John A. Schreifels

1
Chapter 3

• Calculations involving Chemical Formulae and
  Equations

2
Contents

• Mass and Moles of a Substance
• Molecular weight
• Moles
• Determining Chemical Formulae
• Mass from formula
• Elemental Analysis gives C, H, O
• Determining formula from elemental analysis
• Stoichiometry
• Amounts of substances in a reaction
• Limiting reagents

3
Atomic and Molecular Weights

• Molecular Weight (Formula weight) Summed masses
  of all atoms in the formula unit of the compound.
• How?
• For the general formula AaBbCcDd...
• fm formula mass
• a?MA b?MB c?MC d?MD ...
• E.g. determine the formula mass (formula weight)
  of the following
• NaOH, H2O, CaCO3, C2H6O
• Percentage composition from formula (mass
  contribution of each element to the total mass of
  the compound). For AaBbCc
• E.g. determine the mass composition of each
  element in NaOH, H2O, CaCO3, C2H6O

4
The Mole

• Since individual atoms are very small, they weigh
  very little. It is not convenient deal with
  masses that small, since we can typically measure
  gram amounts in the lab.
• Mole Number of atoms (or molecules) as there
  are in 12.00 g Carbon-12 1 mol 6.02x1023 and
  is called Avogadro's number.
• Mole is used to indicate a certain number of
  particles (like a dozen or bushel might be used).
• 1 mol of
• water occupies approximately 18 mL and has
  6.02x1023 molecules.
• gold occupies approximately 10 mL and has
  6.02x1023 atoms.

5
Molar mass

• 1 mol FM 6.02x1023 particles
• The definition of a mol allows us to perform a
  variety of conversions.
• Let n mol, n atoms (molecules), m mass
  and FM formula mass then we can convert to
• m from n, FM
• E.g. Determine the mass of NaCl needed to have
  5.0 mol of it.
• n from m, FM
• E.g. Determine the number of mol of NaCl in 15.00
  g of it.
• n from n
• E.g. Determine the number of molecules in 3.222
  mol of NaCl
• n from m, FM
• E.g. Determine the number of atoms in 4.32 g of
  NaCl
• m from n, FM
• E.g. Determine the mass of one formula unit of
  NaCl.

6
EMPIRICAL FORMULA

• Empirical Formula Simplest formula where all
  coefficients are integers.
• In earlier example Fe2O3, Fe4O6 Fe6O9 Fe8O12
  possible formulas for iron oxide. Its empirical
  formula Fe2O3.
• Often obtained from composition data.
• Assume 100 g of compound-
• Determine mol of each element.
• Divide by smallest of moles.
• Multiply by smallest whole number to make all
  coefficients whole numbers.
• E.g. Determine the empirical formula of a
  compound with the following compositions
• MassO 34.7
• MassC 52.1
• MassH 13.1

7
COMBUSTION ANALYSIS

• Experimental mass also determined. For organic
  compounds the sample is combusted. Analyzes the
  amount of C,H, and O in compounds.
• C converted to CO2 and mass measured.
• H converted to H2O and mass measured
• E.g. Combustion analysis of a 1.621 g sample of
  ethanol, which contains only C,H,O, was
  performed. Calculate the mass composition of
  each element in ethanol.
• Results Mass CO2 3.095 g
• Mass of H2O 1.902 g.

8
MOLECULAR FORMULA

• Measure molecular mass
• Multiply empirical mass by integer to obtain
  molecular mass.
• Multiply all the co-efficients by this integer.
• E.g. Determine the molecular mass of a compound
  with the empirical formula NO2 and a molecular
  mass of 92.00 g/mol.

9
STOICHIOMETRY

• The relative amounts of reactants and products in
  a reaction are given by the ratio of
  stoichiometric coefficients. (Conservation of
  mass).
• E.g. For the reaction
• 2Na(s) Cl2(g) ? 2NaCl(s)
• 2 mol Na 1 mol of Cl2 2 mol of NaCl.
• E.g. 2 Determine mol of Cl2 needed to react
  with 4.2 mol of Na. How many mol of NaCl will
  form?
• Mol of Cl2
• Mol NaCl
• Summary For aA bB ? cC

10
STOICHIOMETRY2

• Mass of one reactant can be related to the mass
  of another reactant or product. (Law of definite
  proportions).
• E.g. Determine the mass of Na needed to react
  with 34.45 g of Cl2 and the maximum mass of NaCl
  that could be produced.
• Write reaction and express the moles of one
  compared to the moles of the other.
• Substitute for mol in terms of mass and formula
  mass in each part.
• Let n mol FM formula mass then
• E.g. 1 Determine the mass of oxygen consumed
  when it reacts with 10 g CH3CHO.
• E.g. 2. Calculate the mass of oxygen needed to
  react with 100 g Al to for Al2O3.

11
LIMITING REAGENTS

• Limiting reagent the reactant which limits the
  maximum amount of product that can be produced.
  It will be completely consumed in the reaction
  before any other reactants.
• Limiting reagent must be known before theoretical
  yield can be determined.
• E.g. 1 Determine limiting reagent if 3.00 moles
  of Al react with 2.15 moles of O2 to form Al2O3.
• Strategy
• Determine mol of Al2O3 that could be produced
  from Al
• Determine mol of Al2O3 that could be produced
  from O2
• Reactant producing smallest amount of Al2O3 is
  limiting reagent.
• E.g.2 Calculate the theoretical yield when 20 g
  Al react with 25 g O2.
• Strategy
• Determine expected yield from each reactant.
• Compare. If mass of Al2O3 from produced from Al
  is less than from O2, then Al is limiting.
  Otherwise O2 is limiting.

12
YIELDS IN CHEMICAL REACTIONS

• Theoretical yield the maximum amount of product
  that can be obtained from given amounts of
  reactants.
• Actual Yield the actual amount of product
  obtained from a reaction.
• Theoretical yield calculated from amount of
  reactants and stoichiometry
• Actual yield is the amount of product actually
  obtained.
• yield is the yield as a percent of the
  theoretical yield
• E.g. The reaction for the synthesis of acetic
  acid from methanol is shown below. What is the
  yield if 15.0 g of methanol were combined with a
  stoichiometric amount of carbon monoxide to
  form19.1 g of product?
• CH3OH(l) CO(g) ? HC2H3O2(l)

13
Summary

• Moles are used to describe measurable quantities
  of a substance (1 mol 6.02x1023 particles).
• Reactions occur in well defined proportions and
  are represented by balanced chemical equations.
• The ratio of stoichiometric coefficients tells
  how much of one compound will react if we know
  the amount of the other
• aA bB ? cC
• Limiting reagent is one that limits the amount of
  product. Use it to determine the theoretical
  yield.
• Empirical formula simplest formula all
  integers.
• Molecular formula actual formula of a compound.