1 STOICHIOMETRIC RELATIONSHIPS
1.2 THE MOLE CONCEPT
1.2.1
Relative masses
All measurements are made relative to an accepted standard. For chemical quantities the standard is
exactly 1 of the mass of a carbon-12 atom.
12
Relative Atomic Mass, Ar

 the average mass of all isotopes of an element compared to 1 of the mass of a carbon-12 atom.
12
This is a weighted average based on the relative proportions of each isotope of that element.
For all calculations, use the IB Chemistry Data Booklet values (show two decimal places)
Relative Molecular Mass, Mr

 the sum of all of the relative atomic masses of the atoms in one molecule
ex.
Mr of water, H2O = 2H + 1O = 2(1.01) + 1(16.00) = 18.02
Mr of sulfuric acid, H2SO4 = 2H + 1S + 4O = 2(1.01) + 1(32.00) + 4(16.00) = 98.02
The relative formula mass is the equivalent of the molecular mass used for an ionic compound
using its ionic formula.
The mole
Mole
the amount of a substance that contains the same number of particles as there are in
exactly 12 g of carbon-12.
Metric (SI) Units of Measure
Variable
Particles
Mass
Volume
Amount
MOLE
Unit
atoms, ions, molecules, formula units
kilogram (kg)
cubic decimetre (dm3)
mole (mol)
Substance
any substance in any state
solids
liquids & gases
ANY SUBSTANCE IN ANY STATE
the convenient unit of measure for any chemical substance
the SI unit for the amount of a substance
the central unit for converting between units or between substances in a chemical reaction
1 mol of carbon-12 = exactly 12 g carbon-12
1 mol of any substance = 6.02× 1023 particles of that substance
1 mol of an ideal gas = 22.4 L of that gas @ STP
Avogadro’s Number, L or NA
the number of carbon-12 atoms in exactly 12 g of carbon-12
NA = 6.022 141 99 × 1023
Converting Moles to Number of Particles
Different chemical substances are made of different basic particles that have the properties of that substance.
1 mol Al = 6.02× 1023 atom Al
Elements
atoms
Molecules
molecules (do NOT abbreviate as mol!)
1 mol H2 = 6.02× 1023 molecules H2
1 mol H2O = 6.02× 1023 molecule H2O
1 mol Ag = 6.02× 1023 atom Ag
1 mol CH4 = 6.02× 1023 molecule CH4
1 mol hemoglobin = 6.02× 1023 molecules hemoglobin
Ionic Compounds
formula units (do not abbreviate this either!) 1 mol NaCl = 6.02× 1023 formula units NaCl
1 mol Fe2O3 = 6.02× 1023 formula units Fe2O3 1 mol (NH4)3PO4 = 6.02× 1023 formula units (NH4)3PO4
Factor Label Method for Converting
 use every time you convert from one unit to another in Chemistry or Physics
 works with converting
 metric units (mm ↔ m ↔ km)
 speeds (m/s ↔ km/h)
 currency ($US ↔ $CAD ↔ £UK)
 amounts of a substance (particles ↔ mole ↔ mass)
 between substances (mol A ↔ mol B)
1. Write the value, the units and the substance
2. Multiply by conversion factors so that undesired units reduce to one and only desired units remain in proper position
3. Multiply each top value and divide by each bottom value
ex. Sample Problem – Moles to Atoms
Given: 1.25 mol NO2
Want: molecules of NO2
Conversion Factor (relating moles to molecules):
1 mol NO2 = 6.02× 1023 molecules NO2
1.25 mol NO2 = 1.25 mol NO2 × 6.02 × 1023 molecule NO2 = 7.525×1023 molecule NO2 = 7.52×1023 molecule NO2
1 mol NO2
 1.25 mol NO2 = 7.52×1023 molecule NO2
Given: 1.25 mol NO2 = 7.525×1023 molecule NO2
Find: atoms in sample
Conversion Factor: 3 atoms in one molecule of NO2  3 atoms = 1 molecule NO2
1.25 mol NO2 = 7.525×1023 molecule NO2 × 3 atoms
= 22.575 ×1023 atoms = 22.6 ×1023 atoms
1 molecule NO2
23
 1.25 mol NO2 = 22.6 × 10 atoms
.
Practice
Euro to US dollars
Euro to Japanese Yen
British pounds to Japanese yen
British pounds to Canadian dollars
1 Euro = $1.29599 USD
1 Euro = 157.256 ¥ JPY
1£ GBP = 236.429 ¥ JPY
1£ GBP = $2.29034 CAD
If one ounce of pure platinum is worth $1141,60 USD, how much would it be worth in Canadian dollars?