Mass Calculations and Stoichiometry

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Mass Calculations and Stoichiometry

• Sections 9.3-9.5

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Review

• N2 (g) H2 (g) ? NH3 (g)
• Balance the reaction
• How many moles of NH3 in the reaction?
• If I said there were 1.3 moles of H2, how many
  moles of NH3 would there be?

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Stoichiometry

• The process of using a chemical equation to
  calculate the relative masses of the reactants
  and product involved in the reaction is called
  Stoichiometry.
• The word stoichiometry derives from two Greek
  words stoicheion (meaning "element") and metron
  (meaning "measure").
• Jeremias Benjamin Richter (1762-1807) was the
  first to lay down the principles of
  stoichiometry. In 1792 he wrote
• "Die stöchyometrie (Stöchyometria) ist die
  Wissenschaft die quantitativen oder
  Massenverhältnisse zu messen, in welchen die
  chymischen Elemente gegen einander stehen."
• Stoichiometry is the science of measuring the
  quantitative proportions or mass ratios in which
  chemical elements stand to one another.

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Stoichiometric Calculations

• So lets try one
• H2O (l) ? H2 (g) O2 (g)
• Balance the reaction
• How many moles do I have of H2O?
• How many molecules do I have of H2O?
• How many grams do I have of H2O?
• What about H2? ( of molecules and grams)
• What about O2? ( of molecules and grams)

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Stoichiometric Calculations cont

• Next, what do we know?
• 2 moles of H2O will yield 2 moles of H2 and yield
  1 mole of O2.
• Last, make the necessary conversions
• 2 moles H2O x 6.022 x 1023 1.204 x 1024
  molecules H2O
• 2 moles of H2 x 6.022 x 1023 1.204 x 1024
  molecules H2
• 1 mole of O2 x 6.022 x 1023 6.022 x 1023
  molecules O2.
• 2 moles H2O x 18.02 amu 36.04 grams H2O
• 2 moles H2 x 2.016 amu 4.032 grams H2
• 1 mole O2 x 32.00 amu 32.00 grams O2

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Stoichiometric Calculations cont

• You can also use this to find out how much matter
  (mass) can be used or produced!!!
• Al I2 ? AlI3
• Calculate the mass of I2 needed to react with 35
  grams of Al
• How do you start this problem?
• Balance the equation!
• Remember that Stoich is MOLE to MOLE ratio. (Not
  Mass to Mass)
• So we need to convert the mass to moles!!!
• 35 grams of Al ? MOLES
• 35 26.98 1.3 moles of Al
• So 2 moles of Al 3 moles of I2
• So we can set up a conversion factor or cross
  multiply
• 2 moles Al 1.3 moles Al
• 3 moles I2 x moles I2
• X 1.95 moles I2
• 1.95 x 253.8 495 grams I2

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Stoichiometric Calculations cont

• The RULES
• Always start with what you know.
• Next balance the equation
• Then find out what you want to calculate
• ExIf you are looking for Grams then you will
  have to convert to MOLES
• Use the MOLE RATIOS to find the desired reactant
  or product
• Convert from moles back to mass

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Stoichiometric Calculations cont

• Example 1
• NaOH CO2 ? Na2CO3 H2O
• If there are 10.0 grams of NaOH, how many grams
  of CO2 are required?
• Go through the steps
• 1. We know that we have 10.0 grams NaOH
• 2. Balance the equation
• 2NaOH CO2 ? Na2CO3 H2O
• 3. Were looking for mass of CO2 in grams
• 4. Convert 10.0 grams NaOH to moles.
• 0.250 moles of NaOH
• 5. Use Mole Ratios. 0.250 moles NaOH is 0.125
  moles CO2.
• 6. Convert back to grams of CO2.
• 0.125 moles CO2 x 44.01 amu 5.50grams CO2.

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Stoichiometric Calculations cont

• Another way to do the same thing THE MOLE
  TRAIN!!!
• Example 1 If there are 10.0 grams of NaOH, how
  many grams of CO2 are required?
• NaOH CO2 ? Na2CO3 H2O

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Stoichiometric Calculations cont

• Example 2
• FeO (s) O2 (g) ?Fe2O3 (s)
• What if I said there was 24.00g of FeO, how many
  moles of Fe2O3 would there be?
• Write down what you know. 24.00g of FeO
• Next, Balance the equation!
• 4FeO (s) O2 (g) ? 2Fe2O3 (s)
• Write down what you are looking for
• FeO converts to how many moles of FeO?
• Convert the grams to moles
• 24 grams FeO x ___1__ 0.3340 moles FeO
  
• 71.85 amu
• Now we have MOLES of FeO, what is the MOLE RATIO
  compared to Fe2O3?
• 0.3340 moles FeO x 2 Fe2O3 0.1670 moles of
  Fe2O3
• 4 FeO

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Stoichiometric Calculations cont

• Mole train way
• Example 2 What if I said there was 24.00g of
  FeO, how many moles of Fe2O3 would there be?
• FeO (s) O2 (g) ?Fe2O3 (s)

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Comparing two reactions

• When would we want to compare two reactions?
• Effectiveness of the reaction (cost effective)
• Do you have enough reactants to get the job done?
  Maybe you should use a different reaction?
• Example
• Stomach Acid Neutralizer what s the best
  bargain/solution?

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Comparing two reactions cont

• NaHCO3 HCl ? NaCl H2O CO2
• If I start with 1.00 grams of NaHCO3, how much
  HCl (stomach acid) will it neutralize?
• Did you balance the equation?
• How many moles of NaHCO3 do you have?
• How many moles of HCl do you have?
• Answer 0.0119 moles HCl

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Comparing two reactions cont

• Mg(OH)2 HCl ? H2O MgCl2
• If I start with 1.00 grams of Mg(OH)2, how much
  HCl will it neutralize?
• Did you balance the equation?
• How many moles of Mg(OH)2 do you have?
• How many moles of HCl do you have?
• Answer 0.0342 mol

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Homework

• Read pages 258-268. Problems 2 and 3 on
  page 268. Problems 13,25, and 28 on pages
  283-284