1 / 27

Percentage Composition:

Percentage Composition:. is the percent mass of each element present in a compound. Percent Composition. Can be calculated if given: the chemical formula OR masses of elements in compound. By Chemical Formula. % mass = molar mass of an element X 100%

babette
Télécharger la présentation

Percentage Composition:

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Percentage Composition: is the percent mass of each element present in a compound.

  2. Percent Composition Can be calculated if given: the chemical formula OR masses of elements in compound

  3. By Chemical Formula % mass = molar mass of an element X 100% total molar mass of the compound

  4. Example: What is the % composition of CaCO3? CHEMICAL FORMULA • Step 1: Find the molar mass of CaCO3 : Ca x 1 = 40.1 g/mol • C x 1 = 12.0 g/mol • O x 3 = 48.0 g/mol • CaCO3 = 100.1 g/mol

  5. Step 2: Find the % composition: % Ca = 40.1 g/mol x 100 % = 40.1 % Ca 100.1 g/mol % C = 12.0 g/mol x 100% = 12.0 % C 100.1 g/mol % O = 48.0 g/mol x 100 % = 48.0 % O 100.1 g/mol

  6. Example: Calculate the percent composition of the compounds that is formed from this reaction: 29.0g of Ag combines completely with 4.30g of S. STEP 1: find the total mass of the elements. 29.0g + 4.30g = 33.30g STEP 2: find the % composition Ag = 29.0g x 100% = 87.1% 33.30g S = 4.30g x 100% = 12.9% 33.30g Masses of elements in compound

  7. Try These: • Find the percent composition of KMnO4. • Calculate the % composition of the compound that results from 9.03g Mg reacting completely with 3.48g N.

  8. ANSWERS: • 1) K = 24.7% Mn = 34.8% O = 40.5% 2) Mg = 72.2% N = 27.8%

  9. Do Problem 17 on page 131 • Do Problem 18 & 19 on page 133 CHECK YOUR ANSWERS!

  10. Example 2 How much carbon is present in 15.2 g of carbon dioxide gas? % carbon = 12.0 g/mol x 100 % = 27.3 % 44.0 g/mol Xg carbon = 15.2 g of CO2 X 27.3 g C 100. g CO2 = 4.15 g of Carbon

  11. Percent Composition • Can be used to:  calculate the mass of elements in a compound  determine the empirical formula of a compound  determine the molecular formula of a compound

  12. Empirical Formula • shows the simplest mole ratio of the elements. • CO is a 1:1 ratio of carbon to oxygen • H2O is a 2:1 ratio • CO2 is a 1:2 ratio • Empirical formulas can’t be reduced.

  13. Molecular Formula • shows the actual number of atoms in a molecule. • The molecular formula for hydrogen peroxide is H2O2. Its empirical formula would be HO. • Often the molecular formula is the same as the empirical formula: H2O, CO2

  14. Empirical? • CH4O • yes, cannot be reduced further • C2H6 • no, empirical would be CH3 • C3H10O • yes • C6H6O2 • no. What would empirical be? • C3H3O

  15. Calculating Empirical Formulas • A chemist with an unknown compound can easily figure out its percent composition, but it is much more meaningful to know its formula. • EXAMPLE: What is the empirical formula for a compound that is 25.9% nitrogen and 74.1% oxygen?

  16. Method • Write the mass (g) of each element in the compound. So….we assume that it is a 100g sample: 25.9% N = 25.9g 74.1% O = 74.1g

  17. 2. Convert the mass of each element to moles, by dividing by the molar mass. • N = 25.9g = 1.85 mol 14.0g/mol • O = 74.1g = 4.63 mol 16.0g/mol

  18. Calculate the simplest whole number ratio by dividing the number of moles by the smallest number of moles. 1.85:4.63 = 1 : 2.5 1.85 1.85 (If the result is not within 0.1 of a whole number, multiply all numbers by a whole number) 2 ( 1 : 2.5) = 2 : 5

  19. Write the empirical formula using the numbers you obtained. N2 O5 NOTE: • For inorganic compounds, write the most positive element first. • For organic compounds, write C first, H second and all others alphabetically.

  20. A special present just for you…….. • Page 135, Problems #20 & 21 • Check your answers

  21. Molecular Formula Given the empirical formula and the gram formula mass (gfm) OR Given the percent composition and the gram formula mass (gfm)

  22. Example #1 Calculate the molecular formula for NaO having a gfm of 78g.  Determine the efm (empirical formula mass). NaO = 23.0g + 16.0g = 39.0 • Divide the efm into the gfm. 78.0 = 2 39.0 • This is the conversion factor used to determine the molecular formula. Na2O2

  23. Example #2 Find the molecular formula for a compound having a composition of 58.8% C, 9.8% H and 31.4% O and a gmm of 102g/mol. • Determine the mass of each component. C = 102g/mol x 58.8% = 60.0g/mol H = 102g/mol x 9.8% = 10.0g/mol O = 102g/mol x 31.4% = 32.0g/mol

  24.  convert to moles C = 60.0g/mol = 5 12.0g H = 10.0g/mol = 10 1.0g O = 32.0g/mol = 2 16.0g

  25.  Use moles as subscripts for components of compound C5H10O2 • Check the gmm of this compound…does it equal 102.0g/mol? • 5(12.0) + 10(1.0) + 2(16.0) = 102.0g/mol • YES!

  26. And Now….. • Oh Yeah! And there’s more… • Page 136, Problems #22 & 23

  27. Now Try page 139, #41 44

More Related