1.2 amount of substance (notes)
relative atomic mass (Ar)
the mean mass of an atom of element, relative to the mass of 1/12th of the mass of an atom of the 12C isotope
average mass of an atom of an element x 12
mass of one atom of 12C
or
∑ (isotope abundance x isotope mass number) x 100
∑ (isotope abundance)
relative molecular mass (Mr)
the mean mass of a molecule of a compound relative to 1/12th of the mass of an atom of the 12C isotope
relative formula mass
the sum of the relative atomic masses of the atoms in the chemical formula of a compound
ionic compound have a giant ionic lattice with a very large number of ions, so the relative formula mass is the Mr of the empirical/chemical formula
the mole:
a unit of measurement that represents a specific quantity of a substance
it is the amount of pure substance that contains the same number of particles as there are in exactly 12g of 12C
1 mole of a substance contains 6.022 × 1023 molecules/atoms/ions
Avogadro’s constant, L
there are 6.022 × 1023 atoms of 12C in 12g of 12C
no. of molecules = nL
n = mass/Mr
n = cv
molarity = moles per dm3 = concentration in moldm-3
molar mass = the mass of one more of a substance = Mr or Ar
gases and volatile liquids follow the ideal gas equation under standard conditions
pV = nRT
p = pressure in Pa
V = volume in m3
m3 ← ÷1000 — dm3 ← ÷1000 — cm3
n = amount in moles
R = ideal gas constant = 8.314 J K-1 mol-1
T = temperature in K
°C —+273 → K
p is proportional to V
V is proportional to T
p and V are inversly proportional
assumptions:
gas molecules move randomly
there are no forces acting between gas molecules
gas molecules are tiny compared to the spaces between them
collisions between gas molecules are elastic
1 mole of a gas at room temp = 24 dm3
volume = n x 24 dm3
room temperature = 25°C = 298 K
room pressure = 101 325 Pa ≈ 101 00 Pa = 101k Pa = 1 atmosphere
empirical formula = the simplest whole number ratio of the atoms of each element in a compound
molecular formula = the true number of atoms of each element in a compound
a molecular formula is not practical for giant crystal structures because giant lattices contain a vast number of atoms of each element, and larger crystals will have more atoms than smaler crystals
percentage yield: a measure of the efficient conversion of reactants to products / idea of getting as much product as possible in the reaction
(mass of product actually made / maximum theoretical mass of product) x 100
percentage yield may be low if:
the product is gas so easily escapes
impure reactants are used which cause side products OR not all reactants react (especially in reversible reactions)
there are side reactions making other products/there are by-products
some of the product is lost during filtration/separation/transferring
atom economy: maximising mass of reactants that end up as desired product, minimising amount of by-products
(Mr of desired product / sum of Mr of all reactants) x 100
for percentage atom economy calculations, use the coefficients
high percentage atom economy =
little to no waste product, only the desired product
so more economically viable for industrial scale manufacture
less mass of reactants that end up as waste
so the reaction is more sustainable.
less natural resources used
so less energy used, so beneficial for the environment
improving percentage atom economy:
cannot be improved.
sell the by-product to increase profits.
if there is only product in a reaction, the atom economy is 100%
it’s hard to separate 2 solids/liquids/gases produced by a reaction, which makes it difficult to obtain a pure sample