Would extra mass in the crucible cause the "mass of oxygen" to come out too high or too low? explain.

 Assuming that the experiment started with the magnesium and did not combine with oxygen, this extra mass would actually decrease the amount of oxygen. This is because it only artificially increases the original mass. Therefore this actually increases the error.

too low

one negative thing it that it can kill water animals. another negative thing is it can cause the destruction of ecosystem. one way water pollution can be reduced is to conserve energy.