Video Transcript
Which of the following is a
hydrogen-containing compound with 11 atoms in its composition? (A) Calcium bicarbonate, (B)
calcium carbonate, (C) sodium carbonate, (D) sodium bicarbonate.
In this question, we need to select
the compound which contains hydrogen and is composed of 11 atoms. We can see that two of the
compounds contain calcium, symbol Ca, and two of the compounds contain sodium,
symbol Na. We can also see that two of the
compounds contain carbonate and two contain bicarbonate. Carbonate and bicarbonate are
atomic groups.
An atomic group is a set of atoms
joined together that have a combined valence and behave as one unit during a
chemical reaction. The carbonate atomic group has the
chemical formula CO32‒. It contains one carbon atom and
three oxygen atoms and has a negative two charge. A chemical formula is an expression
of chemical symbols and numerical subscripts that represents the composition of a
molecule or one unit of a compound. The bicarbonate atomic group has
the chemical formula HCO3‒. It contains a hydrogen atom, a
carbon atom, three oxygen atoms, and a negative one charge. We can see that bicarbonate
contains hydrogen. Since bicarbonate contains
hydrogen, we know that either (A), calcium bicarbonate, or (D), sodium bicarbonate,
is the correct answer.
Now we need to determine which of
these two compounds is composed of 11 atoms. We need to determine the chemical
formula of each compound and then count up the number of atoms in each. Both compounds are ionic. That is, they each consist of a
metal bonded with nonmetals. We can write the chemical formula
of ionic compounds from their names by following three simple steps.
First, we write the symbol of the
metal and the chemical formula of the atomic group and their valence values. Valence is the number of electrons
lost, gained, or shared by an atom, or atomic group, during a chemical reaction. The valence of sodium is one, and
for calcium it is two. The valence of the bicarbonate
atomic group is one.
Step two is to cross over the
valence values and make them the subscripts of the opposite chemical symbol. We get the following. Note that we can leave off
subscripts of one. We need to remember to put
parentheses around the bicarbonate atomic group in calcium bicarbonate, since the
subscript for this whole atomic group is larger than one.
Lastly, we can simplify the valence
values if they contain common factors. In this case, the valence values
cannot be simplified further. Now, we can count up the number of
atoms in each compound. In calcium bicarbonate, we have one
calcium atom. There are two hydrogen atoms, which
we get by multiplying the subscript of hydrogen, which is one, with the number two
outside the parentheses. And there are two carbon atoms. And there are six oxygen atoms. This gives a total of 11 atoms.
Calcium bicarbonate must be our
answer. In sodium bicarbonate, there is one
sodium atom, one hydrogen atom, one carbon atom, and three oxygen atoms, giving a
total of six atoms. Finally, the compound which
contains hydrogen and is composed of 11 atoms is (A), calcium bicarbonate.
