Worked Example: Percentage Change in Mass (Osmosis Practical)
Part of Cell Transport — GCSE Biology
This worked example covers Worked Example: Percentage Change in Mass (Osmosis Practical) within Cell Transport for GCSE Biology. Diffusion, osmosis, active transport, factors affecting transport, surface area to volume ratio, and practical investigations It is section 13 of 18 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 13 of 18
Practice
18 questions
Recall
20 flashcards
🧮 Worked Example: Percentage Change in Mass (Osmosis Practical)
In the required osmosis practical, you cut potato chips, weigh them, put them in different salt solutions, then weigh them again. You calculate the percentage change in mass to compare results fairly (since chips may not all start at the same mass).
Formula
Percentage change in mass = (Final mass − Initial mass) ÷ Initial mass × 100
Worked Example
Given: A potato chip has an initial mass of 2.5 g. After being placed in distilled water for 24 hours, its final mass is 2.8 g.
- Change in mass = 2.8 − 2.5 = +0.3 g
- Percentage change = 0.3 ÷ 2.5 × 100 = +12%
- The positive sign shows the chip gained mass — water entered by osmosis (distilled water is hypotonic relative to the potato cells).
A Second Example (Mass Lost)
Given: A potato chip has an initial mass of 3.0 g. After 24 hours in 1.0% salt solution, its final mass is 2.7 g.
- Change in mass = 2.7 − 3.0 = −0.3 g
- Percentage change = −0.3 ÷ 3.0 × 100 = −10%
- The negative sign shows the chip lost mass — water left the cells by osmosis (salt solution is hypertonic relative to the potato cells).
Why Use Percentage Change, Not Just Change?
If one chip started at 2.0 g and another started at 4.0 g, a raw change of 0.3 g means very different things for each. Percentage change standardises the result, allowing fair comparison between chips of different starting masses.
Quick Check: A potato chip has an initial mass of 4.0 g. After 24 hours in 0.8% salt solution its final mass is 3.6 g. Calculate the percentage change in mass and explain what happened to the potato cells.
Percentage change = (3.6 − 4.0) ÷ 4.0 × 100 = −0.4 ÷ 4.0 × 100 = −10%. The potato chip lost 10% of its mass. This is because the 0.8% salt solution is more concentrated (hypertonic) than the potato cell cytoplasm. Water therefore moved out of the potato cells by osmosis — from an area of higher water concentration (inside the cells) to lower water concentration (the salt solution) — through the partially permeable cell membrane. The cells became flaccid as their vacuoles shrank.