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Wize High School Grade 11 Chemistry Textbook > Stoichiometry

Stoichiometry

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Mole Ratios

  • Stoichiometry is the relationship between the amount of reactants used in a chemical reaction and the amount of products made.

  • The stoichiometric coefficients tells us that if we wanted to make one grilled cheese sandwich, we need two slices of bread and one slice of cheese
  • We can represent this using a mole ratio
2 slices of bread1 slice of cheese \dfrac{{\text {2 slices of bread}}}{ {\text {1 slice of cheese}}} or 1 slice of cheese1 grilled cheese sandwich\dfrac{ {\text {1 slice of cheese}}}{ {\text {1 grilled cheese sandwich}}} or 2 slices of bread1 grilled cheese sandwich\dfrac{ {\text {2 slices of bread}} }{ {\text {1 grilled cheese sandwich}}}
  • Mole ratios can then be used to convert between amounts of any two substances in a chemical reaction
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Stoichiometry of a Reaction

  • We use the coefficients of the balanced reaction along with the our equations that convert mass, volume, and concentration into moles to predict the quantities of reactants and products in a chemical reaction.
  • To answer any stoichiometry problem, focus on converting to and from moles! Moles are the central unit!

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General Steps to Solving a Stoichiometry Problem:

  1. Convert the values given in the problem about a reactant or product to a number of moles
  2. Use the stoichiometric coefficients from the balanced reaction to find the number of moles of the unknown you are being asked for
  3. Convert the number of moles of your unknown to a mass, or whatever quantity you are being asked for


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Example: Gravimetric Stoichiometry

Silver metal can be recovered from waste silver nitrate solutions by reaction with copper metal. What mass of silver can be obtained using 50 g of copper?

2AgNO3(aq)+Cu(s)→2Ag(s)+Cu(NO3)2(aq)2AgNO_3(aq)+Cu(s)→2Ag(s)+Cu(NO_3)_2(aq)

  1. Find the number of moles of Cu in 50g
nCu=mM=50g63.546g/mol=0.8mol Cun_{Cu}=\dfrac{m}{M}=\dfrac{50g}{63.546 g/mol}=0.8mol\ Cu
  1. Use the stoichiometric coefficients to find the number of moles of Ag produced
nAg=nCu×2mol Ag1mol Cu=0.8mol×2mol Ag1mol Cu=1.6mol Agn_{Ag}=n_{Cu}\times \dfrac{2mol\ Ag}{1mol\ Cu}=0.8mol\times\dfrac{2mol\ Ag}{1mol\ Cu}=1.6mol\ Ag
  1. Find the mass of silver from the moles
m=n×M=1.6mol×107.87gmol=170gm=n\times M=1.6mol\times107.87\dfrac{g}{mol}=170g


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Example: Solution Stoichiometry

What volume of 0.125mol/L NaOH(aq) is required to react completely with 15.0mL of 0.100mol/L Al2(SO4)3(aq)?

6NaOH(aq)+Al2(SO4)3(aq)→2Al(OH)3(s)+3Na2SO4(aq)6 NaOH(aq) + Al_2(SO_4)_3(aq) → 2 Al(OH)_3(s) + 3 Na_2SO_4(aq)
  1. Find the number of moles of Al2(SO4)3 originally in solution
nAl2(SO4)3=cV=0.015L×0.1molL=0.0015moln_{Al_2(SO_4)_3}=cV=0.015 L\times0.1\dfrac{ mol}{L}=0.0015mol
  1. Use the stoichiometric coefficients to find the number of moles of NaOH needed to react with 0.0015mol Al2(SO4)3
nNaOH=nAl2(SO4)3×6mol NaOH1mol Al2(SO4)3=0.0090moln_{NaOH}=n_{Al_2(SO_4)_3}\times \dfrac{6mol\ NaOH}{1mol\ Al_2(SO_4)_3}=0.0090mol
  1. Find the volume of NaOH using the concentration and number of moles
V=nc=0.0090mol0.125mol/L=0.072L=72mLV=\dfrac{n}{c}=\dfrac{0.0090mol}{0.125mol/L}=0.072L=72mL


