How many grams of lithium is formed when 12 g of lithium oxide is decomposed?

Given:
m (Li2O) = 12 g

To find:
m (Li) -?

Decision:
1) Write the reaction equation:
2Li2O => 4Li + O2;
2) Calculate the molar masses of Li2O and Li:
M (Li2O) = Mr (Li2O) = Ar (Li) * N (Li) + Ar (O) * N (O) = 7 * 2 + 16 * 1 = 30 g / mol;
M (Li) = Mr (Li) = Ar (Li) = 7 g / mol;
3) Calculate the amount of substance Li2O:
n (Li2O) = m (Li2O) / M (Li2O) = 12/30 = 0.4 mol;
4) Determine the amount of substance Li:
n (Li) = n (Li2O) * 4/2 = 0.4 * 4/2 = 0.8 mol;
5) Calculate the mass Li:
m (Li) = n (Li) * M (Li) = 0.8 * 7 = 5.6 g.

Answer: The mass of Li is 5.6 g.