[LHM]

A sample of copper metal is dissolved in 6M nitric acid contained in a round bottom flask. This reaction yields a blue solution and emits a colorless gas which is found to be nitric oxide. The water is evaporated from the blue solution to leave a blue solid. When the blue solid is heated further, a second reaction occurs. This reaction produces a mixture of nitrogen dioxide gas, oxygen gas, and a black oxide of copper. If all of the blue solid were not decomposed into the black oxide during the final heating, what would be the effect on the calculated percentage of copper in the oxide?

The solution says: "The mass of CuO will be too high. Therefore the percentage determination for copper will be too low." I know that the original amount of copper will be the same, but how is the mass of CuO too high if the blue solid isn't completely decomposed into the black oxide?

[Borek]

If the salt was not completely decomposed, whatever you weight is not just a CuO, but CuO and something.