2Mg + O2 → 2MgO How many grams of MgO are produced when 40.0 grams of O2 react completely with Mg?

101 g

Explanation:

M (O2) = 2g*16.0 = 32.0 g / mol

M (MgO) = 24.3 + 16.0 = 40.3 g/mol

2 Mg + O2 → 2MgO

from reaction 1 mol 2 mol

from reaction 1 mol*32.0 g/mol 2 mol*40.3 g/mol

given 40 g x g

1 mol*32.0 g/mol - - - 2 mol*40.3 g/mol

40 g - - - x g

x = (40*2*40.3) / (1*32.0) = 100.75 g ≈ 101 g