74 WATER AND SOLUTIONWater as a Solvent: Concentration of Solutions
The importance of water depends in very large measure on its
ability to dissolve other substances. Oftentimes substances
which, in the dry state, will not react, do react when they are dis-
solved and their solutions mixed. If a reaction is to be brought
about between two substances in solution it is a matter of impor-
tance to know how much of each solution to take, and to do this
it is necessary to know the concentration of the solutions.
Concentration is the ratio of the amount of dissolved sub-
stance either to the volume of the solution or to the weight of the
solvent. In the problems in this chapter, and generally in analy-
tical and synthetic chemistry, the concentration is based on the
volume of the solution, usually 1 liter. In Chapter III, on the
other hand, concentrations are based on weight of solvent, usually
1,000 grams. The amount of dissolved substance may be expressed
in weight — grams — but it is usual in chemical work to express it
in moles, formula weights, or equivalent weights.
Mole. A mole is one gram molecular weight of a substance,
e.g. 36.5 grams of HC1.
Molal Solution. A molal solution contains one mole of dis-
solved substance in 1 liter of solution, e.g., 40 grams of NaOH or
98 grams of H2SO4. One liter of the solution contains less than
1,000 grams of water, but the weight of the whole solution is
usually more than 1,000 grams.
Formula Weight. A formula weight of a substance means
exactly the same as a mole if the formula is the molecular formula.
Sometimes, however, the molecular weight of a substance is not
known although its composition is known and an empirical formula
is given. In such a case the meaning of the term mole is not cer-
tain, but the meaning of formula weight is perfectly definite: it is
the number of grams obtained by adding up the total of the atomic
weights in the formula as it is written.
Formal Solution. A formal solution contains one formula
weight of the dissolved substance in 1 liter of solution.
Equivalent Weight. The gram equivalent weight of a sub-
stance is that amount which is equivalent in a reaction to 1.008
grams of hydrogen. The gram equivalent weight of an acid is
found by dividing the molecular weight by the number of hydro-
gens available for a reaction such as neutralization. The gram
36 5 98
equivalent weight of HC1 is -rr- grams, and of H 2 SO 4 -^- grams.