1. Choice A is obviously a correct formula --- it has four hydrogen atoms
QUESTION:
A molecule of acetic acid consists of four hydrogen atoms, two carbon atoms, and two oxygen atoms. Which of these cannot possibly be the formula of acetic acid? A. H4C2O2, B. H2CO, C. HC2H3O2, D. CH3COOH
The formula of a molecular compound reflects the composition of each molecule in the compound. Subscripts indicate how many atoms of each kind are present in the molecule.
SCRIPT:
A molecule of acetic acid consists of four hydrogen atoms, two carbon atoms, and two oxygen atoms. Which of these cannot possibly be the formula of acetic acid? A. H4C2O2, B. H2CO, C. HC2H3O2, D. CH3COOH
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The formula of a molecular compound reflects the composition of each molecule in the compound. Subscripts indicate how many atoms of each kind are present in the molecule.
Choice A is obviously a correct formula --- it has four hydrogen atoms
HIGHLIGHT “four hydrogen atoms” in question and subscript “4” in choice A
… two carbon atoms..
HIGHLIGHT …and two oxygen atoms.
It doesn’t have to be written this way. For example, it could be written as C2H4O2
CALLOUT “same as C2H4O2”
The correct answer is B. It cannot be the formula for acetic acid because it has the wrong subscripts.
What about choices C and D?
Let’s examine choice C. The sum of the subscripts for hydrogen is 4.
The subscript for the first H is implied to be one.
CALLOUT “1 implied”
The subscript for the other H is 3.
HIGHLIGHT
1+3 equals 4.
The subscripts for C and O are also correct. They’re both two.
For choice D, the sum of the subscripts for hydrogen is 4.
The first H has a subscript of 3, and the second has an implied subscript of 1.
HIGHLIGHT.
Both Oxygens have an implied subscript of 1.
HIGHLIGHT 1+1 equals 2.
In fact, the formulas in choices C and D are more commonly used to represent acetic acid.
Formulas written like these tell us more than just the composition of the molecule.
The formula in choice C is often used when referring to the compounds behavior as an acid.
One thing acids have in common is that they have hydrogen atoms that can break off as hydrogen ions when
they acid is dissolved in water. In the case of acetic acid, only one of the four hydrogen atoms can break off as a hydrogen ion in water.
The formula in choice C reflects the way the atoms are connected in the molecule.
Here’s CH3…
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Here’s COOH
And here’s the hydrogen atom that can break off as a hydrogen ion when acetic acid is dissolved in water.
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2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08