Chemistry Reference
In-Depth Information
fraction of a mole of AgCl precipitated? (c) What mass of AgCl was
produced? (d) Is it possible to tell what mass of the original compound
chloride was present? (e) If the original solution contained
MgCl
2
,
could
its mass be calculated?
Key Terms
Key terms are defined in the Glossary.
completion (10.4)
excess (10.4)
limiting quantity (10.4)
percent yield (10.5)
reacting ratio (10.1)
stoichiometry (10.1)
theoretical yield (10.5)
Summary
T
he ratios of the numbers of moles of reactants and
products involved in any chemical reaction are given by
the coefficients in the balanced equation for the reaction.
Each ratio of moles may be used as a factor to convert the
number of moles of one reactant or product to the number
of moles of any other (Section 10.1).
If the quantity of any substance is given in terms of
mass instead of in moles, the mass must be changed to
moles before calculating the number of moles of another
substance in the reaction. If the mass of a substance is re-
quired as an answer to a problem, its number of moles
must be converted to a mass. (Conversions between mass
and moles are presented in Chapter 7.) (Section 10.2) If
some other measure of the quantity of a substance is
given or required (for example, the number of molecules
of a substance), an appropriate conversion factor is
needed to convert to or from moles (Section 10.3).
For problems in which the quantities of two (or
more) reactants are given, we must determine if one of
the reactants is present in a quantity less than, equal to, or
greater than that required to react with
all
the other reac-
tants. Determine which reactant is in limiting quantity
and use that quantity to calculate the quantities of the sub-
stances that will be used up and produced. A table of re-
actant and product quantities is useful. If masses are
given, rather than moles, they must be converted to moles
first (Section 10.4).
The theoretical yield is the quantity of product calcu-
lated from the quantity of reactant used (or the limiting
quantity if more than one quantity is given). In some re-
actions, not all of the calculated product can be collected.
The percent yield is the ratio of the actual yield to the the-
oretical yield, converted to a percentage:
Actual yield
Theoretical yield
Percent yield
a
b
100%
(Section 10.5)
Net ionic equations can be used to calculate mole ra-
tios but often cannot be used directly with masses. Al-
though spectator ions do not react, they do have mass,
and the molar mass of the compound cannot be deter-
mined if all the ions are not specified (Section 10.6).
The small number of key terms for this chapter sug-
gests that this chapter does not introduce many new con-
cepts. However, this chapter may seem difficult because
it draws extensively on background material from earlier
chapters. The concepts presented in this chapter are ex-
tremely important because they are applied in later chap-
ters on gas laws, electrochemistry, and equilibrium,
among others.
Items for Special Attention
The balanced chemical equation gives the
mole ratios
of
reactants and products
involved in the reaction,
not the
mass ratios and not the numbers of moles
present
.
The coefficient in the balanced chemical equation is
not
used in conversions between grams and moles of a sub-
stance.
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Include the formulas of the substances involved with the
units when applying the factor label method to solve stoi-
chiometry problems. For example, write “g NaCl” or “mol
NaCl,” rather than just “g” or “mol.”
The substance present in limiting quantity may be present
in a greater number of moles than the substance in excess
(but always less than would be required to react with all of
the substance in excess).
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