The proportion by mass will always be the same! Percent by mass (%) = mass of element x 100 mass of compound
Magnesium combines with oxygen to form magnesium oxide. 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?
The entire sample of magnesium oxide is: 10.57 g magnesium + 6.96 g oxygen = 17.53 g magnesium oxide The percent of magnesium in magnesium oxide is: 10.57 g magnesium_____ X 100 = 60% 17.53 g magnesium oxide The percent of oxygen in magnesium oxide is: 6.96 g oxygen_______ X 100 = 40% 17.53 g magnesium oxide
Your answer should equal 100% of the sample: 60% + 40%= 100%