Determine the mass of silicon (Si) and the volume of oxygen (O2) reacted to form 20 g of SiO2

Given:
m (SiO2) = 20 g

Find:
m (Si) -?
V (O2) -?

1) Si + O2 => SiO2;
2) M (SiO2) = Mr (SiO2) = Ar (Si) * N (Si) + Ar (O) * N (O) = 28 * 1 + 16 * 2 = 60 g / mol;
3) n (SiO2) = m (SiO2) / M (SiO2) = 20/60 = 0.3 mol;
4) n (Si) = n (SiO2) = 0.3 mol;
5) m (Si) = n (Si) * M (Si) = 0.3 * 28 = 8.4 g;
6) n (O2) = n (SiO2) = 0.3 mol;
7) V (O2) = n (O2) * Vm = 0.3 * 22.4 = 6.72 liters.

Answer: The mass of Si is 8.4 g; O2 volume – 6.72 liters.