Answer:
1) The sample may not have been dried properly
2) presence of other impurities in the copper
Explanation:
When the sample is not properly dried, water molecules remain attached to the sample and when the sample is weighed, its weight will be higher than it should be due to the water molecules present. This will make the percentage yield of copper to exceed 100%.
Other impurities present in the copper may make it to weigh higher than it should thereby raising the percent yield of copper above 100%.
