Question

When 2.50 grams of copper reacts with oxygen?

Answer 1

 Mass oxigen = 2.81 - 2.50 = 0.31 g 
Moles O = 0.31 g / 15.9994 g/mol =0.0194 

moles Cu = 2.50 g / 63.546 g/mol =0.0393 

we divide by the smallest number 
0.0194 / 0.0194 = 1 => O 
0.0393 / 0.0194 = 2 => Cu 
the formula is Cu2O ( Copper (I) oxide )
Ya got this