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Stoichiometry
Mass Relationships in Chemical Processes
Stoichiometry
“Stoi” = mass, heaviness, solidity (stoic)“Chio”= chemistry, chemical, component“Metry” = measure
Hence, Stoichiometry = the measurement of chemical masses
Stoichiometry
“Stoicheîon” = element, component“Metry” = measure
Hence, Stoichiometry = the measurement of elements or components
Oh, well...
Stoichiometry
Equations provide the necessary relationships
Calculations require assumption of certain chemical process(es), identified by the balanced equation
Fundamental quantity of chemical substance in the SI system is the mole.
Mole (chemical context)
Not the blind, long-nosed animal that burrows into the ground, leaving a pile (or mole-hill) of dirt at its entrance
Not an infiltrator into espionage organizationNot a skin blemishNot the spicy, chocolaty-peanut sauce so
good on chicken and enchiladas
Mole
A certain number of “things”Like “dozen,” except lots more than 12 things602 sextillion (approximately) things, or 6.02
x 1023 thingsSo many things that only atomic- or molecular-
sized things exist in mole quantities (a mole of paper clips, for example, has the earth’s total mass)
Mole: Official Definition
The precise number of C-12 atoms that have total mass exactly 12 grams
Must be determined experimentallyKnown to several significant figures, but
not exactly
Mole: Working DefinitionQuantity of substance equal to the
substance’s formula weight (molar mass) in grams.
Equivalent to a mass amountConversion factors of form:
one mole X = certain grams Xone mole Cl = 35.45 g Clone mole H2O = 18.0 g H2O
one mole NaCl = 58.5 g NaCl
Mole: Formula Weight
FW = the sum of the atomic weights of all atoms in the formula (as often as they appear)
FW H2O = 18.0 2 x 1.0 (H’s) + 16.0 (oxygen) one mole H2O = 18.0 g H2O
FW NaCl = 58.5 23.0 (Na) + 35.5 (Cl) one mole NaCl = 58.5 g NaCl
One mole CaCl2 = grams
Mole: You Do One...
Formula weight of CaCl2:
At wt Ca = 40.0At wt Cl = 35.5FW CaCl2 = 40.0 + (2 x 35.5) = 111.0
One mole CaCl2 = 111.0 g
111.0
One mole (NH4)2SO4 = grams
Mole: You Do Another One...
Formula weight of (NH4)2SO4 :
At wt N = 14.0 At wt H = 1.0 At wt S = 32.1 At wt O = 16.0 FW (NH4)2SO4 =
(2 x 14.0) + (8 x 1.0) + 32.1 + (4 x 16.0) = 132.1 One mole (NH4)2SO4 = 132.1 g
132.1
0.514 g Mg = mole(s)
Moles: Counting Atoms and Molecules
0.0211
0.514 g Mg x = 0.0211 moles1 mole Mg
24.305 g Mg
75.0 g H2O = mole(s)
Moles: Counting Atoms and Molecules
Formula weight of H2O:At wt H = 1.0At wt O = 16.0FW H2O = (2 x 1.0) + 16.0 = 18.0
One mole H2O = 18.0 g
75.0 g H2O = mole(s)
Moles: Counting Atoms and Molecules
4.17
75.0 g H2O x =OH g 18.0
OH mole 1
2
24.17 moles
1.97 g C4H10 = mole(s)
Moles: Counting Atoms and Molecules
Formula weight of C4H10 :At wt C = 12.0At wt H = 1.0FW C4H10 = (4 x 12.0) + (10 x 1.0) = 58.0
One mole C4H10 = 58.0 g
1.97 g C4H10 = mole(s)
Moles: Counting Atoms and Molecules
0.0340
0.0340 moles1.97 g C4H10 x =104
104
HC g 58.0
HC mole 1
Moles and Stoichiometry
Chemical equations specify quantities as well as substances
Quantities are indicated in mole amounts in balanced equations
Moles and Stoichiometry
Consider the reaction of methane with oxygen which we discussed previously:
CH4 + 2 O2 CO2 + 2 H2O
Moles and Stoichiometry
Quantities are specified in terms of (a) individual molecules or (b) moles of molecules
CH4 + 2 O2 CO2 + 2 H2O
Two molecules of water
One molecule of carbon dioxide
Two molecules of oxygen
One molecule of methane
Moles and Stoichiometry
Quantities are specified in terms of (a) individual molecules or (b) moles of molecules
CH4 + 2 O2 CO2 + 2 H2O
Two moles of water
One mole of carbon dioxide
Two moles of oxygen
One mole of methane
Moles and Stoichiometry
The mole scale is 602 sextillion times as big as the molecule scale!
CH4 + 2 O2 CO2 + 2 H2O
Two moles of water
One mole of carbon dioxide
Two moles of oxygen
One mole of methane
CH4 + 2 O2 CO2 + 2 H2O
Quantities can be at various scalings:
1 molecule + 2 molecules 1 molecule + 2 molecules
1 mole + 2 moles 1 mole + 2 moles
5 moles + 10 moles 5 moles + 10 moles
0.2 mole + 0.4 mole 0.2 mole + 0.4 mole 4 grams + 16 grams 11 grams + 9 grams (0.25 mole)
(0.50 mole) (0.25 mole) (0.50 moles)
Last example represents usual information about reaction quantities.
CH4 + 2 O2 CO2 + 2 H2O
Practice Example
Based on the equation below, how many grams of water should be formed by reaction of 21.0 grams of methane, CH4?
Three-step conversion-factor sequence applies in most instances.
? moles ? moles
? grams21.0 gramsUse FW of CH4 for conversion factor
Use FW of H2O for conversion factorUse equation
coefficients for conversion factor
CH4 + 2 O2 CO2 + 2 H2O
Practice Example
Based on the equation below, how many grams of water should be formed by reaction of 21.0 grams of methane, CH4?
? moles ? moles
? grams21.0 gramsUse FW of CH4 for conversion factor
Use FW of H2O for conversion factorUse equation
coefficients for conversion factor
21.0 g CH4 x1 mole CH4
16.0 g CH4
x2 mole H2O1 mole CH4
x18.0g H2O1 mole H2O
= 47.3 g H2O
CH4 + 2 O2 CO2 + 2 H2O
Practice Example
Based on the equation below, how many grams of CH4 should be required to react with 125 grams of oxygen, O2?
? moles CH4
? gramsUse FW of CH4 for conversion factor
? moles O2
125 gramsUse FW of O2 for conversion factor
Use equation coefficients for conversion factor
125 gO2 x1 mole O2
32.0 g O2
x1 mole CH4
2 mole O2
x16.0 g CH4
1 mole CH4
= 31.3 g CH4
Fe2O3 + 3 CO 2 Fe + 3 CO2
Practice Example
Based on the equation below, how many grams of Fe2O3 would be required to form 5.00 grams of iron, Fe?
? moles ? moles
? grams 5.00 gramsUse FW of Fe2O3 for conversion factor
Use FW of Fe for conversion factor
Use equation coefficients for conversion factor
5.00 g Fe x1 mole Fe55.85 g Fe
x1 mole Fe2O3
2 mole Fex
159.7 g Fe2O3
1 mole Fe2O3
= 7.15 g Fe2O3
