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Chapter 11 Notes IV

Chapter 11 Notes IV. Empirical/Molecular Formula. Empirical Formula. The empirical formula gives you the lowest, whole number ratio of elements in the compound. The empirical formula may or may not be the same as the molecular formula. For example:.

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Chapter 11 Notes IV

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  1. Chapter 11 Notes IV Empirical/Molecular Formula

  2. Empirical Formula • The empirical formula gives you the lowest, whole number ratio of elements in the compound. • The empirical formula may or may not be the same as the molecular formula.

  3. For example: • For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms. • The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.

  4. What is the empirical formula for: • C6H12O6 • C6H12O2 • N2H2 • CH4

  5. Obj. 10-11…Empirical Formulas • empirical formulas are the lowest whole # ratio of a compound. • i.e. the empirical formula for glucose (C6H12O6) would be… CH2O • to calculate from % composition… 1. ÷ given % by atomic mass (to find moles) 2.÷ each mole in step 1 by lowest mole value (to find ratio) 3. values from step 2 = subscripts ~subscripts MUST be a whole # (multiply if they are not!)

  6. To Find the Empirical Formula from % Composition: • If given the percentages, assume there are 100.0 grams of the compound. • Convert the grams of each element to moles. • Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.

  7. Obj. 10-11 cont… • practice… 1.What is the empirical formula of a compound that contains 36.5% sodium, 25.4% sulfur and 38.1% oxygen? Na=36.5% =1.59 moles =2 23 0.794 S=25.4% =0.794 moles *** Na SO =1 subscripts 32 3 2 0.794 O=38.1% =2.38 moles =3 16 0.794

  8. Obj. 10-11 cont… • more practice… 2.What is the empirical formula of a compound that contains 72.4% iron (Fe) and 27.6% oxygen? Fe=72.4% =1.293 moles *** =1 x 3 = 3 56 1.293 O=27.6% =1.725 moles x 3 = 4 =1.33 16 1.293 MUST be a whole #!!! Fe O 4 3

  9. Two Examples: • What is the empirical formula of a compound that is 27.3% carbon and 72.7% oxygen? • What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

  10. More to try… • Calculate the empirical formula of a compound that is 94.1% oxygen, 5.9% hydrogen. • Calculate the empirical formula of a compound that is 79.8% carbon, 20.2% hydrogen.

  11. From there, find it’s molecular formula: • If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula. • Take the compound’s empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formula’s mass.

  12. Example: • If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?

  13. Try these: • What is the molecular formula of a compound whose molar mass is 60.0 g and whose empirical formula is CH4N? • What is the molecular formula for a compound whose molar mass is 78 g and whose empirical formula is CH?

  14. Obj. 10-11 cont… • calculate hydrated compounds (bonded to water) same way… ~use ‘’ to separate water from crystal ~since water is a compound, you are finding the coefficient… NOT the subscript! • practice… What is the formula of a compound that contains 18.28% Ca, 32.36% Cl and 49.36% H2O? Ca=18.28% =0.457 moles *** =1 40 0.457 Cl=32.36% CaCl  H2O 6 =0.925 moles 2 =2 35 0.457 H2O=49.36% =2.742 moles =6 18 0.457

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