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Stoichiometry summary
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StoichiometryChemical equations
⚛ chemical reactions are processes in which change occurs and are characterised by the
formation of new substances. Arrangement of atoms is changed to produce new substances
but atoms are neither created nor destroyed
⚛ Balancing chemical equations
→ to represent a chemical reaction correctly, the equation must be balanced
→ to be balanced the number of atoms of each element on the left-hand and right-hand sides
of the equation must have the same number and type of each atom
→ balancing equations is achieved by changing the coefficients in front of the formulas
⚛ Steps for chemical calculations:
→ 1. Balanced chemical equations (correct formulas and valencies)
→ 2. Determine the molar ratio
→ 3. Determine the number of moles of the known substance
→ 4. Use the known to determine the number of moles of the unknown
→ 5. Answer the question
Stoichiometry
⚛ quantities and ratios of chemical species in a chemical reaction. Enables us to predict the
yield of products whether a reactant is limiting or in excess
⚛ mass is conserved, so the total mass in a chemical reaction must be the same before and
after the reaction occurs. Knowing this, you can work out the mass of product formed in a
reaction or how much mass of reactants has been used up in a reaction
⚛ sometimes too much reactant will be in a chemical reaction and not all of it will be consumed
in the reaction. This is a reactant which is in excess
The mole
⚛ constant value that is essential to chemical analysis involving calculations of masses and
volumes in chemical reactions
⚛ allows for the comparison of chemical species involved in a reaction since it is a quantity of
materials based on a constant number of particles
⚛ Avogadro’s number (NA): 6.022 x 10²³
⚛ the mole scale, a basic mass scaled, based on grams was developed by Amadeo Avogadro
⚛ the mole scale is based on the following concepts:
→ 12g exactly of the C-12 isotope contains 6.022 x 10²³ atoms
→ an Avogadro number of C-12 atoms is called a mole of C-12 atoms
⚛ a mole is an amount of a substance that contains the same number of particles as there are
atoms in exactly 12g of carbon-12
⚛ it was found that 1 mole of any chemical substance is equal to its:
→ atomic weight for elements
→ molecular/formula weight for compounds
⚛ the atomic weight or molecular/formula weight is known as its molar mass (MM)
⚛ the number of moles of any chemical substance can be calculated with the equation:
→ n = m ÷ MM n = number of moles (mol)
m = mass of substance (g)
MM = molar mass (g)
⚛ particle units being counted should be stated as atoms, molecules, ions or formula units
(used for ionic metals and some network substance that don’t exist as simple molecules)
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