a 6 g sample of magnesium was burned in a crucible producing 10 g of magnesium oxide. What mass of oxygen reacted with magnesium to form the 10 g of magnesium oxide?

Answer 1

2Mg + O2 ==> 2MgO
mols MgO = grams/molar mass = approx 0.25 but you need a more accurate answer.
Then convert mols MgO to mols O2 using the coefficients in the balanced equation. That is 0.25 mols MgO x (1 mol O2/2 mols MgO) = 0.25 x 1/2 = 0.125 mols O2.
g O2 = mols O2 x molar mass O2 = ?
IF you knew that all of the 6g Mg starting material was consumed, then you could have used the law of Conservation of mass or
6g Mg + ?g O2 = 10 g and g O2 = 4 grams.