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Percent by Mass Miss Fogg Fall 2015.

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Presentation on theme: "Percent by Mass Miss Fogg Fall 2015."— Presentation transcript:

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: %


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