Is osmosis stronger or weaker than gravity, and by how much? Suppose you perpare a jar of salt water and another of sugar water and invert one on top of the other with a divider between them, and then carefully remove that divider so the liquids are in contact.
Will the concentrations of salt and sugar reach equilibrium, despite the fact that the salt or sugar in the bottom jar has to overcome gravity to rise into the upper jar? How would you calculate the relevant forces here?
 A: Osmotic pressure is not really relevant here. In osmosis, only the solvent can move. In your scenario, both the solvent and the solutes can move. You are asking about ordinary mixing.
It will always be possible for equilibrium to be reached eventually, but in general we don't know how to calculate from first principles how long it will take. The solute particles in the bottom jar are not confined to the bottom jar; they have kinetic energy and will occasionally cross over into the top jar. The mixing will be more rapid at higher temperatures.
At equilibrium, the solute will have a very slightly higher concentration closer to the ground. The heavier the solute is, the more pronounced the gradient. If we treat the solution as an ideal solution, we can calculate the difference in concentrations explicitly using the Boltzmann distribution. For example, let's assume that each jar has a height of 10 cm. The mass of a sucrose molecule is $5.68 \times 10^{-25}$ kg. Let's assume an ambient temperature of 293 K. The ratio of probabilities for it to be found in the bottom jar rather than the top jar is $\exp(\frac{mgh}{kT})$ which comes out to about 1.000138. That means the sugar will be more concentrated in the bottom jar by a ratio of 1.000138; a very slight difference. You might be able to measure it using a very sensitive spectrophotometer.
