Question

The amount of copper was determined in 10 g sample. copper was precipitated, filtered and calcined, obtaining a final precipitate of 0.79 g oxide of copper , What % in copper mass in this sample?

Answer 1

Your post isn't complete. You don't say if the oxide is CuO or Cu2O. At lower temperatures (200-400 C) the product is CuO but at higher temperature (say1000 C) is Cu2O. I will assume the product is CuO and you can make the change if that is necessary.
0.79 g CuO x (atomic mass Cu/molar mass CuO) = ? = mass Cu in the 10 g sample.
Then % Cu in sample = (grams Cu from above/10)*100 = ?

Answer 2

Thank u , you save my life !!