ical formula for water is H 2 O, in which there are two atoms of hydrogen (H) bonded to
an atom of oxygen (O). The subscript 2 indicates that there are two atoms of hydrogen
in the molecule; if there is no subscript number, as with the oxygen (O), a subscript of
1 is implied. (Remember, not all compounds are molecular; for example, NaCl, or sodi-
um chloride [regular table salt] is called an ionic compound. In these cases, the for-
mula shows the proportion of the atoms of each element making up the compound.)
There are other types of formulas in chemistry, but this is the most familiar.
Equations in chemistry also differ from those in mathematics. Chemical equations
represent the reaction relationship between two or more chemical compounds—along
with the products of the chemical reaction. For example, the chemical equation 2H 2
O 2 $2H 2 O is the reaction of hydrogen with oxygen to form water. The arrow indicates
the direction of the reaction toward the product; the reactants (or the substances that
react) are hydrogen and oxygen. There is also a methodology in writing chemical equa-
tions. Simply put, first determine the reactants and outcome; next, determine the for-
mula for each substance; and finally, balance the equation.
What is the pH scale?
The “pH” scale stands for p(otential of) H(ydrogen) scale, or the logarithm of the reci-
procal of hydrogen-ion concentration in gram atoms per liter. In simpler terms, the
pH is merely the measure of the hydrogen ion concentration of a solution. The pH
numbers are based on a scale from 0 to 14, in which numbers less than 7 represent
acidic solutions and numbers greater than 7 represent alkaline (base) solutions. A
reading of 7 is considered neutral.
Mathematically speaking, once the concentration of hydrogen ions is determined
chemically (based on moles per liter), the pH value is established by taking the expo- 285
MATH IN THE PHYSICAL SCIENCES
What is Avogadro’s number?
A
vogadro’s number (also called Avogadro’s constant or Avogadro’s figure) was
determined by Italian physicist Lorenzo Romano Amedeo Carlo Avogadro,
Count of Quarengna and Cerreto (1776–1856), who was also the first one to use
the term “molecule” in chemistry. It represents the number of elementary enti-
ties, such as atoms, molecules, or formula units, in a mole of any chemical sub-
stance (a mole is approximately 6.02214199 1023 atoms, according to the most
recent number from the National Institute of Standards and Technology). To
translate even further, a mole is the molecular weight of a substance in grams;
one mole is the amount of a substance that contains Avogadro’s number. For
example, the number of carbon atoms in 12 grams of the substance carbon-12 is
equal to one mole.