How can a molecule be at equilibrium across a membrane if the concentrations are different?

There is less solvent on one side because of the nature of the membrane. Therefore the concentration is raised even if there is the same amount of molecules on each side

Most likely in this situation the membrane is either permeable or semi permeable (both meaning "allowing molecules to go through it"). Now, because the concentration of a molecule is higher at one side, this means that it is unbalanced. A concentration gradient (the level of how unbalanced the concentration is between the 2 sides) is formed. Diffusion is when molecules go from a higher concentration to an area of lower concentration due to higher pressure at one end (because there are more molecules at one side, there is a greater mass (heavier) which pushes through the membrane to the part where there are less molecules (lighter)). This balances out the concentration of both sides. There are equal volume of molecules at both ends which means that it is at equilibrium. I hope I helped!

EDIT: Basically, diffusion of molecules on both sides of the membrane causes equilibrium.

equilibrium means that the current situation doesn't change. It doesn't mean that the concentration is the same on two sides of a membrane.

You can get an equilibrium situation (where the current distribution is stable and won't change unless some conditions change) where there a different concentration on an ion across a membrane. Since you usually have not molecules but ions in a solution their distribution will get affected by the other ions in the solution. If you have e.g. a lot of sodium on one side of the membrane, the sodium moving through the membrane will also carry a charge when they move through the membrane. If the solution was such, that
a) before there wasn't a charge across the membrane
b) the anions can't move through the membrane
then the charge across the membrane will grow the more sodium ions cross. Eventually the sodium ions won't be able to cross the membrane since they will be "held back" by the negative charge of the anions despite the concentration of the sodium is not the same on both sides of the membrane.

In short, there are additional forces in addition to entropy.

You are right.

however, you may include other factors to determine equilibrium.  The most common for cells is the voltage of the membrane.  So if you are talking about the equilibrium of sodium ions across a membrane.  If the membrane voltage is 0 [and the membrane is permeable to sodium ions] then the equilibrium concentration of sodium ions will be equal on both sides of the membrane.

If there is a voltage applied across the membrane and one side is positive and the other is negatively charged, the the positively charged sodium ions will equilibriate with more positive sodium ions on the negative side of the membrane than on the positive side of the membrane, in order to negate the charge of the membrane.

So the sodium ions are actually subject to two forces as they try to obtain equlibrium, they try to equilibrate by concentration but they also try to equilibrate by charge as well.  

If the molecule is uncharged then the equilibrium will be when the concentration on both sides of teh membrane is equal regardless of the voltage of the membrane.

you can apply a voltage across the mebrane either using an electrical source, or by the presence of other molecules which may be charged.