Video Transcript
Many household cleaning products
contain hydroxides. What is the chemical formula of
zinc hydroxide?
A chemical formula is an expression
of chemical symbols and numerical subscripts that represents the composition of one
molecule or unit of a compound. H2O is an example of a chemical
formula.
In this question, we need to use
the name of a compound to construct the chemical formula. First, we need to determine if the
compound is ionic or covalent. The name of a covalent compound
will contain at least one prefix: mono-, di-, or tri-. Looking at the name “zinc
hydroxide,” we don’t see any of these prefixes. This indicates that the compound is
ionic.
To write the chemical formula of an
ionic compound, we first write the chemical symbol and valence for each part of the
name. Valence is the number of electrons
that an atom gains, loses, or shares during a chemical reaction. Zinc has the chemical symbol Zn and
a valence of two. Hydroxide is an atomic group. An atomic group is a set of atoms
joined together that have a combined valence. Atomic groups behave as one unit
during a chemical reaction. The name “hydroxide” sounds similar
to the elements hydrogen and oxygen. In fact, the atomic group hydroxide
contains one oxygen atom and one hydrogen atom and has the chemical symbol OH. This atomic group has a valence of
one.
Next, we crisscross the valence
values, writing each as a subscript to the right of the opposite chemical
symbol. When atomic groups are part of a
chemical formula, we need to remember that if the valence value placed after an
atomic group is not one, we need to place the group in parentheses.
In this question, a valence value
of two is being placed after the atomic group hydroxide. So, we need to place the atomic
group hydroxide in parentheses first, then crisscross the valence value to the
outside of the parentheses. This gives us the chemical formula
Zn one parenthesis OH parenthesis two. In a chemical formula, subscripts
of one are not written. Removing the one gives us the
chemical formula of zinc hydroxide, Zn parenthesis OH parenthesis two.
