Students investigated the effect of carbon dioxide concentration on the rate of photosynthesis in the aquatic plant Elodea canadensis by measuring the volume of oxygen gas produced per minute. The carbon dioxide concentration was varied using different concentrations of sodium hydrogencarbonate (NaHCO3\text{NaHCO}_3NaHCO3) solution. The results are shown in the table below:
| Concentration of NaHCO3\text{NaHCO}_3NaHCO3 (%) | Rate of photosynthesis in mm3/minute\text{mm}^3/\text{minute}mm3/minute | |||
|---|---|---|---|---|
| Trial 1 | Trial 2 | Trial 3 | Mean | |
| 0.05 | 8.2 | 7.9 | 8.5 | YYY |
| 0.10 | 15.4 | 14.8 | 15.1 | 15.1 |
| 0.15 | 22.1 | 23.3 | 22.4 | 22.6 |
| 0.20 | 28.5 | 29.1 | 28.8 | 28.8 |
| 0.25 | 31.2 | 30.8 | 31.6 | 31.2 |
| 0.30 | 32.1 | 12.5 | 32.9 | 32.5 |
Calculate the mean value YYY shown in the table. Show your working.
Identify the anomalous result in the table by stating its NaHCO3\text{NaHCO}_3NaHCO3 concentration and trial number.
Explain how the students should deal with this anomalous result when calculating the mean for that concentration.
State one key factor that the students should have kept constant in this investigation to ensure valid results.