The mass of lithium oxide formed during the combustion of 3.5 g of lithium in an excess of oxygen is equal to ..

Reaction: 4Li + O2 = 2Li2O
The amount of lithium substance n = m / M = 3.5 g / 7 g / mol = 0.5 mol. (M is the molar mass determined by the periodic table).
According to the reaction from 4 mol of lithium, 2 mol of oxide is obtained, 0.5 / 4 = n (Li2O) / 2. n (Li2O) = 0.5 * 2/4 = 0.25 mol.
Lithium oxide mass m = n * M = 0.25 mol * 30 g / mol = 7.5 g.
Answer: the mass of lithium oxide is 7.5 g.