Multiple Choice
Which of the following structural formulas represents a polar molecule?
13. Liquids, Solids & Intermolecular Forces
Molecular Polarity
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How many of the following molecules are polar? CO2, H2O, BF3, NH3
A
2
B
3
C
1
D
4
Verified step by step guidance1
Step 1: Determine the molecular geometry of each molecule using the VSEPR theory. For CO2, the molecule is linear; for H2O, it is bent; for BF3, it is trigonal planar; and for NH3, it is trigonal pyramidal.
Step 2: Identify the polarity of each bond by comparing the electronegativities of the atoms involved. For example, in CO2, the C=O bonds are polar due to the difference in electronegativity between carbon and oxygen.
Step 3: Analyze the symmetry of each molecule to determine if the bond dipoles cancel out. In CO2, the linear shape causes the dipoles to cancel, making the molecule nonpolar. In H2O, the bent shape causes dipoles to add up, making it polar.
Step 4: Apply the same logic to BF3 and NH3. BF3 has a trigonal planar shape with symmetrical bond dipoles that cancel out, so it is nonpolar. NH3 has a trigonal pyramidal shape with a lone pair, causing an overall dipole moment, so it is polar.
Step 5: Count the number of polar molecules based on the above analysis. H2O and NH3 are polar, CO2 and BF3 are nonpolar, so the total number of polar molecules is 2.
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