Osmosis: use percentages

The idea

A percentage expresses a quantity as a fraction of 100, so it allows fair comparison between samples of different starting sizes. In osmosis practical work, you may need to calculate what proportion of a plant tissue's original mass has been gained or lost, or express a change relative to the starting value. To use percentages, identify the whole or original value, divide the part by that whole, then multiply by 100. For example, if a potato cylinder starts at 2.0 g and ends at 2.4 g, the change is 0.4 g; as a percentage of the original mass this is (0.4 ÷ 2.0) × 100 = 20%. Always state whether the change is a gain or a loss and give the percentage with appropriate units or context.

What earns the marks

• Identify the original or starting value as the whole amount before calculating any percentage.
• Calculate the change by subtracting the original value from the final value, or the final value from the original value, depending on whether mass is gained or lost.
• Divide the change by the original value, not by the final value, to find the proportion of the original mass.
• Multiply the proportion by 100 to convert it to a percentage.
• State clearly whether the result represents a percentage gain or a percentage loss.
• Use consistent units for mass throughout the calculation, such as grams for both original and final masses.

Common mistakes and how to put them right

• Dividing the change by the final mass instead of the original mass.

  Put it right by using the starting mass as the denominator.

• Forgetting to multiply by 100 and leaving the answer as a decimal or fraction.

  Put it right by completing the conversion to a percentage.

• Omitting the direction of change and simply giving a percentage.

  Put it right by stating whether the tissue gained or lost mass.

Exam tips

• Write down the original mass and final mass before doing any arithmetic so the values are clear.
• Show the calculation step by step, including the division and the multiplication by 100, to gain method credit where available.
• Check that your percentage is sensible: a small change in mass should give a small percentage, while a doubling of mass gives a 100% gain.