For the reduction of 240 g of magnesium oxide, 80 g of coal were consumed

For the reduction of 240 g of magnesium oxide, 80 g of coal were consumed: MgO + C = Mg + CO. What is the mass fraction of pure carbon in this coal?

Given:
m (MgO) = 240 g
m (coal) = 80 g

Find:
ω (C) -?

1) MgO + C => Mg + CO;
2) n (MgO) = m (MgO) / M (MgO) = 240/40 = 6 mol;
3) n (C) = n (MgO) = 6 mol;
4) m (C) = n (C) * M (C) = 6 * 12 = 72 g;
5) ω (C) = m (C) * 100% / m (coal) = 72 * 100% / 80 = 90%.

Answer: The mass fraction of C is 90%.