Percent by Mass Miss Fogg Fall 2015.

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Presentation transcript:

Percent by Mass Miss 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%