Answer
$mass_{O_{2}}$ = 0.268 g
Work Step by Step
The mass of the products and the mass of the reactants are equal. This is stated in the Law of Conservation of Mass.
So $mass_{products}$ = $mass_{reactants}$
$mass_{Mg} + mass_{O_{2}} $ = $mass_{MgO}$
0.406 + $mass_{O_{2}}$ = 0.674
$mass_{O_{2}}$ = 0.674 - 0.406
$mass_{O_{2}}$ = 0.268 g
Rounding to three significant figures gives $mass_{O_{2}}$ = 0.268 g
