Using Percentiles and Osmosis.
You need to be able to use percentiles when analysing osmosis data.
A percentile is a term that is used to describe the percentage of data values that fall at or below another data value.
Percentiles example
Example: The 90th percentile is the value below which 90% of the data falls.
Suppose exam scores (out of 100) for 10 students are:
20, 35, 40, 50, 55, 60, 65, 70, 80, 90
(make sure the scores are placed in numerical order, smallest to biggest).
25th percentile (P25):
A score that’s higher than 25% of the data (and lower than the other 75%).
Look for the score that’s about ¼ of the way through the list.
¼ of 10 = 2.5 → so around the 2nd or 3rd score → ≈ 40.
50th percentile
This is special — it’s also called the median.
Take the middle two numbers (the 5th and 6th).
Median = (55+60)/2 = 57.5
75th percentile (P75):
A score that’s higher than 75% of the data (and lower than the top 25%).
Look for the score that’s about ¾ of the way through the list.
¾ of 10 = 7.5 → so around the 7th or 8th score → ≈ 70.
Osmosis and Percentiles.
Potato cylinders were placed into solutions of different sucrose concentrations (0.0, 0.2, 0.4, 0.6, 0.8 mol/dm³) and the percentage change in mass was calculated.
| Concentration (mol/dm³) | % Change in Mass |
|---|---|
| 0.0 (distilled water) | +15 |
| 0.2 | +10 |
| 0.4 | 0 |
| 0.6 | -8 |
| 0.8 | -14 |
Step 1: Order the data
Arrange % change in mass from lowest → highest:
–14%, –8%, 0%, +10%, +15%
Step 2: Find percentiles
25th percentile (P25): 25% of data below → between –14% and –8% → ≈ –11%
50th percentile (P50 / median): middle value → 0%
75th percentile (P75): 75% of data below → between 0% and +10% → ≈ +5%
Step 3: Link to osmosis
P25 (–11%): 25% of potato samples lost more than 11% of their mass (strong movement of water out).
P50 (0%): the concentration where mass doesn’t change → potato’s internal concentration (isotonic point).
P75 (+5%): 75% of samples gained less than +5% mass (weaker movement of water in).
Practice Question
1.Define the term percentile.
2. Use the following data series to answer
55, 25, 65, 15, 35, 45, 75
2a. Place the number series in numerical order, smallest to biggest
2b. The 25th percentile is……..
2c. The 50th percentile is……..
2d. The 75th percentile is………