Chemical Formulas and the Mole

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Chemical Formulas and the Mole
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Introduction

• 84 potatoes
• 63 carrots
• 15 onions
• 3 heads of garlic
• 27 turnips
• 42 pieces of celery
• 9 cans of green beans
• 6 cans of tomato puree
• 6 cans of diced peppers
• 9 cans of corn
• 6 cans of lima beans
• Serves 81

• If only 1 head of garlic is available, how many
  of each ingredient will be needed to adjust the
  recipe in order to make the stew correctly and
  how many people will it feed?
• If you wanted to feed 324 people, how much of
  each ingredient would you need?

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How are formulas represented?

• Calculate the composition of the formula you
  were given.
• composition comes from experimental analysis
  and provides information about the make up of the
  compound.
• Compare your calculations with the students
  around you having a different colored card.
• What did you find regarding your compositions?
• Although the formulas are different, are there
  any similarities among your formulas that could
  account for the similarities of your
  calculations?

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Empirical Formulas

• The molecular formula of a compound gives the
  actual number of atoms of each element making up
  a compound.
• (The molecular formula is the formula that is
  written on your card)
• The empirical formula of a compound is the
  formula with the smallest whole number ratio of
  the elements making up the compound.
• What is the empirical formula for the compound
  written on your card?
• The empirical formula may or may not be the same
  as the actual molecular formula.
• If the two formulas are different, the molecular
  formula will always be a simple multiple of the
  empirical formula.

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Practice Problems

• Write the empirical formula for each of the
  following molecular formulas.
• C6H12O6
• H2O2
• C4H10
• H2O
• N2O4
• C3H6O2

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Determining Empirical Formulas

• If composition can be determined from the
  formula, then the formula can be determined from
  the composition of the compound.

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Rules for determining Empirical Formulas

• Assume a 100 g sample, so becomes g.
• Convert grams to moles by dividing by the atomic
  mass of each element.
• Divide each result (mole) by the smallest result
  present (mole ratio).
• Look for whole number ratios.
• The whole number ratios become the subscripts for
  the formula.

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Rhyme for Remembering Rules

• Percent to massMass to moleDivide by
  smallMultiply 'til whole
• A Simple Rhyme for a Simple Formulaby Joel S.
  ThompsonJournal of Chemical EducationVol. 65,
  No. 8 August 1988, p. 704

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Sample Problems

• Determine the empirical formula for a compound
  that is 27.3 carbon and 72.7 oxygen.
• Determine the empirical formula for a compound
  that is 56.6 K, 8.7 C, and
• 34.7 O.
• Determine the empirical formula for a compound
  that is 69.9 Fe and 30.1 O.

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Multiples for use when ratio is not initially a
whole number
Decimal Multiplier
0.5 X 2
0.30-.35 X 3
0.63 - .67 X 3
0.22-0.25 X 4
0.72-0.75 X 4
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Molecular Formulas

• Molecular formulas are multiples of empirical
  formulas.

Compound Empirical Formula Molar Mass Molecular Formula
Formaldehyde CH2O 30 CH2O
Acetic acid CH2O 60 C2H4O2
Glucose CH2O 180 C6H12O6
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Sample Problems

• The empirical formula for a compound containing
  phosphorus and oxygen was found to be P2O5.
  Experiments show that the molar mass of the
  compound is 284 g/mol. What is the molecular
  formula and name of the compound?
• Determine the molecular formula of a compound
  having an empirical formula of CH and a molar
  mass of 78.11 g/mol.

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Putting it All Together

• A compound with a molar mass of 92 g/mol contains
  0.608 g of nitrogen and 1.388 g of oxygen. What
  is the empirical and molecular formula of the
  compound?
• A compound with a molar mass of 86 g/mol contains
  83.62 C and 16.38 H. What is the molecular
  formula of the compound?

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Hydrates

• Hydrates are solid ionic compounds in which water
  molecules are trapped.
• Some products, such as electronic equipment, are
  boxed with small packets labeled dessicant. These
  packets control moisture by absorbing water.
  Some contain ionic compounds called hydrates.
• Each hydrate has a specific number of water
  molecules bound to its atoms.

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Example

• An opal is hydrated silicon dioxide (SiO2)
• 
  The presence of water and
• 
  various mineral impurities
• 
  accounts for the variety of
• 
  colors.

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Naming Hydrates

• In the formula for a hydrate, the number of water
  molecules associated with each formula unit of
  the compound is written following a dot.
• For example Na2CO3 10 H2O
• This compound is called sodium carbonate
  decahydrate.
• The prefix deca- means ten and the root word
  hydrate refers to water.
• When naming hydrates use the same prefixes for
  the water as used with covalent molecules.
• A decahydrate has ten water molecules associated
  with each formula unit of compound.
• The number of water molecules associated with
  hydrates varies widely.

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Examples of Hydrates
The hydrate CoCl2 6 H2O
Anhydrous (without water) CoCl2
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Hydrating CuSO4
Anhydrous CuSO4
Hydrated CuSO4 5H2O
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Calculating the Formula Mass of Hydrates

• Na2CO3 10 H2O
• Na 2 x 23 46
• C 1 x 12 12
• O 3 x 16 48
• H2O 10 x 18 180
• (H 20 x 1 20 O 10 x 16 160 20 160
  180)
• Total 286 g/mol
• Reminder-the dot in the formula means to add
  the mass of the water (not multiply as in math)

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Practice Problem

• Calculate the formula mass of the following
• 1. CuSO4 5 H2O
• Answer 249.5 g/mol
• BaCl2 2 H2O
• Answer 244 g/mol

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Determining the Formula of Hydrates from
Experimental Data

• The composition of a hydrate is determined to
  contain 48.8 MgSO4 and 51.2 H2O.
• What is the formula of the hydrate?
• 48.8/120 (mass of MgSO4) 0.407 moles MgSO4
• 51.2/18 (mass of H2O) 2.84 moles H2O
• 0.407/0.407 1
• 2.84/0.407 7
• Formula is MgSO4 7 H2O

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Lets Try Another

• A 1.628 g sample of a sample of a hydrate of
  magnesium iodide (MgI2) heated until its mass is
  reduced to 1.072 g and all water has been
  removed. What is the formula of the hydrate?
• Answer
• 1.072/2780.00386 moles MgI2
• 1.628-1.072 0.556 g water/18 0.0309 moles H2O
• 0.00386/0.00386 1
• 0.0309/0.00386 8
• Formula MgI2 8 H2O