of molar concentration
The molar concentration is a quantity
characterizing any solution, it reflects the proportion of a
dissolved chemical species (solute) in a solvent. The molar
concentration (C) of a chemical species is the ratio of the quantity
(n) of this chemical species by the volume (V) of the solution, which
results in the following formula:
• C is
usually expressed in moles per liter (mol / L), other units are
possible (mmol / L, mol / mL etc.) but the mol / L is the most common
unit (most relations intervening molar concentration require this
n is mole
V is in liter
notation C is often accompanied by an indication of the concerned
chemical species whose name or formula may be mentioned in
parentheses or in subscript.
volume V used to calculate a molar concentration is the volume of the
solution and not the volume of the solvent but when the quantity of
matter is low, it is often possible to make an approximation that the
volume of the solution is the same as that of the solvent.
the term "concentration" is used without precision it
usually refers to the molar concentration and not to the mass
concentration. Similarly, if the notation "C" is used
without precision then it refers to the molar concentration and not
to the mass concentration.
a 100 mL solution was obtained by dissolving 0.050 mol of glucose in
0.5 mol / L
molar concentration influence on a solution
concentration can influence the appearance of the solution:
• if the
solute is a colored chemical species then the color is more intense
as the concentration of this species is high.
• If the
dissolved chemical species has a taste then this taste will be
"strong" as the well as concentration is high.
• If the
dissolved chemical species is acidic then the acidity of the solution
will be stronger (low pH) as well as the concentration is higher.
if a chemical species is basic then the basicity of a solution will
be stronger (high pH) as well as the molar concentration is high.
• If the
dissolved chemical species is a reagent then the reaction will be
faster as the concentration is high.
a molar concentration
If we know
the volume of the solution and the quantity of matter, then the molar
concentration can be calculated using the relation which defines it
as the ratio of these two quantities:
by a hue scale
dissolved chemical species is colored, then it is possible to provide
a framing of its concentration by performing a color scale: the
solution of unknown concentration is compared to a series of
solutions having a decreasing known concentrations. If its hue is
intermediate between two solutions (lighter than one but darker than
the others) then the concentrations of these two solutions provide a
framing of the desired concentration.
is an experimental technique that exploits chemical or physical
properties of a chemical species in order to determine its
concentration, it is possible to perform pH-metric dosage, redox,
conductimetric and spectrophotometric dosage.
concentration and mass concentration
these concentrations reflects the proportion of solute dissolved in a
solution, the first refers to the quantity of matter and the second
to the mass. It is possible to establish a relation between the two
and to express one according to the other.
C the molar
concentration of a chemical species in a solution (in mol / L)
Cm the mass
concentration of this chemical species in the same solution (in g /
matter quantity of this chemical species (in mol)
m mass of
the chemical species in this solution (in g)
volume of the solution (in L)
M the molar
mass of the chemical species (in g / mol)
molar concentration is defined by the following relation
= m .
concentration of a chemical species can be obtained by the ratio of
the mass concentration divided by the molar mass of the chemical
We can also
deduce the mass concentration from the molar concentration thanks to
the following relation:
formula indicates that the mass concentration is the product of the
molar concentration by the molar mass of the chemical species.
the molar concentration to calculate a quantity of solute
have a solution with a known concentration, it is possible to express
the quantity of matter contained in the totality of the solution or
in a sample of volume V from a relation.
the following formula: n
that the quantity of solute corresponds to the product of its
concentration by the volume of solution.
the molar concentration to calculate the volume of the solution
determine the volume to be taken from a solution in order to obtain a
given quantity of matter, we can adapt the previous relations to
obtain the following formulas:
V = n
concentration and dilution
definition, the dilution of an aqueous solution consists in adding
solvent to it without modifying the quantity matter that it contains.
Since the concentration of this solution is given by the relation C
= n / V ,
the increase of the term "V" without variation of "n"
indicates that the concentration decreases:
of a solution induces a decrease in the concentration
example, if a solution is diluted by doubling the volume of solvent
then the concentration C2
of this diluted solution is:
the solvent volume is dividing the concentration by two,
but it could
also be shown that multiplying the volume by 3 leads to dividing the
concentration by 3, multiplying the volume by 4 divides the
concentration by 4, etc.
dilution induces a decrease in concentration there can also be:
lightening of the hue (for a colored solution)
• a taste
(salty, sweet or other) less pronounced (if the solute has a taste)
decrease in acidity (for an acidic solution)
decrease in basicity (for a basic solution)
concentration and vaporization
leads to a loss of solvent and is accompanied by a decrease in the
volume of the solution while the amount of material does not vary (in
general the solute is not vaporized). A vaporization has the inverses
consequences of a dilution:
concentration and saturation
concentration of a chemical species in a given solvent is always
limited, when this limit is reached it is said that the solution is
saturated and the concentration takes on a particular value: it
corresponds to the solubility of the chemical species in the solvent.
of a solution with a known molar concentration
preparation of a solution is generally carried out following this
for a volumetric flask whose V capacity corresponds to the volume of
solution that you want to prepare.
the quantity of material "n" needed to obtain a given
concentration "C" in a volume solution "V": n =
the mass "m" of solute to dissolve: m = M.n (M is the molar
mass of the solute)
• If the
solute is solid, it is removed with a spatula and placed in a cup or
directly in a beaker. The beaker placed on a scale and after a tare
(reset) a mass "m" of solute is added.
begins in the beaker by adding a much smaller volume of solvent than
the solution. Stirring is preferably done using a magnetic stirrer.
complete dissolution, the homogeneous mixture is transferred to the
volumetric flask of volume V. The beaker is rinsed with the solvent
and then poured into the flask.
solvent completes the solution; it is added until the lower part of
the meniscus (on the surface) coincides with the mark.
vial is then shaken (with a stopper) to homogenize the solution
solution is ready.
solvent is liquid, it is necessary to determine the volume V 'to be
taken from the mass "m" (already calculated) and the