Video Transcript
Acetic acid is a compound in
household vinegar. Its formula is CH3COOH. Which of the following best
describes a molecule of acetic acid? (A) It is composed of eight atoms
and six elements. (B) It is composed of eight atoms
and three elements. (C) It is composed of seven atoms
and four elements. (D) It is composed of seven atoms
and three elements. (E) It is composed of four atoms
and two elements.
This question gave us the chemical
formula of acetic acid, which is CH3COOH. 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. Chemical symbols are a letter or
combination of letters that represent an element. So the chemical symbols in a
chemical formula tell us the elements that are in the compound. The numerical subscripts tell us
the number of atoms of each element that are in one molecule or one unit of the
compound.
In this chemical formula, the
chemical symbol C appears twice. C is the chemical symbol for
carbon. The chemical symbol H also appears
twice, which is the chemical symbol for hydrogen. Finally, we have the chemical
symbol O, which is the chemical symbol for oxygen. So, this means atoms of the three
elements — carbon, hydrogen, and oxygen — are all found in one molecule of acetic
acid. Since we’ve identified that there
are three elements in a molecule of acetic acid, we can eliminate answer choices
(A), (C), and (E).
To figure out if answer choice (B)
or (D) is the correct answer here, we need to know how many atoms are in a molecule
of acetic acid. To do that, we’ll take a look at
the subscripts. We can see that there is a
subscript of three next to one of the Hs, but there are also implied ones next to
the other chemical symbols in this formula. As there’s an implied one next to
each of the carbon atoms in the chemical formula, there are a total of two carbon
atoms in a molecule of acetic acid.
There’s a subscript of three next
to one hydrogen and a subscript of one next to the other hydrogen in the chemical
formula. That gives us a total of four
hydrogen atoms. Finally, there is a one next to
both appearances of oxygen, giving us a total of two oxygen atoms in this chemical
formula. Two carbon atoms plus four hydrogen
atoms plus two oxygen atoms gives us a total of eight atoms in a molecule of acetic
acid.
The answer option with eight atoms
and three elements is answer choice (B). The answer choice that best
describes a molecule of acetic acid is answer choice (B): it is composed of eight
atoms and three elements.
