Mole Concept and Stoichiometry

Unit Overview

In this unit, the concept of the mole is developed. That concept, together with the concepts of ratio and proportion, relative masses of the atoms, and conservation of mass, is key to understanding the analysis of molecules and chemical reactions.

Use concrete examples, analogies and models as much as possible, to enable students to become abstract thinkers in stoichiometric analysis.

Factors of scientific literacy which should be emphasized

• A1 ..........public/private
• A3 ..........holistic
• A5 ..........empirical
• A6 ..........probabilistic
• A8 ..........tentative

• B1 ..........change
• B3 ..........orderliness
• B8 ..........quantification
• B9 ..........reproducibility of results
• B10 ..........cause-effect
• B11 ..........predictability
• B13 ..........energy-matter
• B15 ..........model
• B20 ..........theory
• B21 ..........accuracy
• B22 ..........fundamental entities
• B23 ..........invariance

• C2 ..........communicating
• C4 ..........working cooperatively
• C5 ..........measuring
• C6 ..........questioning
• C7 ..........using numbers

• D3 ..........impact of science and technology
• D4 ..........science, technology, and the environment
• D7 ..........variable positions

• E1 ..........using magnifying instruments
• E3 ..........using equipment safely
• E4 ..........using audio visual aids
• E5 ..........computer interaction
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E11 ..........measuring mass
• E12 ..........using electronic instruments
• E13 ..........using quantitative relationships

• F1 ..........longing to know and understand
• F3 ..........search for data and their meaning
• F5 ..........respect for logic
• F7 ..........demand for verification

• G1 ..........interest
• G3 ..........continuous learner
• G6 ..........response preference
• G9 ..........valuing contributors

Foundational Objectives for Chemistry and the Common Essential Learnings

Explore the concepts which relate to Avogadro's number .

• Use an atomic mass (atomic weight) table to compare the relative masses of atoms.
• Realize that the absolute mass of an atom is very small.
• Describe how Avogadro's number is obtained.
• Explain the concept of a mole.
• Calculate the number of moles in a given mass of atoms or molecules.
• Calculate the masses of various numbers of moles of atoms and molecules

Apply knowledge about atomic mass to calculations dealing with molecules.

• Calculate the percentage composition of elements, by mass, in inorganic and organic molecules from molecular formulas and from mass measurements.
• Calculate the empirical formulas for molecules from data on percentage composition and from mass measurements.
  
• Calculate the concentrations of solutions in mols×L^-1.

Perform stoichiometric calculations.

• Extract qualitative and quantitative information from balanced chemical equations.
• Use balanced chemical equations to determine the relative number of moles of reactants and products.
• Manipulate the relationship among molar mass, number of moles and mass of chemicals to solve stoichiometric problems.
• Manipulate the relationship among concentration, number of moles and volume of solution to solve stoichiometric problems.
• Identify limiting reagents in chemical reactions involving non-stoichiometric masses of reactants.

Strengthen understanding within chemistry through applying knowledge of numbers and their interrelationships. (NUM)

• Read and interpret charts and tables.
• Collect, organize and analyze quantitative information from activities.
• Appreciate value of being able to calculate required masses of reactants or anticipated quantities of products.

Develop an understanding of how knowledge is created, evaluated, refined and changed within chemistry. (CCT)

• Make careful observations during active involvement in constructing knowledge.
• Discuss observations, hypotheses, predictions and generalizations with others.
• Recognize how mathematical skill and logical thinking ability are critical to constructing models of how chemical reactions work.

Suggested activities and research project ideas

• Some examples which illustrate the size of Avogadro's number:

  • a mole of sheets of ordinary paper, divided into a million equal piles, is enough so that each pile would reach from the earth to beyond the sun.

  • a mole of pennies, placed side by side would stretch for a million light years.

  • Avogadro's number of grains of sand would cover an area approximately the size of Ontario, Manitoba, Saskatchewan, Alberta, British Columbia, Yukon and the Northwest Territories) to a depth of 2 m.

  • a Cray S-1 supercomputer running at 1 000 MIPS (million instructions per second) would take 1.9 million years to execute Avogadro's number of steps. (These are taken from the article "A Mole of Heartbeats" by Arthur Last, Athabasca AB in CHEM13 NEWS, #195, May 1990, page 6 )

• Here are some ideas for raising awareness about moles in particular and the chemistry program in general.

  • Celebrate Mole Day from 6:02 a.m. to 6:02 p.m. on October 23rd.

  • Make an display small stuffed moles. (Pattern in CHEM13 NEWS, #205 (September 1991).

  • Buy and wear a mole t-shirt. (#T27 from Chemsmiles, Box 411, Waterloo N2J 4A9 - $13.00 plus $1.50 postage for one shirt. Postage is 15% of total if more than one shirt is ordered.)

  • Make and wear mole buttons. Give them to other teachers.

  • Make Mole Day posters and post them around the school.

  • Make Mole Day certificates to present to people for various reasons on Mole Day.

  • Join the National Mole Day Foundation. $10 US to NMDF, Box 373, Prairie du Chien, WI 53821.

