Calculate the mass of copper oxide obtained by reacting copper with oxygen with an amount of 0.25 mol.

Given: n (O2) = 0.25 mol; it is necessary to find the mass of copper oxide m (CuO) =?
We are also given a reaction, write it down and arrange the coefficients:
2Cu + O2 = 2CuO
Since the amount of О2 entered into the reaction of 0.25 mol, then two times more copper oxide was formed (since the coefficient in front of the formula is 2).
n (CuO) = 0.25 * 2 = 0.5 mol.
The calculation of the mass of copper oxide is carried out according to the formula m = M * n, where M is the molar mass (we find it according to the periodic table), for CuO it is equal to 80 g / mol
m (CuO) = 80 * 0.5 = 40g.