Stoichiometry

1
Stoichiometry

• Calculations with Chemical Formulas and Equations

2
Molecular Formulas

• Analysis will give you empirical formulas
• Use the molecular mass to get the molecular
  formula
• Divide molecular mass by empirical mass to get
  factor to multiply

3
Combustion Analysis

• Use combustion apparatus to combust a hydrocarbon
  (in theory)
• Products are CO2 and H2O
• H and C come completely from the sample.
• Determine this by 5 of each in the two products.
• Subtract from sample mass to get original mass of
  O
• Determine empirical formula as before.

4
Meaning of an equation

• Consider O2 (g) 2NO(g) ? 2NO2 (g)
• What is (g)?
• What is conserved?
• What can coefficients stand for?

5
Kinds of Reactions

• Combination or synthesis
• A B ? AB
• 2H2 O2 ? 2H2O
• Decomposition
• AB ? A B
• 2H2O ? 2H2 O2
• Which usually takes energy?

6
Combustion

• Rapid reaction that produces a flame.
• In air, oxygen is a reactant
• Hydrocarbons are the most common fuel (CH4, C3H8
  etc.)
• These will produce H2O and CO2 in the presence of
  ample oxygen
• What happens when oxygen becomes limited?

7
Formula Weights

• Try C6H12O6

8
Percent Composition

• What is the percent of each element in the
  glucose molecule?

9
The Mole

• Avogadros Number 6.02 X 1023
• Mass in grams of relative mass from the periodic
  table
• What is molar mass of glucose?

10
Conversions

• Moles in the middle!
• How many moles in 23 grams of glucose?
• What is the mass of 2 million glucose molecules?
• How many carbon atoms in 2 grams of glucose?

11
Empirical Formula from Analysis (usually percent
composition)

• Find grams of each element in 100-g sample
• Convert to moles of each element
• Divide each by smallest mole amount
• Express as a whole number ratio
• Try PE on p.93

12
Combustion Analysis

• Hydocarbon is burned in apparatus
• H2O and CO2 are produced
• Trace origins of each element
• H and C can be determined from these products
• O is found from mass left in original sample

13
Combustion Apparatus
Try PE (a) on page 95
14
Stoichiometric Relationships

• C6H12O6(s) 6O2(g) ? 6H2O(l)
• 
  6CO2(g)
• How many grams of water are produced from the
  oxidation of 1 gram of glucose?

15
Limiting Reagent

• Problem gives amounts of two reactants
• Find a product with each reactant
• Smallest amount of product produced from the
  limiting reagent
• This determines the product formed
• PE page 102

16
Percent Yield

• Shows how good your lab technique is
• Actual yield is what you get in the lab
• Theoretical yield is the calculated yield from
  stoichiometry
• Percent yield (actual yield/theoretical yield)
  X 100
• PE page 103