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

3 hours because it is a decent distance.