  • Contact The Mole Company, 1012 Fair Oaks Avenue, #356, Pasadena, CA 91030 for a catalog. (This activity was adapted from "Mole Day, October 23" in CHEM13 NEWS (#205, September 1991, page 8.)

• Have the students record their predictions, observations and interpretations of the following demonstration:

  Onto the sidearm of each of two 500 mL sidearm flasks, fit a #4 one hole stopper, with the small end of the stopper toward the flask.

  Blow up two large (20 cm diameter) round balloons. Remove the air from them and reinflate several times, so that they will inflate easily during the demonstration. Then place the mouths of the deflated, stretched balloons over the ends of the #4 sto ppers.

  Add 200 mL of 6.0 M HCl solution to each flask. Into one flask, place one 2.5 cm by 30 cm strip of aluminum foil. Stopper the flask immediately. Add two strips of foil to the other flask. Stopper it.

  Ask the students to predict which balloon will become larger. Compare the balloons' diameters when the reaction is complete. How do their volumes compare?

  Twist the stem of the balloons, remove them from the stoppers and tie them. Use a candle taped to a metre stick to explode the balloons.

  What gas is produced? What is the limiting reagent? Why does the reaction start slowly and then speed up? What is the relationship between the amount of hydrogen produced and the amount of aluminum used? Would a definite mass of aluminum give a defi nite volume of hydrogen? (This activity was adapted from CHEM13 NEWS, #199, December 1990, page 6, based on a demonstration contributed by Kerro Knox, Amherst, MA)

• Use the displacement of copper from copper chloride by zinc to determine which sample provide is copper(I) chloride and which is copper(II) chloride. (Hint: zinc should be used as the limiting reagent in this reaction. Why?)

  Devise a procedure to accomplish this task. Include a sketch of the apparatus and an outline of the data analysis. During a class discussion of the various designs proposed by you and your classmates, be prepared to defend and modify your design.

  Teacher's note: Copper(II) chloride absorbs water from the air to form a bright blue hydrated form. Using the brown anhydrous form will simplify the analysis in this activity. Anhydrous copper(II) chloride can be produced by heated th e hydrated crystals in a lab drying oven or under a heat lamp. A subsidiary investigation might be to determine the value of n in the hydrated crystal CuCl₂·nH₂O.

• Devise a procedure to determine the percentage of oxygen in the air you exhale. Draw a diagram of the apparatus you would use to discover this, and outline the calculations which would be needed.

Sample ideas for evaluation and for encouraging thinking

• A class or small group discussion on the significance of the Law of Conservation of Mass can centre about the students' own bodies. Every atom in every cell - skin, muscle, bone. etc. - has come from somewhere else.

  Trace the origin of the component atoms of the students' bodies back to plants and then to the air, the soil, and the water. A source of information to get the discussion going might be a list of the elements in a human body. Students could prepare posters or charts which indicate from which foods each of the elements come. This could be related to the Ecological Organization unit in grade 11 biology.

  The discussion can be extended to consider the matter of recycling. This has been adapted from an article "The Law of Conservation of Mass Revisited - With a Look to STSE" by Warren Wessell in June 1990 issu e of The Accelerator.

• Balance these equations.

  FeS_(S) + O_2(g) Fe₂O_3(S) + SO_2(g)

  Na_(S) + H₂O_(l) NaOH_(aq) + H2_(g)

• CrO₄^2-_(aq) + H⁺_(aq) Cr₂O₇^2- _(aq) + H₂O_(l)

  Balance this equation. In the reaction represented by the equation above, why must there always be 2 moles of H⁺_(aq) which react for every 1 mole of Cr₂O₇^2- _(aq) which forms? (What is the H⁺_(aq) species doing during the reaction?)

• One mole of nitrogen gas reacts with three moles of hydrogen gas to form two moles of ammonia. Write the balanced equation for the reaction and determine what mass of ammonia can be formed if 56.0 grams of nitrogen gas react.

• When coal is burned, sulphur impurities in the coal also burn. The sulphur oxides which form react with oxygen and water vapour in the air to form acids. If the major oxide of sulphur which forms is SO_2(g), write a balanced equation for the production of sulphuric acid from this oxide.

  What are some other formulas of sulphur oxides? Why would SO_2(g) be the most common oxide produced when coal is burned as a fuel?

  Assume that the average sulphur content of Western Canadian coal is 0.4% by mass. (The range is from 0.2% to 1%.) What volume of 1.0 mol·L^-1 H₂SO_4(aq) could be made from the SO_2(g) produced when 1 tonne of coal is burned in an electrical generation plant?

  Approximately 11 million tonnes of coal are burned in Saskatchewan to supply 76% of our electrical supply. Assuming that the average sulphur content is 0.4%, how many tonnes of sulphur are found in the Saskatchewan coal burned each year. How many moles of sulphur is this? How many litres of concentrated (18 mol·L^-1)could be made from this sulphur if it were all captured as S_8(s)?

  How is sulphur removed from coal before the coal is burned? How are the sulphur oxides removed from the flue emissions after the coal is burned?