2. What Proust and Dalton contributed to Modern Atomic Theory

3. Regardless of the amount, a compound is always composed of the same elements in the same proportion by mass.

4. If there is one H 2 O molecule

5. Or many H 2 O molecules

6. The proportion by mass will always be the same! Percent by mass (%) = mass of element x 100 mass of compound

7. 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?

8. 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

9. Your answer should equal 100% of the sample: 60% + 40%= 100%