LESSON II-Physical Properties of Solution
In the previous lessons we discover the phase changes in terms of the
accompanying changes in energy and forces between particles. Many chemical
reactions take place between ions and molecules dissolved in water or other
solvents, rather than between pure solids, liquids, or gases. Previously, we looked
at the properties of gases, liquids, and solids. In this lesson, we will examine the
properties of solutions, concentrating mainly on the role of intermolecular forces
in solubility and other physical properties of solution. Quantitative study of a
solution requires knowing its concentration, that is, the amount of solute present
in each amount of solution. Several different concentration units are used by
chemists, each of which has strengths as well as drawbacks. Let us examine the
units of concentration: percent by mass, mole fraction, molarity, molality,
percent by volume, and parts per million (ppm).
The mole fraction is also unitless, because it too, is a ratio of two similar
quantities. Example 1.2 What is the mole fraction of the solute in a 40% by mass
ethanol (C2H6O) solution in water? Strategy We are given the percentage by
mass (40%) of the solute in the solution. Hence, we can calculate the mole
fraction through (1) converting the concentration units based on the mass or
moles of a solute and solvent or mass percentage, it is useful to assume a
certain total mass of solution; (2) changing the masses of the component’s
ethanol and water to number of moles; (3) substituting the values obtained in
the formula and solve for the mole fraction of the solute ethanol, and solvent
water. Solution We write: