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# Composition stoichiometry

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### Composition stoichiometry

1. 1. Composition Stoichiometry Percent Composition & Chemical Formulas
2. 2. • Law of Definite Proportions – In samples of any chemical compound, the masses of the elements are always in the same proportions. – Compares the different elements in a single compound – H2O • mass of H : mass of O is always the same• Law of Multiple Proportions – Whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers – Compares the same elements found in different compounds – H2O and H2O2 • Mass of O in H2O : mass of O in H2O2 = small whole number
3. 3. Percent Composition• Percent composition – Percent by mass of each element in the compound – Always the same for a given compound
4. 4. Percent Composition from Mass Data• The number of grams of the element divided by the mass in grams of the compound mass of element% mass of element 100 mass of compound
5. 5. When a 13.60-g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound?Mass of oxygen = 5.40 gMass of magnesium = 13.60 g – 5.40 g = 8.20 gMass of compound = 13.60 g mass of Mg 8.20g%Mg 100 100 60.3% Mg mass of compound 13.60g mass of O 5.40g%O 100 100 39.7% O mass of compound 13.60g
6. 6. Percent Composition from Chemical Formula • Number of grams of element in 1 mole of compound divided by the molar mass of the compound times 100 mass of elementin 1 molof compound% mass 100 molarmass of compound
7. 7. Calculate the percent composition of propane (C3H8).C: 3 x 12.0g = 36.0g / 44.0 X 100 = 81.8%CH: 8 x 1.0g = 8.0g / 44.0 X 100 = 18.2%H 44.0g Number of atoms ofelement from formula Molar mass of Molar mass of compound element from periodic table
8. 8. • Hydrates – A compound that has a specific number of water molecules bound to each formula unit – When writing the formula, use a dot to connect the formula of the compound and the number of water molecules per formula unit • CuSO4 5H2O – When naming a hydrate, name the compound then use a prefix followed by the word “hydrate” to indicate the number of water molecules per formula unit • Copper(II) sulfate pentahydrate
9. 9. • Finding the percent of water in a hydrate mass of water% H 2O 100 mass of hydrate
10. 10. Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate.Formula of compound = Na2CO3 10 H2ONa : 2 23.0g 46.0gC : 1 12.0g 12.0gO : 3 16.0g 48.0gH 2O :10 18.0g 180.g 286g 100 62.9%H 2O T otal 286g