If 10.57 g of magnesium reacts completely with 6.96 g of oxygen, what is the percent by mass of oxygen in magnesium oxide? round to the nearest tenth of a percent.

The percent by mass of oxygen in the magnesium oxide is 39.5%.

The given parameters;

The chemical reaction of the magnesium and oxygen is given as;

Mg    +     O₂    ----->   MgO  

10.57 g of Mg = 40 g of MgO

10.57 g of Mg =  ?

The percentage by mass of the oxygen is calculated as;

Thus, the percent by mass of oxygen in the magnesium oxide is 39.5%.

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39.7 %

Explanation:

magnesium + oxygen ⟶ magnesium oxide

   10.57 g         6.96 g               17.53 g

According to the Law of Conservation of Mass, the mass of the product must equal the total mass of the reactants.

Mass of MgO = 10.57 + 6.96

Mass of MgO = 17.53 g

The formula for mass percent is

% by mass = Mass of component/Total mass × 100 %

In this case,

% O = mass of O/mass of MgO × 100 %

Mass of O = 6.96 g

Mass of MgO = 17.53 g

% O = 6.96/17.53 × 100

% O = 0.3970 × 100

% O = 39.7 %

b is the correct