CH 3: Stoichiometry

1
CH 3 Stoichiometry

• Moles

2
Molar Mass

• Molar mass mass in grams of one mole of a
  substance
• Aka molecular weight
• Calculate molar mass by summing the masses of the
  component atoms.

3
Molar Mass Related Calculations

• Molar mass of a substance.
• Moles present in a given mass of substance.
• Mass of a given number of moles of a substance.
• Number of particles in a given mass or a given
  number of moles of a substance.

4

• What is the molar mass of glucose. The formula
  for glucose is C6H12O6.
• What is the mass of 0.023 moles of glucose?

5

• How many glucose molecules are present in 0.023
  moles of the compound?
• How many moles of glucose are present in 3.5 x
  10-3 grams of the substance?

6
Mass Percent

• Compounds are typically described by either their
  chemical formula or their percent by mass of the
  component elements.
• Mass X n (molar mass X) x 100
• molar mass of compound

7
Mass Percent Calculations

• Calculate the mass percent of each element in
  C6H12O6
• Assume one mole of the substance.

8
Empirical and Molecular Formulas

• Molecular formula ratio of atoms in a molecule
• Empirical formula simplest ratio of elements in
  a molecule
• Glucose
• Molecular formula C6H12O6
• Empirical Formula
• Molecular formula n (empirical formula)

9
Mass ? Empirical Formula

• Given mass percent data
• Calculate mass in grams of each element in 100
  g of compound.
• Convert each mass into moles of the element.
• Divide each molar answer by the smallest of the
  values.
• If the numbers obtained in step 3 are not whole
  numbers, multiply all by an integer so the
  results are whole numbers
• Whole numbers obtained in step ¾ are the
  subscripts in the empirical formula.

10
Empirical Formula ? Molecular Formula

• Molar mass needed
• Calculate mass of one mole of the empirical
  formula.
• Molar Mass
  integer (n)
• empirical formula weight
• (n)(empirical formula) molecular formula

11
Caffeine

• Molar mass 194 g/mol
• 49.49 C
• 5.19 H
• 28.85 N
• 16.48 O
• What is the empirical and molecular formula of
  caffeine?

12
Acetaminophen (Tylenol)

• Molar mass 151 g/mol
• 63.56 C
• 6.00 H
• 9.27 N
• 21.17 O
• What is the empirical and molecular formula of
  acetaminophen?

13
72 page 120

• Urea Molar Mass 60 g/mol (I looked this up
  not in question)
• 1.121 g N
• 0.161 g H
• 0.480 g C
• 0.640 g O
• What is the empirical and molecular formula of
  urea?

14
3.7/3.8 Chemical Reactions

• Writing chemical reactions
• Write the correct chemical formula for each
  reactant and product.
• Use a subscript after each formula to indicate
  the state of each substance
• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous (dissolved in water)

15

• Chemical equations must obey the law of
  conservation of matter balancing does this.
• Balance the equation by adding coefficients in
  front of reactants and products as needed
• DO NOT CHANGE THE FORMULAS FOR THE COMPOUNDS

16

• One of my favorite chemical reactions
• Mg(s) HCl(aq) ? MgCl2 (aq) H2
  (g)
• As written the reaction does not obey the law of
  conservation of matter reaction needs to be
  balanced

17

• Mg(s) 2 HCl(aq) ? MgCl2 (aq) H2
  (g)
• Now the reaction is balanced!

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Balance the Reactions

• N2 (g) H2 (g) ? NH3 (g)
• Na(s) Cl2 (g) ? NaCl(s)
• Pb(NO3)2(aq) NaCl(aq) ? PbCl2(s) NaNO3(aq)

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Meaning of the Balanced Reaction

• Atomic level
• Molar level
• Stoichiometry - relationship between moles, the
  balanced reaction and mass
• Page 104 outlines the needed steps

20

• 2 Na Cl2 ? 2 NaCl
• 2 mol Na 1 mol Cl2 ? 2 mol NaCl

21
Cookie example!

• 2 eggs 1 bag chips ? 50 cookies
• This example was much more enjoyable in person!

22
Molar Ratios

• Balanced reactions lead to molar ratios
• N2 3 H2 ? 2 NH3

23
N2 (g) 3 H2 (g) ? 2 NH3 (g)

• If 3.5 grams of N2 to react, how many moles and
  how many grams of NH3 would be made?

24
N2 (g) 3 H2 (g) ? 2 NH3 (g)

• How many grams of H2 are needed to make 37.8
  grams of NH3?

25
2 LiOH CO2 ? Li2CO3 H2O

• How many grams of LiOH are needed to react 750.
  grams of CO2?
• How many grams of Li2CO3 will be made if 750.
  grams of CO2 react?