PERCENT BY MASSMiss Fogg
Fall 2015
Joseph Proust (France 1799)
•Law of Definite Proportions:
A given compound always contains elements in a certain proportion by mass. (Constant composition).
Percent by Mass Equation
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 + 4.02g hydrogen = 36.02g water
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 100 = 88.8% 36.02g water
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 100 = 11.2% 36.02g water
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: 100%
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.
Using atomic mass to find percent by mass
• Atomic mass = mass of an atom measured in amu (atomic mass units)
Mass of oxygen = 15.999 16.00amu
Mass of hydrogen = 1.008 1.01amu
Mass of molecules
Q: What is the mass of one water molecule?
• H2O = 2 hydrogen atoms; 1 oxygen atom
Mass of oxygen: 16.00amu x 1 = 16.00amu
Mass of hydrogens: 1.01amu x 2 = 2.02amu
Total mass: 18.02amu
Using atomic mass to find percent by mass
Mass of oxygen: 16.00amu
Mass of hydrogen: 2.02amu
(Total) Mass of water: 18.02amu
Q: What is the oxygen percent by mass in water?
% oxygen = 16.00amu oxygen x 100 = 88.79% 18.02amu water
Using atomic mass to find percent by mass
Mass of oxygen: 16.00amu
Mass of hydrogen: 2.02amu
(Total) Mass of water: 18.02amu
Q: What is the hydrogen percent by mass in water?
% hydrogen = 2.02amu hydrogen x 100 = 11.21% 18.02amu water
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: 88.79%• Percent by mass hydrogen: 11.21%
• Sum: 100.00%
