Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 348

Especially for acids and bases, the normality ( N ) rather than the molarity of a solution is often
reported. Normality is the number of equivalents of solute per liter of solution (1 equivalent of a
substance reacts with 1 equivalent of another substance):

N = Number of equivalentsofsolute
Number of liters of solution

(14.6)

In acid/base terms, an equivalent (or gram equivalent weight) is the amount that will react with
one mole of H + or OH - ; for example,

• One mole of HCl will generate 1 mole of H + ; therefore, 1 mole HCl = 1 equivalent.
• One mole of Mg(OH) 2 will generate 2 moles of OH - ; therefore, 1 mole of Mg(OH) 2 = 2
equivalents.

HCl ⇒ H + + Cl -

Mg(OH) +2 ⇒ Mg +2 + 2OH -

By the same principle:

• A 1- M solution of H 3 PO 4 is 3 N .
• A 2- N solution of H 2 SO 4 is 1 M .
• A 0.5- N solution of NaOH is 0.5 M .
• A 2- M solution of HNO 3 is 2 N .

Chemists titrate acid/base solutions to determine their normality. An endpoint indicator is used
to identify the point at which the titrated solution is neutralized.

Key Point: If it takes 100 mL of 1- N HCl to neutralize 100 mL of NaOH, then the NaOH solution
must also be 1 N .

14.1.2.4 Predicting Solubility
Predicting solubility is difficult, but there are a few general rules of thumb, such as “like dissolves
like.”

• Liquid-liquid solubility— Liquids with similar structure and hence similar intermolecular
forces will be completely miscible. For example, we would correctly predict that methanol
and water are completely soluble in any proportion.
• Liquid-solid solubility— Solids always have limited solubilities in liquids, in general
because of the difference in magnitude of their intermolecular forces. Therefore, the closer
the temperature is to its melting point, the better the match between a solid and a liquid.

Key Point: At a given temperature, lower melting solids are more soluble than higher melting solids.
Structure is also important; for example, nonpolar solids are more soluble in nonpolar solvents.

• Liquid-gas solubility— As with solids, the more similar the intermolecular forces, the
higher the solubility. Therefore, the closer the match between the temperature of the sol-
vent and the boiling point of the gas, the higher the solubility. When water is the solvent,
an additional hydration factor promotes solubility of charged species. Other factors that
can significantly affect solubility are temperature and pressure. In general, raising the tem-
perature typically increases the solubility of solids in liquids.