Video Transcript
Which of the following pairs of
compounds have the same number of nitrogen atoms? (A) NH4OH and NaHCO3, (B) NaNO2 and
MgNO32, (C) NaNO3 and AlNO33, (D) NH4NO2 and CaNO32.
In this question, we’ve been given
eight different chemical formulas. 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.
Let’s use AlNO33 as an example to
look at the different components of a chemical formula. First, chemical formulas consist of
chemical symbols. Chemical symbols may be a capital
letter or a capital letter followed by a lowercase letter. Every element has a unique chemical
symbol which we can find on the periodic table. The second component of a chemical
formula is numerical subscripts. In this chemical formula, there is
a subscript that appears directly after a chemical symbol and a subscript that
appears outside of a set of parentheses. Let’s look at these, one at a
time.
A subscript written directly after
a chemical symbol tells us the number of atoms of that element. So this subscript tells us that
there are three oxygen atoms. We should notice that a subscript
is not written after every chemical symbol. When no subscript is written only
one atom is present. A subscript outside of parentheses
tells us how many of that atomic group are in one unit of the compound. So one unit of this compound
contains three NO3 groups.
To determine the number of atoms of
each element in a compound, we multiply the number of atoms of each element inside
of the parentheses by the subscript value outside of the parentheses. So in this compound, there’s one
atom of aluminum; one times three, or three, atoms of nitrogen; and three times
three, or nine, atoms of oxygen.
To answer the question, we need to
determine how many nitrogen atoms each compound has then find the pair of compounds
that have the same number of nitrogen atoms. We already know that the chemical
symbol of nitrogen is N. So we can identify nitrogen in each
of the compounds. When identifying nitrogen in each
of the compounds, we need to take care to avoid the chemical symbol of sodium:
capital N lowercase a. Now we can count the atoms of
nitrogen in each compound.
In answer choice (A), we see that
the first compound contains the chemical symbol N without a subscript. So this compound contains one atom
of nitrogen. The second compound doesn’t contain
any atoms of nitrogen.
In answer choice (B), the first
compound contains one atom of nitrogen. The second compound contains one
times two or two atoms of nitrogen.
In answer choice (C), the first
compound contains one atom of nitrogen. And the second compound contains
three atoms of nitrogen.
In answer choice (D), the first
compound has one atom of nitrogen written in two locations in the chemical
formula. Adding these together gives us a
total of two atoms of nitrogen. The second compound also contains
two atoms of nitrogen.
We can now see that the pair of
compounds that have the same number of nitrogen atoms is answer choice (D), NH4NO2
and CaNO32.
