Environmental Modeling - Mathematics and Statistics for the Environment - page 79

3.4 MEDIA MATERIAL CONTENT
Media material content is a measure of the material contained in a bulk medium, quantified by the
ratio of the amount of material present to the amount of the medium. The terms mass , moles , or
volume can be used to quantify the amounts. Thus, the ratio can be expressed in several forms such
as mass or moles of material per volume of medium, resulting in mass or molar concentration; moles
of material per mole of medium, resulting in mole fraction; and volume of material per volume of
medium, resulting in volume fraction.
When dealing with mixtures of materials and media, the use of different forms of measures in
the ratio to quantify material content may become confusing. With regard to mixtures, the ratio
can be expressed in concentration units. The concentration of a chemical (liquid, gaseous, or solid)
substance expresses the amount of substance present in a mixture. There are many different ways
to express concentration.
Chemists use the term solute to describe the substance of interest and the term solvent to describe
the material in which the solute is dissolved. For example, in a can of soft drink (a solution of sugar
in carbonated water), there are approximately 12 tablespoons of sugar (the solute) dissolved in the
carbonated water (the solvent). In general, the component that is present in the greatest amount is
the solvent.
Some of the more common concentration units are

1. Mass per unit volume. Some concentrations are expressed in milligrams per milliliter (mg/
mL) or milligrams per cubic centimeter (mg/cm 3 ). Note that 1 mL = 1 cm 3 and that a cubic
centimeter is sometimes denoted as a “cc.” Mass per unit volume is handy when discussing
how soluble a material is in water or a particular solvent—for example, “the solubility of
substance x is 4 grams per liter.”

2. Percent by mass. Also called weight percent or percent by weight, this is simply the mass
of the solute divided by the total mass of the solution and multiplied by 100%:

Mass of component
Mass of so

Percentbymass = =100% (3.1)

lution

The mass of the solution is equal to the mass of the solute plus the mass of the solvent. For
example, a solution consisting of 30 g of sodium chloride and 70 g of water would be 30%
sodium chloride by mass: [(30 g NaCl)/(30 g NaCl + 70 g water)] × 100% = 30%. To avoid
confusion as to whether a solution is percent by weight or percent by volume, “w/w” (for
weight to weight) is often added after the concentration—for example, “10% potassium
iodide solution in water (w/w).”

3. Percent by volume . Also called volume percent or volume/volume percent, this is typically
only used for mixtures of liquids. Percent by volume is simply the volume of the solute
divided by the sum of the volumes of the other components multiplied by 100%. If we mix
30 mL of ethanol and 70 mL of water, the percent ethanol by volume will be 30%, but the
total volume of the solution will not be 100 mL (although it will be close), because ethanol
and water molecules interact differently with each other than they do with themselves. To
avoid confusion as to whether we have a percent by weight or percent by volume solution,
we can label this mixture as “30% ethanol in water (v/v),” where v/v stands for “volume to
volume.”

4. Molarity . Molarity is the number of moles of solute dissolved in 1 liter of solution. For
example, a quantity of 90 g of glucose (molar mass = 180 g/mol) is equal to (90 g)/(180 g/
mol) = 0.50 moles of glucose. If we place this glucose in a flask and add water until the
total volume is 1 liter, we would have a 0.5 molar solution. Molarity is usually denoted with