1. Percent by Mass
Miss Fogg
Fall 2015

2. Joseph Proust (France 1799)
Law of Definite Proportions:
A given compound always contains elements in a certain proportion by mass. (Constant composition).

3. Percent by Mass Equation

4. Example: Water You have a sample of water. 32.00g is oxygen
4.02g is hydrogen
Mass of compound = sum of the masses of all elements
Q: What is the overall mass of your water sample?
32.00g oxygen g hydrogen = 36.02g water

5. Example: Water Your sample of water is 36.02g 32.00g is oxygen
4.02g is hydrogen
Q: What is the oxygen percent by mass in water?
% oxygen = 32.00g oxygen x = % g water

6. Example: Water Your sample of water is 36.02g 32.00g is oxygen
4.02g is hydrogen
Q: What is the hydrogen percent by mass in water?
% hydrogen = 4.02g hydrogen x = % g water

7. Example: Water Check your answer!
The percent by mass of all the elements should total 100% (all the elements make up the compound)
Percent by mass oxygen: 88.8%
Percent by mass hydrogen: 11.2%
Sum: %

8. Law of Definite Proportions
Atoms combine in whole number ratios, so their proportion by mass will always be the same.
Example: H2O is always made up of 2 atoms of H and one atom of O.

9. Using atomic mass to find percent by mass
Atomic mass = mass of an atom measured in amu (atomic mass units)
Mass of oxygen =  16.00amu
Mass of hydrogen =  1.01amu

10. Mass of molecules Q: What is the mass of one water molecule?
H2O = 2 hydrogen atoms; 1 oxygen atom
Mass of oxygen: amu x 1 = amu
Mass of hydrogens: 1.01amu x 2 = amu
Total mass: amu

11. Using atomic mass to find percent by mass
Mass of oxygen: amu
Mass of hydrogen: amu
(Total) Mass of water: amu
Q: What is the oxygen percent by mass in water?
% oxygen = 16.00amu oxygen x = % amu water

12. Using atomic mass to find percent by mass
Mass of oxygen: amu
Mass of hydrogen: amu
(Total) Mass of water: amu
Q: What is the hydrogen percent by mass in water?
% hydrogen = 2.02amu hydrogen x = % amu water

13. Example: Water Check your answer!
The percent by mass of all the elements should total 100% (all the elements make up the compound)
Percent by mass oxygen: %
Percent by mass hydrogen: 11.21%
Sum: %