Unit 3: Gravimetric Stoichiometry

1
Unit 3 Gravimetric Stoichiometry

• Notes and Examples

2
Introduction

• What is stoich-i-o-metry?
• It is the methods used to calculate the
  quantities of substances in a chemical reaction.
• Gravimetric (mass of solids)
• Solution (volume and concentration of solutes)
• Gas (volume, pressure and temperature of gases)
• What do you need to know?
• good problem-solving skills
• experience with lab procedures and techniques
• good background chemistry knowledge (both
  theoretical and empirical)

3
Review

• You must have a solid understanding of the
  following to achieve success in this unit
• The Mole
• Molar Mass
• how to calculate molar mass of elements and
  compounds using values found on the periodic
  table
• The Mass formula
• m nM, n m/M, M m/n
• know how to rearrange formula for any variable
  required
• Writing Correct Chemical Formulas
• for all elements and compounds, including their
  states
• Balancing Chemical Equations
• this is the first step in every stoichiometry
  problem and therefore mastery of this skill is
  vital to answering a question correctly.

4
Significant Digits

• this is to ensure common communication and
  accuracy in all calculated and measured values in
  chemistry
• we will use the common rules used on the diploma
  exam in Chemistry 30
• Here are a few of the basics
• your answer cannot be more accurate than the
  values given in the question (use the least
  number of sig digs from the question).
• Sig digs include all digits correctly reported
  from a measurement, except leading zeros
• exact values have an unlimited number of sig digs
• When adding values, use the least number of
  decimal places found in the measurements
• molar mass is recorded to 2 decimal places
• you may need to use scientific notation to write
  correct number of significant digits.

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Stoichiometry Concept Map
6
Stoichiometry Procedure
Balanced Chemical Equation
Given quantity, find moles of given (mass,
volume, concentration)
From moles calculated, find quantity of
required (mass, volume, concentration)
From moles calculated, find moles of
required (use mole ratio formula)
7
Stoichiometry Steps

• Write a balanced equation for the reaction.
• Determine the two substances involved in the
  reaction, the given and the unknown.
• Calculate the number of moles of the given
  substance.
• Set up the mole ratio using coefficients of the
  balanced equation for the two substances.
• Substitute values in mole ratio for the given and
  the unknown.
• Solve the equation for the unknown number of
  moles.
• Convert required moles into mass by multiplying
  the unknowns molar mass.

8
Note

• You may not always use steps 3 and 7 (only in 2
  or 3 step stoich problems)
• A correct balanced equation is vital for full
  marks as it effects all other steps and
  calculations
• Always list the given and unknown variables under
  each substance in your chemical equation for step
  2.
• Check for correct significant digits and include
  all units when recording your final answer in a
  box
• Marks distributed as follows on any stoich
  question
• 1 mark for balanced chemical equation
• 1 mark for each step of the procedure
• 2 marks for final answer (1 mark for value, ½ for
  units, ½ for correct significant digits)
• Show ALL your work for full marks!!!

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1 step problems Mole to Mole

• The Mole Ratio
• A mathematical relationship of the of moles
  between the given and the unknown found in the
  balanced chemical equation.
• Example
• Given the following equation, write the molar
  ratios for
• a)
• c)
• b)

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Example 1

• During the production of ammonia,
• If 3.00 moles of H2 were used, how many moles of
  NH3 would be made?
• If 0.600 moles of NH3 were produced, how many
  moles of N2 are required?

11
More Examples and Practice

• Go through examples on p.160 of your workbook.
• Do Practice Problems 1-9 on p.160-164 of
  workbook (random HW check show all your work)
• Work on Stoichiometry Assignment 1
• due Thursday, November 2nd

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2 step problemsMole to Quantity or Quantity to
Mole

• An additional step is needed
• to find the of moles of given, or
• to find the mass of the unknown.
• Need to use the mass formula, mnM, to solve for
  either ngiven or munknown
• May use unit conversion for this step if desired

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Example 2

• During the combustion of methane,
• how many grams of carbon dioxide are produced if
  2.0 moles of methane are used?
• how many moles of methane are needed to produce
  12.2 grams of water vapour?

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More Examples and Practice

• Go through examples on p.165-166 of your
  workbook.
• Do Practice Problems 1, 6, 8, and 9 on p.166-167
  of workbook (random HW check)
• Finish Stoichiometry Assignment 1
• due Thursday, November 2nd
• Go through example 2 on p.169 and do Practice
  Problems 1-4, 6, 8-11 on p.169-171 of workbook

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3 step Problems Quantity to Quantity

• In this type of question, you are provided with a
  quantity of one substance, and asked to calculate
  a quantity such as mass of another substance.
• All 7 stoichiometry steps are needed and each
  question is worth 6 marks
• Note these are never asked in a multiple choice
  question
• Show ALL your work in each step

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Example 3

• During a reaction between lithium hydroxide and
  hydrobromic acid, if you start with 3.7 grams of
  lithium hydroxide, how many grams of lithium
  bromide will be produced?

17
Example 4

• If you start with lead (II) nitrate and 15.0
  grams of sodium iodide, how many grams of sodium
  nitrate can be formed?

18
More Examples and Practice

• Go through example 1 on p.172 of your workbook.
• Do Practice Problems 3, 5, 6, and 9-11 on
  p.173-175 of workbook (random HW check)
• Finish Stoichiometry Assignment 2
• due Wednesday, November 8th
• Stoich Quiz on Thursday, Nov. 9th

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Limiting Reagants

• During a chemical reaction, one reactant will
  usually run out first. This substance is called
  the limiting reagant and stops the formation of
  new products.
• An additional step is needed to compare the moles
  of both reactants to find which reagent will be
  used up first
• usually the smallest mole amount but be careful
  and check the mole ratio amounts

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Example 5

• If you start with 25.0 grams of lead (II) nitrate
  and 15.0 grams of sodium iodide, how many grams
  of sodium nitrate can be formed?

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Example 5 contd

• What is the limiting reagent?
• How much of the excess (nonlimiting) reagent will
  be left over from the reaction?

22
Practice Problems and Review

• Do Practice Problems 1c, 4, 5, and 7 (use answer
  from 6) on p.177-178 of your workbook (random HW
  check)
• Finish Vocabulary Assignment
• due day of unit test
• Practice for the Unit Test by doing Review
  Problems 2-4, 6-10 and the multiple choice test
  1-6 on p.179-181 of your workbook
• Unit Test November ____