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Using Analytical Data

Section 3 Formulas and Percentage Composition. Chapter 7. Using Analytical Data. The percentage composition is the percentage by mass of each element in a compound. Percentage composition helps verify a substance’s identity.

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Using Analytical Data

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  1. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data • The percentage composition is the percentage by mass of each element in a compound. • Percentage composition helps verify a substance’s identity. • Percentage composition also can be used to compare the ratio of masses contributed by the elements in two different substances.

  2. Percentage Composition of Iron Oxides Chapter 7

  3. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data, continued Determining Empirical Formulas • An empirical formula is a chemical formula that shows the simplest ratio for the relative numbers and kinds of atoms in a compound. • An actual formula shows the actual ratio of elements or ions in a single unit of a compound. • For example, the empirical formula for ammonium nitrate is NH2O, while the actual formula is NH4NO2.

  4. Empirical and Actual Formulas Chapter 7

  5. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data, continued Determining Empirical Formulas, continued • You can use the percentage composition for a compound to determine its empirical formula. • Convert the percentage of each element to g. • Convert from g to mol using the molar mass of each element as a conversion factor. • Compare these amounts in mol to find the simplest whole-number ratio among the elements.

  6. Visual Concepts Chapter 7 Percentage Composition Click below to watch the Visual Concept. Visual Concept

  7. Section3 Formulas and Percentage Composition Chapter 7 Determining an Empirical Formula from Percentage Composition Sample Problem G Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.

  8. Section3 Formulas and Percentage Composition Chapter 7 Determining an Empirical Formula from Percentage Composition • Sample Problem G Solution • Assume that you have a 100.0 g sample, and convert the percentages to grams. • for C: 60.0%  100.0 g = 60.0 g C • for H: 13.4%  100.0 g = 13.4 g H • for O: 26.6%  100.0 g = 26.6 g O

  9. Section3 Formulas and Percentage Composition Chapter 7 Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued Convert the mass of each element into the amount in moles, using the reciprocal of the molar mass.

  10. Section3 Formulas and Percentage Composition Chapter 7 Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued The formula can be written as C5H13.3O1.66, but you divide by the smallest subscript to get whole numbers. The empirical formula is C3H8O.

  11. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data, continued Molecular Formulas Are Multiples of Empirical Formulas • The formula for an ionic compound shows the simplest whole-number ratio of the large numbers of ions in a crystal of the compound. • A molecular formula is a whole-number multiple of the empirical formula. • The molar mass of any compound is equal to the molar mass of the empirical formula times a whole number, n.

  12. Visual Concepts Chapter 7 Comparing Molecular and Empirical Formulas Click below to watch the Visual Concept. Visual Concept

  13. Section3 Formulas and Percentage Composition Chapter 7 Determining a Molecular Formula from an Empirical Formula Sample Problem H The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.

  14. Section3 Formulas and Percentage Composition Chapter 7 Determining a Molecular Formula from an Empirical Formula • Sample Problem H Solution • Find the molar mass of the empirical formula P2O5. • 2  molar mass of P = 61.94 g/mol • + 5  molar mass of O = 80.00 g/mol • molar mass of P2O5 = 141.94 g/mol

  15. Section3 Formulas and Percentage Composition Chapter 7 Determining a Molecular Formula from an Empirical Formula Sample Problem H Solution, continued n (empirical formula) = 2 (P2O5) = P4O10

  16. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data, continued Chemical Formulas Can Give Percentage Composition • If you know the chemical formula of any compound, then you can calculate the percentage composition. • From the subscripts, determine the mass contributed by each element and add these to get molar mass. • Divide the mass of each element by the molar mass. • Multiply by 100 to find the percentage composition of that element.

  17. Section3 Formulas and Percentage Composition Chapter 7 Using Analytical Data, continued Chemical Formulas Can Give Percentage Composition • CO and CO2 are both made up of C and O, but they have different percentage compositions.

  18. Section3 Formulas and Percentage Composition Chapter 7 Using a Chemical Formula to Determine Percentage Composition Sample Problem I Calculate the percentage composition of copper(I) sulfide, Cu2S, a copper ore called chalcocite.

  19. Section3 Formulas and Percentage Composition Chapter 7 Using a Chemical Formula to Determine Percentage Composition • Sample Problem I Solution • Find the molar mass of Cu2S. • 2 mol  63.55 g Cu/mol = 127.10 g Cu • + 1 mol  32.07 g S/mol = 32.07 g S • molar mass of Cu2S = 159.17 g/mol

  20. Section3 Formulas and Percentage Composition Chapter 7 Using a Chemical Formula to Determine Percentage Composition Sample Problem I Solution, continued Calculate the fraction that each element contributes to the total mass by substituting the masses into the equations below and rounding correctly. 79.852% Cu

  21. Section3 Formulas and Percentage Composition Chapter 7 Using a Chemical Formula to Determine Percentage Composition Sample Problem I Solution, continued 20.15% S

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