Multiple Choice
Which of the following molecules is IR-active?
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13. Liquids, Solids & Intermolecular Forces
Molecular Polarity
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Which of the following compounds is polar?
A
BF_3
B
CO_2
C
NH_3
D
CCl_4
Verified step by step guidance1
Step 1: Understand that polarity in molecules depends on both the polarity of individual bonds and the overall molecular geometry, which determines if the bond dipoles cancel out or add up to a net dipole moment.
Step 2: Analyze BF_3: Boron trifluoride has three B-F bonds which are polar, but the molecule is trigonal planar and symmetrical, so the dipoles cancel out, making BF_3 nonpolar.
Step 3: Analyze CO_2: Carbon dioxide has two polar C=O bonds arranged linearly (180°), so the dipoles are equal and opposite, canceling each other out, resulting in a nonpolar molecule.
Step 4: Analyze NH_3: Ammonia has three N-H bonds and a lone pair on nitrogen, giving it a trigonal pyramidal shape. This asymmetry causes the bond dipoles to not cancel, resulting in a net dipole moment, so NH_3 is polar.
Step 5: Analyze CCl_4: Carbon tetrachloride has four C-Cl bonds arranged tetrahedrally symmetrically. The dipoles cancel out, making CCl_4 nonpolar.
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