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Empirical Formula and Molecular Formula

Empirical Formula and Molecular Formula. For Ionic cmpds we can predict formulas based on common ion charges Sodium + Oxygen together Na + O 2-  Na 2 O For molecular cmpds, charges are not involved, and this makes it harder to predict. If you know the % comp it can be done!!.

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Empirical Formula and Molecular Formula

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  1. Empirical Formula and Molecular Formula

  2. For Ionic cmpds we can predict formulas based on common ion charges Sodium + Oxygen together Na+ O2- Na2O • For molecular cmpds, charges are not involved, and this makes it harder to predict. If you know the % comp it can be done!!

  3. Empirical Formula • Smallest whole number ratio of elements in a cmpd. (not necessarily the real formula!!)

  4. Can be same! Real Formula E.F. H2O H2O C6H12O6 CH2O

  5. Steps • Change % to grams • Change grams to mole • Divide all by smallest number of moles • Use answers from step 3 as subscripts

  6. What is the empirical formula for a compound that is 79.9% C and 20.1% H 1) 79.9%  79.9g +20.1%  +20.1g 100% 100.0g 2) 3) 79.9g x 1 mol = 6.66mol C  6.66 = 1 12gC 20.1 g x 1 mol = 20.1 mol H 6.66= 3 1gH 4) CH3

  7. Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN

  8. Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN 54.5gC x 1molC = 4.54molC 12g 13.6gH x 1 mol H = 13.6molH 1g 31.8gN x 1mol N = 2.27mol N 14g 2.27 =2 2.27 =6 2.27 =1 C2H6N

  9. Molecular formula This formula has the actual number of atoms in the cmpd. (the EF could be the same or have less atoms)

  10. Steps • Find molar mass of the E. F. • Divide M.F.’s molar mass E. F. molar mass 3) If greater than 1, multiply all the subscripts in the E.F. by your answer in # 2 Given in problem

  11. Ex. What is the M.F. if: NO2 = E.F Molar mass is 92g/mol

  12. M.F.’s molar massE. F. molar mass 92 (MF) (14+2(16)= 46 (EF) =2 NO2 becomes N2O4

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