122 DIY Science: Illustrated Guide to Home Chemistry Experiments
dILUTECo/ NCENTRATEd vERSUS wEAk/STRoNG
It’s a mistake to refer to a dilute solution as “weak” or a
concentrated solution as “strong.” The words weak and
strong have different meanings to a chemist. For example,
a concentrated solution of acetic acid (a weak acid) is still
a weak acid solution, and a dilute solution of hydrochloric
acid (a strong acid) is still a strong acid solution.dAR. Rm y CHERvENAk CommENTS:
Expressing concentration as ppm or wt/wt percentage
is common in industry. As an example, biocides and
preservatives are added to paint, adhesives, wash water,
cosmetics, oil, metal working fluids, and sizing baths in
ppm. Additionally, the components that form a synthetic
latex or plastics are measured in ppm. The pharmaceutical
industry is, of course, different.The most important characteristic of a solution is its
concentration. Concentration is sometimes referred to in general
terms. A dilute solution is one that contains a relatively small
amount of solute per volume of solvent. A concentrated solution
is one that contains a relatively large amount of solute per
volume of solvent. A saturated solution is one that contains the
maximum amount of solute that the volume of solvent is capable
of dissolving at a specified temperature. (A supersaturated
solution contains more solute than the solvent is capable of
dissolving; see Laboratory 6.3.)
For many applications, it’s important to have a specific value
for concentration. Chemists specify concentration in numerous
ways, using the mass, volume, and/or number of moles of solute
and solvent. Some ways, such as parts per million (ppm) are
used primarily in specialized applications, such as trace metal
analysis or environmental science. Others are obsolescent or
seldom used.
Here are the primary methods chemists use to specify
concentrations:
molarity
Molarity, abbreviated mol/L or M, specifies the number
of moles of solute per liter of solution (not per liter of
solvent). Molarity is the most commonly used way to specify
concentration. Technically, the word “molarity” and the unit
symbol M are obsolete. The official replacements are amount-
of-substance concentration and mol/dm^3 , neither of which
anyone actually uses. The advantage of using molarity to
specify concentration is that it makes it very easy to work
with mole relationships. The disadvantages are that it is
much more difficult to measure volumes accurately than it
is to measure masses accurately, which makes it difficult to
prepare solutions of exact molarities, and that the molarity of a
solution changes with temperature because the mass of solute
remains the same while the volume of the solution changes
with temperature.molality
Molality, abbreviated mol/kg or m, specifies the number of
moles of solute per kilogram of solvent (not per kilogram of
solution). The primary advantage of using molality to specify
concentration is that, unlike its volume, the mass of the solvent
does not change with changes of temperature or pressure,
so molality remains constant under changing environment
conditions. Molality is used primarily in tasks that involve the
colligative properties of solutions, which are covered in the
following chapter. For the dilute aqueous solutions typically
used in laboratories, molarity and molality are nearly the same,
because nearly all of the mass of these solutions is accounted
for by the solvent (water) and the mass of water at room
temperature is almost exactly one kilogram per liter.Normality
Normality, abbreviated N, specifies the number of gram
equivalents of solute per liter of solution (not per liter of
solvent). The concept of gram equivalents takes into account
the dissolution of ionic salts in solution. For example, a 1.0 M
solution of calcium chloride (CaCl 2 ) can be made by dissolving
one mole (111.0 g) of anhydrous calcium chloride in water
and bringing the volume up to 1.0 liter. The calcium chloride
dissociates in solution into one mole of calcium ions and two
moles of chloride ions. That solution is 1.0 N with respect
to calcium ions, but 2.0 N with respect to chloride ions.
Accordingly, it is nonsensical to label such a solution of calcium
chloride as 1.0 N or 2.0 N, unless you specify the ion species to
which the normality refers.Acids and bases are often labeled with their normalities, the
assumption being that normality for an acid always refers to
the hydronium (H 3 O+) ion concentration and the normality
for a base to the hydroxide (OH-) ion concentration. For
monoprotic acids such as hydrochloric acid (HCl) and nitric
acid (HNO 3 ), normality and molarity are the same, because
these acids dissociate in solution to yield only one mole of
hydronium ions per mole of acid. For diprotic acids such as
sulfuric acid (H 2 SO 4 ), the normality is twice the molarity,
because one mole of these acids dissociates in solution to
form two moles of hydronium ions. For triprotic acids such as
phosphoric acid (H 3 PO 4 ), normality is three times molarity,
because in solution these acids yield three hydronium ions per