Osmosis: calculate percentage gain and loss of mass of plant tissue.

The idea

In osmosis experiments, plant tissue such as potato or beetroot cylinders is weighed before and after being placed in solutions of different concentrations. The change in mass shows whether water has entered or left the cells. To calculate percentage gain or loss, first find the change in mass by subtracting the original mass from the final mass. A positive change is a gain; a negative change is a loss. Then divide the change by the original mass and multiply by 100. For example, if a cylinder starts at 5.0 g and finishes at 4.0 g, the change is −1.0 g, so the percentage loss is (1.0 ÷ 5.0) × 100 = 20%. Always report the direction of change clearly.

What earns the marks

• Record the original mass of the plant tissue before placing it in the solution.
• Record the final mass after the tissue has been removed and any surface water has been blotted.
• Calculate the change in mass by subtracting the original mass from the final mass.
• Divide the change in mass by the original mass to find the proportional change.
• Multiply the proportional change by 100 to express it as a percentage.
• State whether the result is a percentage gain or a percentage loss, using the sign or wording to show direction.

Common mistakes and how to put them right

• Using the final mass as the denominator instead of the original mass.

  Put it right by dividing by the starting mass.

• Ignoring the sign of the change and reporting a loss as a gain.

  Put it right by checking whether the final mass is greater or smaller than the original.

• Failing to blot excess water from the tissue before weighing, which makes the final mass too high.

  Put it right: Gently drying the surface before recording the final mass.

Exam tips

• Lay out your calculation clearly: change in mass, then change divided by original mass, then multiply by 100.
• Include the percentage sign and state gain or loss in your final answer, for example '20% loss'.
• Use the same balance and units for both masses to avoid unnecessary conversion errors.