An Example for Determining the Empirical Formula

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An Example for Determining the Empirical Formula
Empirical formula is Na2Cr2O7
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Calculating the Mass of an Element in a Given
Mass of a Compound
Calculate the mass of carbon in 83.5 g of
formaldehyde (CH2O)
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Mass Percentage (Percentage Composition) from the
Molecular Formula
Calculate the percentage composition in
formaldehyde (CH2O)
always check that sum of percentage 100
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Empirical Formula from Composition
A substance give the following mass 17.5 Na,
39.7 Cr, 42.8 O What is the empirical formula?
If we have mass , assume that the total mass is
100 g.
Then mNa 17.5 g mCr 39.7 g mO 42.8 g
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Empirical Formula
Empirical formula is Na2Cr2O7
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Elemental Analysis
Calculating the Percentage Masses of Elements
We want to calculate the following C, H, O
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1. Mass Ratio in one Mole
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2. Mass
mass O in sample 100 (39.96.72) 54.4
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Empirical Formula
Assume that the total mass is 100g
Empirical formula is CH2O
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Molecular Formula from Empirical Formula
In the previous example, If we know that the MW
is 60.0 amu calculate the molecular formula
The sample is acetic acid
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Stoichiometry Quantitative Relations in Chemical
Reactions
Stoichiometry is the calculation of the
quantities of reactants and products involved in
a chemical reaction.
Stoichiometry is based on the following 1. The
balanced chemical equation. 2. The relationship
between mass and moles.
Such calculations are fundamental to most
quantitative work in chemistry
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Molar Interpretation of a Balanced Chemical
Equation and the Law of Conservation of Mass
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An Example for Balancing Chemical
Equations Combustion of Methanol
Balance the equation
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Calculation of Number of Moles of Products from
Number of Moles of ReactantsMethod I

• Suppose we wished to determine the number of
  moles of NH3 we could obtain from 4.8 mol H2.

• Because the coefficients in the balanced equation
  represent mole-to-mole ratios, the calculation is
  simple.

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Calculation of Number of Moles of Reactants from
Number of Moles of ProductsMethod II
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Calculation of Masses of Reactants from Masses of
Products
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Calculation of Masses of Products from Masses of
Reactants
5.0 g
? g
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Limiting Reactant (LR)
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Limiting Reactant (or Limiting Reagent) and the
Theoretical Yield
The limiting reactant (or limiting reagent) is
the reactant that is entirely consumed when the
reaction goes to completion.
The limiting reactant ultimately determines how
much product can be obtained.
The theoretical yield of product is the maximum
amount of product that can be obtained from The
limiting reactant.
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An Example on the Limiting Reactant and the
Theoretical Yield
What is the LR? What is the theoretical yield
of H2(g)
0.30 mol
0.52 mol
Zn is the excess reactant
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Example 2 Combustion of Methanol
Consider combustion of 10.0 ml of methanol in
20.0L of air. What it is the limiting reactant
(LR)? Which reactant remains in excess? What is
the theoretical yield of water? How much
reactant remains in excess?
The density of methanol is 0.791 g/ml, the
density of O2 is 1.31 g/L.
Answer work it out on the board
Download answer (PDF)
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Operational Skills

• Calculating the formula weight from a formula or
  model.
• Calculating the mass of an atom or molecule.
• Converting moles of substance to grams and vice
  versa.
• Calculating the number of molecules in a given
  mass.
• Calculating the percentage composition from the
  formula.
• Calculating the mass of an element in a given
  mass of compound.

• Calculating the percentages C and H by
  combustion.
• Determining the empirical formula from percentage
  composition.
• Determining the molecular formula from percentage
  composition and molecular weight.
• Relating quantities in a chemical equation.
• Calculating with a limiting reactant.