EMPIRICAL FORMULA

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EMPIRICAL FORMULA

• The empirical formula represents the smallest
  ratio of atoms present in a compound.
• The molecular formula gives the total number of
  atoms of each element present in one molecule of
  a compound.
• The empirical formula is the simplest formula and
  the molecular formula is the true formula.

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EMPIRICAL FORMULA

• Assume 100g sample
  Calculate mole ratio
• Use Atomic Masses

Mass of elements
Empirical Formula
Grams of each element
Moles of each element
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EMPIRICAL FORMULA

• Step 1 If given the composition, assume a 100g
  sample then convert to grams.
• Step 2 Use the atomic masses to convert grams to
  moles.
• Step 3 Divide the moles of each element by the
  SMALLEST mole fraction.
• Step 4 The results from step 3 should be a
  whole number, if not, make it so by multiplying
  by a common factor.

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EMPIRICAL FORMULA

• 1. Calculate the empirical formula from a sample
  containing 43.4 Na, 11.3 C, and 45.3 O.
• 
  smallest
• 43.4 ? 43.4 g Na (1 mole / 23 g/mol) 1.887
  moles Na
• 11.3 ? 11.3 g C (1 mole / 12 g/mol) 0.9417
  moles C
• 45.3 ? 45.3 g O (1 mole / 16 g/mol) 2.831
  moles O
• 1.887/0.9417 2.00 Na
• 2.831/0.9417 3.00 O .
• 9417/.9417 1.00 C
• Empirical Formula Na2CO3

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EMPIRICAL FORMULA

• 2. When 8.00 g of calcium metal is heated in
  air, 11.20 g of metal oxide is formed. Calculate
  the empirical formula.
• According to the Law of Conservation of mass,
• 11.20 g Product - 8.00 g Ca 3.20 g Oxygen
  (reactive part of air)
• smallest
• 8.00 g Ca (1 mole / 40 g/mol) 0.200 moles Ca
• 3.20 g O (1 mole / 16 g/mol) 0.200 moles O
• 
  0.200 / 0.200 1
• Empirical Formula CaO

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EMPIRICAL FORMULA

• 3. A compound was found to have a composition
  of 33.0 Sr, 26.8 Cl, and 40.2 water.
  Calculate the empirical formula of this hydrate.
• 
  smallest
• 33.0 ? 33.0 g Sr (1 mole/87.6 g/mol) 0.3767
  moles Sr
• 26.8 ? 26.8 g Cl (1 mole/35.45 g/mol) 0.7560
  moles Cl
• 40.2 ? 40.2 g H2O (1 mole/18.0g/mol) 2.233
  moles H2O
• 0.7560 / 0.3767 2 Cl
  2.233 / 0.3767 5.9 6 H2O
• Empirical Formula SrCl2 . 6 H2O

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EMPIRICAL FORMULA Molecular Formula

• 4. Propylene contains 14.3 H, 85.7 C, and has
  a molar mass of 42.0 g/mol. What is its
  molecular formula?
  smallest
• 14.3 ? 14.3 g H (1 mole/1.01 g/mol) 14.19
  moles H
• 85.7 ? 85.7 g C (1 mole/12.01 g/mol) 7.142
  moles C
• 14.19 / 7.142 1.987 2 H
• Empirical Formula CH2
• Molar mass / empirical mass multipier
• (42.0 g/mol / 14.0 g/mol) 3
• 3 x CH2 becomes the molecular formula
  ? C3H6

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PRACTICE PROBLEM 12
A

• ______ 1. Which contains the larger number of
  MOLES of atoms?
• a) 125.0 g KCl b) 25.0 g CaSO4 c)
  17.0 g of N2
• ______ 2. What is the empirical formula of
  the compound whose composition is 39.7 K, 27.8
  Mn, and 32.5 O?
• ______ 3. Determine the empirical formula of a
  compound that contains 89.7 bismuth and 10.3
  oxygen.
• ______ 4. Write the molecular formula for a
  compound that contains 54.5 C, 9.1 H, and 36.4
  O and has a molar mass of 132 amu?

K2MnO4
Bi2O3
C6H12O3
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GROUP STUDY PROBLEM 12

• ______ 1. Which contains the larger number of
  MOLES of atoms?
• a) 125.0 g HBr b) 25.0 g C6H11O6 c)
  17.0 g of Br2
• ______ 2. A sample of a compound weighing
  4.18 g contains 1.67 g of sulfur and the rest is
  oxygen. What is the empirical formula?
• ______ 3. What is the empirical formula of the
  compound whose composition is 28.7 K, 1.4 H,
  22.8 P, and 47.1 O?
• ______ 4. A compound contains 92.3 C and 7.7
  H and has a molar mass of 78.0 g/mol. Determine
  the molecular formula.