Whether a mixture contains two mixed solids, two mixed liquids, or a solid dissolved in a liquid, the compound present in the greater amount is called the “solvent” and the compound present in the smaller amount is called the “solute.” In a solid/solid mixture, the concentration of solute is most easily expressed as mass percent. If the solute is extremely dilute (i.e., substantially less than 1 percent by mass), then concentration is usually expressed as parts per million (ppm). Some calculations involving concentration require the solute to expressed as a mole fraction.
Calculate concentration in mass percent by dividing the mass of the solute by the total mass of the sample and multiplying by 100. For example, if a sample of metal alloy contains 26 g of nickel (Ni) and the total mass of the sample is 39 g, then
(26 g Ni) / (39 g) x 100 = 67% Ni
Express concentration of dilute solutes in ppm by dividing the mass of the solute by the total mass of the sample and multiplying by 1,000,000. Thus, if a sample of metal alloy contains only 0.06 g of Ni and the total mass of the sample is 105 g, then
(0.06 g Ni) / (105 g) x 1,000,000 = 571 ppm
Calculate mole fraction by dividing moles of solute by the total moles of solute and solvent. This first involves converting the masses of the solute and solvent to moles, which requires knowledge of the amounts of both solute and solvent. The conversion to moles further requires the formula weights of the solute and solvent. For example, consider a nickel/iron (Ni/Fe) alloy that contains 25 g of Ni and 36 g of Fe. The formula weight of the Ni (determined from the periodic table) is 58.69 grams per mole (g/mol) and the formula weight of Fe is 55.85 g/mol. Therefore,
Moles of Ni = (25 g) / (58.69 g/mol) = 0.43 mol
Moles of Fe = (36 g) / (55.85) = 0.64 mol
The mole fraction of Ni is then given by (0.43) / (0.43 + 0.64) = 0.40.
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