Answer :
Final answer:
Approximately 200g of the radioactive substance would be present at 5 pm, assuming the decay from 1000g at 12 pm to 100g at 6 pm follows exponential decay and that the substance's half-life allows for this estimation.
Explanation:
To determine how much of a radioactive substance was present at 5 pm on a certain day, when we know that at 12 pm there was 1000g and after six hours only 100g remained, we can use the concept of half-life. From the information given, the substance reduces to one-tenth of its original mass in six hours. However, we need to find the amount at 5 pm, which is one hour before the six-hour mark.
Since the exact half-life of the substance is not given, we will assume exponential decay within each hour. To find the mass at 5 pm, we should determine the ratio of decay per hour. From 1000g to 100g in six hours implies that the substance goes through multiple half-lives in this period. Without knowing the exact half-life, we can only approximate that the amount of substance at 5 pm would be twice that of 6 pm because one hour is presumably not enough time for it to decay to half if six hours takes it down to one-tenth. Therefore, approximately 200g of the substance would be present at 5 pm
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