What is the strongest intermolecular force in a test tube with isopropanol, water, and ammonium sulfate?

Intermolecular forces ....

Isopropanol, water, (NH4)2SO4

Ammonium sulfate dissolves in water to make two ions: NH4^+ and SO4^2-.  Both of these will be hydrated by water molecules which will surround them due to ion-dipole attraction. Water and isopropanol are both polar and will exhibit ion-dipole attraction.

Isopropanol is polar and exhibits all three van der Waals forces(*) plus hydrogen bonding, and the same is true of water.  Hyrdrogen bonding is stronger than any of the three van der Waals forces. There will be hydrogen bonding between isopropanol molecules, between water molecules and between water and isopropanol.

The bottom line. Normally, hydrogen bonding is the strongest of the intermolecular forces. Ion-dipole attraction is not technically an intermolecular force because ions in solution are not neutral molecules.  Nonetheless, ion-dipole attractions are usually lumped in with the intermolecular forces and will usually be stronger than even hydrogen bonding.

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The van der Waals forces are the intermolecular attractions between molecules and include Keesom forces, Debye forces and London dispersion forces. Together with hydrogen bonding, the van der Waals forces account for many of the properties of compounds which exist as discrete molecules like the melting and boiling points, as well as surface tension, and others.

1. London dispersion forces .... forces between all molecules. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Sometimes London dispersion forces were characterized as the weakest of the three van der Waals forces. In actuality, London dispersion forces are often stronger than Keesom or Debye forces and are second only to hydrogen bonding. The strength of London dispersion forces does not depend on the molar mass. That is only coincidental since "heavier" molecules often contain more electrons and are larger.

2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.

3. Debye forces (induced attraction) .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule. In such a case the induced charge may momentarily increase or decrease the dipole moment of the substance.

4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.