Using the amylase-starch table (20.0 g at 0 min and 11.0 g at 30 min), what is the average rate of enzyme activity over 30 minutes?
0 | 20.0
5 | 17.5
10 | 15.0
15 | 13.7
20 | 12.5
25 | 11.8
30 | 11.0
The average rate of amylase activity over 30 minutes is the average decrease in starch mass per minute: $\frac{20.0\,\text{g} - 11.0\,\text{g}}{30\,\text{min}} = \frac{9.0\,\text{g}}{30\,\text{min}} = 0.30\,\text{g min}^{-1}$. This means amylase broke down starch at an average of $0.30$ g per minute over the 30-minute period.
What the question is asking
You are given how much starch is left at the start and after 30 minutes. The average enzyme activity rate over the whole period is found by taking the total amount of starch broken down and dividing by the total time.
Find the total starch broken down
Initial mass of starch at $0$ min: $20.0\,\text{g}$ Final mass of starch at $30$ min: $11.0\,\text{g}$
Amount broken down: $$20.0 - 11.0 = 9.0\,\text{g}$$
Convert that change into an average rate
Total time $= 30\,\text{min}$
Average rate: $$\text{rate} = \frac{\Delta \text{mass}}{\Delta t} = \frac{9.0\,\text{g}}{30\,\text{min}} = 0.30\,\text{g min}^{-1}$$
Interpreting the result
On average, the mass of starch decreased by $0.30$ g each minute over the 30 minutes (even though the rate may not have been exactly constant at every 5-minute interval).
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