Chemistry Reference
In-Depth Information
number of moles of the other substance may be used to calculate its
concentration, its molar mass, or other quantities.
A. A titration of HCl and NaOH used 10.00 mL of 3.000 M HCl and 29.42 mL
of NaOH. (a) How many millimoles of HCl was used? (b) How many
millimoles of NaOH did that HCl react with? (c) What was the original
concentration of the NaOH?
Key Terms
Key terms are defined in the Glossary.
buret (11.3)
concentration (11.1)
end point (11.3)
Erlenmeyer flask (11.3)
indicator (11.3)
molar (11.1)
molarity (11.1)
pipet (11.3)
solute (11.1)
solvent (11.1)
titration (11.3)
Symbols/Abbreviations
M (molar) (11.1)
M
(molarity) (11.1)
Summary
T
he concentration of a solute depends on the quantities
of both the solute and the solution (or solvent). Molarity
is defined as the number of moles of solute per liter of so-
lution. Molarity is calculated by dividing the number of
moles of solute by the volume of the solution in liters, or
alternatively, by dividing the number of millimoles of
solute by the milliliters of solution. Because molarity is a
ratio, it can be used as a conversion factor to change the
volume of solution into the number of moles of solute, or
vice versa.
If an aqueous solution is diluted with water, the
number of moles of solute does not change, but the mo-
larity does. The final concentration of such a solution is
calculated by dividing the number of moles of solute by
the final volume. (The number of moles might have to be
calculated from the initial volume and concentration.) If
two solutions of the same solute are mixed, the total num-
ber of moles present in the final solution is the sum of the
numbers of moles in the two original solutions. The
mo-
larities
are
not
added.
The number of moles of a reactant involved in a re-
action can be calculated from molarity and volume; that
number of moles can then be used to calculate the number
of moles of product. The number of moles of product can
then be changed to a final molarity if a final volume is
known (Section 11.1).
The individual ions of an ionic compound may be re-
garded as separate solutes. The number of moles of each
ion is calculated from the number of moles of the com-
pound and the formula of the compound. If solutions of
two compounds containing one ion in common are
mixed, the number of moles of that ion is determined by
adding the numbers of moles of the ion in the original so-
lutions. In contrast, if solutions of ions that react with
each other are mixed, the numbers of moles of the ions
that react are subtracted from the original numbers of
moles present, as in a problem involving a limiting quan-
tity. The molarities of the ions in the final solution will be
related to the numbers of moles of the ions remaining in
that solution (Section 11.2).
The experimental technique of titration is often used
to determine the number of moles of a reactant in a given
sample of an unknown, using a measured volume of a
(standard) solution of known concentration. The color
change of an indicator shows when the reaction has been
completed. The concentration and volume of the standard
