonly used. Some of these are listed below. Molarity, M = moles solute/liter of solution Normality, N = equivalents of solute/liter of solution Weight %, Wt % = (mass of solute/mass of solution) x 100% Parts per million, ppm = (mass of solute/mass of solution) x 106 Mass per volume, mg/L = mass of solute/liter of solution molality, m = moles of solute/mass of solvent mole fraction, χ = moles of solute/total moles

Concentrations expressed as ppm and N are less familiar to most students at this stage. Parts per million: The number of milligrams of solute per kg of solution = one ppm, since 1 mg = 10-3 g and 1 kg = 103 g. Assuming the density of water is 1.00 g/mL, 1 liter of solution = 1 kg and hence, 1 mg/L = 1 ppm. This is generally true for freshwater and other dilute aqueous solutions. Parts per million concentrations are essentially mass ratios (solute to solution) x a million (106). In this sense, they are similar to wt %, which could be thought of as parts per hundred (although nobody uses this term). Other variations on this theme include: ppt – parts per thousand (used for common ions in sea water) ppb – parts per billion (used for heavy metals and organics) ppt – parts per trillion (used for trace metals and trace organics) The following table summarizes common mass ratios for solutions and solids. Unit Solutions Solids ppm mg/L mg/kg µg/mL µg/g ppb ng/mL ng/g µg/L µg/kg ppt ng/L pg/mL ng/kg pg/g

F:\courses\CHEM 311\notes and handouts\UNITS OF CONCENTRATION.doc

To convert concentrations in mg/L (or ppm in dilute solution) to molarity, divide by the molar mass of the analyte to convert mass in mg into a corresponding number of moles. What is the molarity of a 6.2 mg/L solution of O2(aq)?

To convert from molarity to mg/L (or ppm in dilute solution), multiply by the molar mass of the analyte to convert moles into corresponding number of...
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