Acid-Base Equivalents
319
Care must be taken to avoid the ambiguity that may arise when a substance
may have more than one equivalent weight. For example, Na^Oa might react
with HCl in either of two ways:
2HC1 + NaaCOs -> 2NaCl + CO 2 + H 2 O
HCl + Na 2 CO 3 -» NaHCO 3 + NaCl
In the first reaction, Na 2 CO 3 acts as though it has two equivalents/mole; in
the second it acts as though it has one. The ambiguity is avoided by stating
which reaction is involved.
When we prepare a solution that contains 1 equivalent weight per liter of
solution, its concentration is said to be 1 normal, designated as 1 N. A
0.200 N Ba(OH) 2 solution contains
0.200
85.7 -
=17. 14 goiter
\
liter / \
eqmv/
&
The normality (N) of the solution is said to be 0.200 equivalents/liter.
Calculations involving equivalents, milliequivalents, normalities, and volumes of solutions are made in just the same way as those involving molarities
of solutions. The unique and useful feature about the use of equivalents is
that, for any chemical reaction, when reactant A has just exactly consumed
reactant B, we can say
equivalents of A = equivalents of B
regardless of the number of moles of A required to react with a mole of B. For
example, at the endpoint in a titration that uses V a liters of acid with normality
N. d to neutralize V b liters of base with normality Af b , we can say that
endpoint equivalents = ( N a
e Q
ulv j (V a liters) = ( Af b
e
.?
U1V
) (V b liters)
regardless of the base that is used. Or, similarly, if a weighed sample (W grams)
of an acid with equivalent weight E is titrated with a standard base solution,
.
uiv\
endpoint equivalents = - - — = I N^ -p- — I (V b liters)
W