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Example: Mixed Stoichiometry

Lithium metal was added to a 25mL of a 1.3mol/L solution of Ag2SO4. Once the reaction has gone to completion, what mass of silver metal is produced?
Ag2SO4(aq)+2Li(s)→Li2SO4(aq)+2Ag(s)Ag_2SO_4(aq)+2Li(s)\to Li_2SO_4(aq)+2Ag(s)

  1. Find the number of moles of Ag2SO4 originally in solution
nAg2SO4=cV=0.025L×1.3molL=0.0325moln_{Ag_2SO_4}=cV=0.025 L\times1.3\dfrac{ mol}{L}=0.0325mol
  1. Use the stoichiometric coefficients to find the number of moles of Ag produced
nAg=nAg2SO4×2mol Ag1mol Ag2SO4=0.065moln_{Ag}=n_{Ag_2SO_4}\times \dfrac{2mol\ Ag}{1mol\ Ag_2SO_4}=0.065mol
  1. Find the mass of silver from the moles
m=n×M=0.065mol×107.87gmol=7.01gm=n\times M=0.065mol\times107.87\dfrac{g}{mol}=7.01g


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Example: Gas Stoichiometry

The first recorded observation of hydrogen gas was made by the famous alchemist Paracelsus when he added iron to sulfuric acid. Calculate the volume of hydrogen gas at STP produced by adding 10.0g of iron to an excess of sulfuric acid.

2Fe(s)+3H2SO4(aq)→3H2(g)+Fe2(SO4)3(aq)2Fe(s)+3H_2SO_4(aq)→3H_2(g)+Fe_2(SO_4)_3(aq)

  1. Find the number of moles of Fe
nFe=mM=10.0g55.845g/mol=0.179moln_{Fe}=\dfrac{m}{M}=\dfrac{10.0g}{55.845g/mol}=0.179mol
  1. Use the stoichiometric coefficients to find the number of moles of H2 produced
nH2=nFe×3mol H22mol Fe=0.269moln_{H_2}=n_{Fe}\times \dfrac{3mol\ H_2}{2mol\ Fe}=0.269mol
  1. Find the volume of H2 from the moles
V=nRTP=0.269mol×8.314L⋅kPa/mol⋅K×273.15K101.325kPa=6.02LV=\dfrac{nRT}{P}=\dfrac{0.269mol\times 8.314 L\cdot kPa/mol\cdot K\times 273.15K}{101.325kPa}=6.02L



How many moles of H2 and N2 can be formed by the decomposition of 12.7mol of ammonia, NH3? Do not include units in your answer.

2NH3→N2+3H22NH_3\rightarrow N_2+3H_2

Practice: Stoichiometry

Calculate the mass of iron (III) oxide (rust), in grams, produced by the reaction of 250 g of iron with oxygen from the air. Round your answer to the nearest integer; do not include units.

The thermite reaction is used for welding railroad rails. The reaction is:

Fe3O3(s)+2Al(s)→3Fe(s)+Al2O3(s){Fe}_3{O}_3({s})+2{Al(s)}\rightarrow 3{Fe(s)}+{Al}_2{O}_3\text{(s)}
If 25g of iron, Fe(s), are produced from a reaction how many grams of aluminum oxide are generated?
MW Fe: 56 g/mol
MW Al: 27 g/mol
MW O: 16 g/mol

Practice: Stoichiometry and Molar Volumes

Suppose that 12.0L of hydrogen gas reacts with excess nitrogen gas to make ammonia gas, all at the same temperature and pressure. What volume of ammonia is expected from this reaction? Give your answer to one decimal point; do not include units

Practice: Balancing Chemical Reactions and Stoichiometry

When solutions of lead (II) nitrate and sodium iodide are mixed, a bright yellow precipitate of lead (II) iodide appears.
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Write the complete balanced equation for this double-replacement reaction.