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Laws and Percent Composition

Laws and Percent Composition. Law of Definite Proportions, 1797. Definition: States that in samples of any chemical compound, the masses of the elements are always in the same proportions Created by Joseph Proust Also called the Law of Constant Composition

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Laws and Percent Composition

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  1. Laws and Percent Composition

  2. Law of Definite Proportions, 1797 Definition: States that in samples of any chemical compound, the masses of the elements are always in the same proportions Created by Joseph Proust Also called the Law of Constant Composition Relates to experimental data of percent composition Use ratios of mass data to prove that composition is constant
  3. Example 1 Two samples of carbon dioxide, obtained from different sources, were decomposed into their constituent elements. One sample produced 4.8 g of oxygen and 1.8 g of carbon, and the other sample produced 17.1 g of oxygen and 6.4 g of carbon. Show that these results are consistent with the law of definite proportions.
  4. Are they consistent?
  5. Example 2 Two samples said to be carbon disulfide (CS2) are decomposed into their constituent elements. One sample produced 8.08 g S and 1.51 g C, while the other produced 31.3 g S and 3.85 g C. Are these results consistent with the law of constant composition?
  6. Are they consistent?
  7. Law of Multiple Proportions Definition: 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 Created by John Dalton
  8. http://honolulu.hawaii.edu/distance/sci122/Programs/p25/p253.gif http://honolulu.hawaii.edu/distance/sci122/Programs/p25/p253.gif
  9. Chemical Formulas can be determined from experimental mass data Percent Composition describes the relative masses of the elements in a compound
  10. Percent Composition Using mass data to determine percent composition % of element E = grams of E * 100 grams of compound
  11. Percent Composition from Mass DataExample 1 When a 13.6-g sample of a compound contains only Mg and O is decomposed, 5.40 g of O is obtained. What is the % composition of this compound?
  12. When a 13.6-g sample of a compound contains only Mg and O is decomposed, 5.40 g of O is obtained.
  13. Con’t
  14. Percent Composition from Mass DataExample 2 What is the percent composition of a compound that is formed from 2.70 g of Al + 5.10 g of O (forms aluminum oxide)
  15. What is the percent composition of a compound that is formed from 2.70 g of Al + 5.10 g of O (forms aluminum oxide) Law of conservation of Mass ….
  16. Percent composition from chemical formula Use when no mass data is given, you can compare the ratio of the molar mass of each element to molar mass of the compound. % mass = grams of element in 1 mole** x 100 molar mass of compound **Need to take into account the subscripts for each atom
  17. Percent Composition from Formula: Example 1 Propane, C3H8, the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the % composition of propane.
  18. % Composition of Propane Do all elements unless noted….
  19. Percent Composition from Formula: Example 2 Ammonium sulfate, (NH4)2SO4 is a commonly used fertilizer. Calculate the % composition of ammonium sulfate.
  20. % Composition of (NH4)2SO4
  21. % Composition of (NH4)2SO4
  22. Hydrate: A solid compound containing water molecules combined in a definite ratio as an integral part of the crystal.
  23. Copper Sulfate•5H2O
  24. Hydrates When hydrates are heated they break down into the anhydrous salt (ionic portion) and water We can use this information to help us find the formula of an unknown hydrate
  25. Section 10-5
  26. Finding the Formula of a hydrate Using the law of conservation of mass, find the mass of the salt and the mass of the water. This may be given or may need to be determined experimentally Convert the mass of salt to moles using molar mass of salt. Convert mass of water to moles of water using molar mass of water (18.02 g/mol)
  27. 4. Determine the ratio of moles of water to moles of salt to find the formula for the hydrate.
  28. Example 1: Hydrate 9.00 grams of an unknown FeCl3 hydrate were heated until all the water was driven off, leaving 5.4 g of anhydrous salt. What is the formula of the hydrate? Law of conservation of mass… 9.00 g  5.4 g salt + X g H2O X g H2O = 3.6 grams
  29. Convert grams to moles Convert grams of salt (FeCl3) to moles Convert grams of water to moles
  30. Find the ratio of moles of water/moles of salt
  31. The formula of the hydrate is…
  32. Hydrates, Example 2 What is the formula of an unknown CuBr2 hydrate that decomposes into 5.67 g of anhydrous salt and 1.83 g of water?
  33. Answer: CuBr2•4H2O Convert grams to moles for each
  34. Practice 10-16 A mass of 2.50 g of blue hydrate is placed in a crucible and heated. After heating, 1.59 g of white anhydrous copper(II) sulfate remains. What is the formula for the hydrate? Name the hydrate. (Hint – calculate grams of water lost and convert this to moles of water; also calculate grams of copper(II) sulfate and convert this to moles; lastly, make the ratio as on the previous slide)
  35. Practice 10-17 A mass of 5.00 g of a hydrate is placed in a crucible and heated. After heating, 4.26 g of anhydrous barium chloride remains. What is the formula for the hydrate? Name the hydrate.
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