A 10.0 g sample of magnesium reacts with oxygen to form 16.6 g of magnesium oxide. How many grams of oxygen reacted?
The idea here is that chemical reactions must obey the Law of Mass Conservation, which states that when a chemical reaction takes place, the total mass of the reactants must be equal to the total mass of the products.
In your case, magnesium metal and oxygen gas are the reactants and magnesium oxide is the product.
#2"Mg"_ ((s)) + "O"_ (2(g)) -> 2"MgO"_ ((s))#
You know that
You can say that the difference between the mass of the product and the total mass of the reactants, i.e. the mass of magnesium and the mass of oxygen gas, will give you the mass of oxygen gas that took part in the reaction.
#color(blue)(ul(color(black)(m_"Mg" + m_("O"_ 2) = m_"MgO"))) ->#the Law of Mass Conservation
You will thus have
#m_ ("O"_ 2) = m_ "MgO" - m_ "Mg"#
#m_ ("O"_ 2) = "16.6 g" - "10.0 g" = color(darkgreen)(ul(color(black)("6.6 g")))#
The answer is rounded to one decimal place.