4g of copper was formed. How many grams of copper oxide reacted?

1. Pure copper can be reduced from oxide by interaction with carbon:
2CuO + C = 2Cu + CO2 ↑;
2. Find the number of moles of the resulting metal:
n (Cu) = m (Cu): M (Cu) = 4: 64 = 0.0625 mol;
3. The amounts of copper and its oxide are equivalent:
n (CuO) = n (Cu) = 0.0625 mol;
4. Let’s calculate the mass of the reacted oxide:
m (CuO) = n (CuO) * M (CuO);
M (CuO) = 64 + 16 = 80 g / mol;
m (CuO) = 0.0625 * 80 = 5 g.
Answer: 5 g.