A solution consists of two components: solute (the dissolved material) and solvent (the liquid in which the solute is dissolved). The amount of solute in a given amount of solvent is known as the concentration. The concentration can be expressed in multiple ways depending on what definitions of "amount" are used.
When considering solutions, the value of moles of solute in a volume of solution is found by using the concentration. This is analogous to using the atomic or molecular weight (a mass-per-mole quantity) to find the moles in a certain mass of a substance.
The concentration (M) of a solution in units of moles of solute (m) per liter of solution (V) is called the molarity:
The units of molarity are mol*L-1, often abbreviated as M.
For example, the number of moles of NaCl in a 123mL of a 0.123M solution of NaCl can be calculated as in .
The concentration (b) of a solution in units of moles of solute (n) per kilogram of solvent (m) is called the molality:
The units of molality are mol*kg-1.
Other Concentration Definitions
Molality and especially molarity are the most common and easily applied definitions of concentration, but there are other ways of expressing amount of solute per amount of solvent.
Percent composition (Pm) by mass is the ratio of mass of solute (msolute) to mass of solvent (msolvent) multiplied by 100:
Similarly, percent by volume (PV) is the ratio of volume of solute (Vsolute) to volume of solvent (Vsolvent) multiplied by 100:
Mole fraction is another form of ratio involving quantities of solute and solvent. The mole fraction (X) of a given solute in a solution is the ratio of the moles of that soute (ms) to the sum of moles of all species in the solution (mt):
Reaction Stoichiometry In Solutions
Reaction stoichiometry in solutions is analogous to reaction stoichiometry in any other state. In the solid state, one can be asked how many grams of Z are produced when x grams of A and y grams of B are mixed. The masses of A and B are converted to moles, and the molar quantities of product that can be produced by each of A and B (assuming excess of the other) are found. The lower value is the amount of Z that can actually be produced by the limiting reagent; that molar quantity can be converted to grams by using the molecular weight.
Analogously, in the aqueous state, one can multiply the volume of an x M solution of A by its concentration and repeat the same for a y M solution of B. When the moles of A and B in their separate solutions are found, one can determine the limiting reagent, moles and mass of Z as if the problem were a solid-state consideration.