To determine which substance has only London dispersion forces as its only intermolecular force, we need to analyze the intermolecular forces present in each substance.
Here's a step-by-step breakdown:
1. PH₃ (Phosphine):
- Phosphine has a molecular geometry that results in a dipole moment. This leads to dipole-dipole interactions.
- Thus, PH₃ has both London dispersion forces and dipole-dipole interactions.
2. CH₃OH (Methanol):
- Methanol contains an OH (hydroxyl) group.
- The presence of the OH group allows for hydrogen bonding, which is a strong type of intermolecular force.
- Besides hydrogen bonding, it also has London dispersion forces and dipole-dipole interactions due to the polar nature of the molecule.
3. H₂O (Water):
- Water contains two OH bonds.
- Water molecules engage in extensive hydrogen bonding due to the presence of these OH groups.
- In addition to hydrogen bonding, water also has London dispersion forces and dipole-dipole interactions.
4. CBr₄ (Carbon Tetrabromide):
- Carbon tetrabromide is a tetrahedral molecule with four bromine atoms symmetrically around a central carbon atom.
- Because the molecule is symmetrical, it is nonpolar.
- Nonpolar molecules with no other type of intermolecular forces will have London dispersion forces as their sole type of intermolecular force.
5. HBr (Hydrogen Bromide):
- Hydrogen bromide consists of a hydrogen atom bonded to a bromine atom, which results in a polar bond.
- This polarity leads to dipole-dipole interactions.
- Therefore, HBr has both London dispersion forces and dipole-dipole interactions.
Evaluating the intermolecular forces in all the given substances, only one of them (CBr₄) is a nonpolar molecule with London dispersion forces as its only intermolecular force.
Thus, the substance that has London dispersion forces as its only intermolecular force is:
[tex]\[
\boxed{CBr_4}
\][/tex]
