Mass Reactants = Mass Products

Law of Conservation of Mass: the mass of the reactants must equal the mass of the

products.

Law of Definite Proportions: a compound (like H2O) is always composed of the same elements in the same proportion by mass (no matter what the sample size).

% by Mass = mass of element (g) x 100 mass of compound (g)

Example: 78.0g of Unknown Compound 12.4g of Hydrogen (H) ? = % by mass of H

% by Mass H = mass of hydrogen (g) x 100 mass of unk compound (g)

= 12.4 g H x 10078.0g Unk Compound

= 15.9%

Practice:A 24.0g sample of unknown compound contains 6.5 g of hydrogen. What is the percent by mass of hydrogen in the compound?

Start by writing down your information.

Practice:A 24.0g sample of unknown compound contains 6.5 g of hydrogen. What is the percent by mass of hydrogen in the compound?

24.0 g of Unknown Compound 6.5 g of Hydrogen (H) ? = % by mass of H

Now, set up the equation & plug in the numbers.

Practice:A 24.0g sample of unknown compound contains 6.5 g of hydrogen. What is the percent by mass of hydrogen in the compound?

24.0 g of Unknown Compound 6.5 g of Hydrogen (H) ? = % by mass of H

% by Mass H = mass of hydrogen (g) x 100 mass of unk compound (g)

Practice:A 24.0g sample of unknown compound contains 6.5 g of hydrogen. What is the percent by mass of hydrogen in the compound?

24.0 g of Unknown Compound 6.5 g of Hydrogen (H) ? = % by mass of H

% by Mass H = 6.5 g Hydrogen x 100 24.0 g Unk Compound

Practice:A 24.0g sample of unknown compound contains 6.5 g of hydrogen. What is the percent by mass of hydrogen in the compound?

24.0 g of Unknown Compound 6.5 g of Hydrogen (H) ? = % by mass of H

% by Mass H = 6.5 g Hydrogen x 100 24.0 g Unk Compound= 27.1% by mass of Hydrogen

Law of Multiple Proportions: when different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the element in a ratio of small whole numbers (can use colons or fractions).

Law of Multiple Proportions