Читать книгу Geochemistry - William M. White - Страница 113

Geochemists concerned with aqueous solutions commonly use a variety of concentration units other than mole fraction. The first is molality (abbreviated as lower-case m), which is moles of solute per kg of solvent (H₂O). Molality can be converted to moles solute per moles solvent units by dividing by 55.51 mol/kg. A second unit is molarity (abbreviated as uppercase M), which is moles of solute per liter of solution. To convert molality to mole fraction, we would divide by the molecular weight of solvent and use the rational activity coefficient. Natural solutions are often sufficiently dilute that the difference between molality and molarity is trivial (seawater, a relatively concentrated natural solution, contains only 3.5 weight percent dissolved solids). Another common unit is weight fraction (i.e., grams per gram solution), which may take several forms, such as weight percentage, parts per thousand, or parts per million (abbreviated %, ppt or ‰, ppm or mg/kg). To convert to mole fraction, one simply divides the weight of solute and H₂O by the respective molecular weights.