Chemistry Reference

Components of a Solution

Solutions are a type of homogeneous mixture in which one or materials (the solute) are dissolved in another material (the solvent). Solutions are most often composed of solid and/or gas solutes and a liquid solvent. When a solution is made of two or more liquids, it is not feasible to identify a solute and solvent. In this situation, liquids are said to be miscible (they dissolve in one another) or immiscible. Oil and vinegar provide an example of two liquids that are immiscible because they do not dissolve in one another.
Concentration - Molarity
Quantitatively, concentration is important for determining the relative amount of solute dissolved per unit of solution. There are many units used for measuring the concentration of a solution. Molarity measures the number of moles of solute dissolved per liter of solution. It is important for stoichiometric calculations as it is one of the few units that quantifies moles of solute, and the only unit that does so in terms of the volume of solution.
Concentration - Molality
Though only one letter different than molarity, molality is a unique unit of concentration. It measures the moles of solute per kilogram of solvent. In an aqueous solution (i.e. one that has water as the solvent), the number of kilograms of solvent is equal to the volume of water used. This relationship is based the density of water, which is 1 g/mL. One liter (1000 mL) of water should have a mass of 1000 g, which is one kilogram. Therefore, one kilogram of water occupies a volume of one liter. This relationship works for water solutions only!
Molality is also important when considering freezing point depression and boiling point elevation. The freezing and boiling point of a liquid can be changed when solutes are dissolved in it. For water, the freezing point can be lowered (depressed) by 1.86 degrees Celsius for per molal of solute. The boiling point is raised (elevated) by 0.51 degrees Celsius for per molal of solute. Additionally, solutes that dissociate in water yield a greater number of particles in solution, and therefore alter the freezing point more. For example, when 1 mol of sugar is dissolved in 1 kg of water (a solution that is 1 molal), the boiling point of the water is raised by 0.51 degrees Celsius to 100.51 degrees Celsius. However, if 1 mol of NaCl is dissolved in 1 kg of water, the boiling point will be raised by 1.02 degrees Celsius. This is twice the change that the sugar caused despite the fact that both solutions contained 1 mol of solute. The reason for the difference is because NaCl will dissociate into two moles of ions. Sugar, a nonelectrolyte, does not dissociate and therefore produces no ions. All electrolytes that dissociate in water will change the boiling point (or freezing point) by a multiple equal to the number of ions produced. In other words, a solution of CaCl2 will generate three moles of ions when dissolved in solution. One mole of CaCl2 dissolved in one kilogram of water will raise the boiling point by 1.53 degrees Celsius, or three times 0.51 degrees Celsius.
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