Barrons SAT Subject Test Chemistry, 13th Edition

because the two sides are not equal. While the left side has two atoms of oxygen,
the right side has only one. Knowing that the Law of Conservation of Matter
dictates that matter cannot easily be created or destroyed, we must get the number
of atoms of each element represented on the left side to equal the number on the
right. To do this, we can only use numbers, called coefficients, in front of the
formulas. It is important to note that in attempting to balance equations THE
SUBSCRIPTS IN THE FORMULAS MAY NOT BE CHANGED.

TIP

You cannot change subscripts of formulas to attempt to balance an equation!

Looking again at the skeleton equation, we notice that if 2 is placed in front
of H 2 O the numbers of oxygen atoms represented on the two sides of the equation

are equal. However, there are now four hydrogens on the right side with only two
on the left. This can be corrected by using a coefficient of 2 in front of H 2. Now

we have a balanced equation:

2H 2 + O 2 → 2H 2 O

This equation tells us more than merely that hydrogen reacts with oxygen to
form water. It has quantitative meaning as well. It tells us that two molecular
masses of hydrogen react with one molecular mass of oxygen to form two
molecular masses of water. Because molecular masses are indirectly related to
grams, we may also relate the masses of reactants and products in grams.

This aspect will be important in solving problems related to the masses of
substances in a chemical equation.
Here is another, more difficult example: Write the balanced equation for the
burning of butane (C 4 H 10 ) in oxygen. First, we write the skeleton equation:

C 4 H 10 + O 2 yields CO 2 + H 2 O.

TIP

First deal with multiatomic reactants.