Mole fraction

The mole fraction of a component of a solution is the number of moles of that component divided by the total number of moles present in solution. In a two-component (binary) solution, the mole fraction of solvent, x1, is given by x j = n1/(n 1 + n2), where n1 and n2 are respectively the numbers of moles of solvent and of solute present in solution. Similarly, the mole fraction of solute, x2, is given by x2 = n2/(nj + n2). The sum of the mole fractions of all components is, of course, unity, i.e. for a binary solution, xj + x 2 = 1.

EXAMPLE 3.1 Units of concentration

Isotonic saline contains 0.9% w/v of sodium chloride (mol. wt. = 58.5). Express the concentration of this solution as: (a) molarity; (b) molality; (c) mole fraction and (d) milliequivalents of Na + per litre. Assume that the density of isotonic saline is 1 g cm 3.

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