In the classic osmosis experiment using potato cubes and placing them into different solutions, there are several different sources of error. These include the concentrations of the solutions, the size of the potato cubes, the measurements of the masses of the cubes before and after the experiment, the drying of the cubes before weighing, the timing of the experiment, and the process of making qualitative observations.

It is best in this experiment to use a hypertonic solution, a hypotonic solution, and an isotonic solution, at a minimum. In this case, 50% and 25% are both hypertonic. It might be better to replace one of these with something closer to isotonic, perhaps 1 or 2% salt. Another approach that would involve more work but would give much better results is to make a series of solutions, such as 0%, 1%, 2%, 5%, 10%, 20%, 50% salt. This would allow you to obtain much more information, and also to determine more closely the actual potato solute concentration.

In terms of the size of the potato cubes, it would be important to standardize their size. Cutting by hand can result in different sizes. Some of this could be alleviated by using 10 potato cubes in each solution or by using a device like a cork borer to provide uniform diameter potato sections. Again, this would be more effort but would provide better results.

In terms of measuring the masses of the cubes, this is a step that might be skipped, but it is worth doing. The cubes need to be patted dry in a uniform way, and then their mass found to the nearest thousandth of a gram. The mass can be determined before and after the cubes sit in the solutions for at least an hour. Careful use of a weighing scale and using 10 cubes for each solution, then basing conclusions only on the average change in mass in each solution will go a long way to reducing error in finding changes in the cubes.

It is, of course, important to be sure that all the cubes are in their solutions for the exact same amount of time. Some careful planning and organization of the work area will make it possible to be sure that each cube is in the solution for the same time. Finally, in making qualitative observations, such as the appearance of the cubes, it would be best to have one person remove the cubes and monitor which ones are which based on a code system. Then another person, the observer, can make the observations without knowing which cubes were in which solution. This will help to prevent biased observation. The same approach could be used in weighing the cubes to prevent bias there as well.

So, as simple as the experiment is, there are many places where errors can occur, but there are also ways to reduce each error significantly.