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Chemical Formulas and the Mole

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Chemical Formulas and the Mole 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 ... – PowerPoint PPT presentation

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Title: Chemical Formulas and the Mole


1
Chemical Formulas and the Mole
2
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?

3
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?

4
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.

5
Practice Problems
  • Write the empirical formula for each of the
    following molecular formulas.
  • C6H12O6
  • H2O2
  • C4H10
  • H2O
  • N2O4
  • C3H6O2

6
Determining Empirical Formulas
  • If composition can be determined from the
    formula, then the formula can be determined from
    the composition of the compound.

7
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.

8
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

9
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.

10
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
11
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
12
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.

13
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?

14
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.

15
Example
  • An opal is hydrated silicon dioxide (SiO2)

  • The presence of water and

  • various mineral impurities

  • accounts for the variety of

  • colors.

16
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.

17
Examples of Hydrates
The hydrate CoCl2 6 H2O
Anhydrous (without water) CoCl2
18
Hydrating CuSO4
Anhydrous CuSO4
Hydrated CuSO4 5H2O
19
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)

20
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

21
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

22
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
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