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 mole 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

for ionic compounds, the chemical formula is always the empirical formula


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 solid product is lost during filtration/separation/transferring/weighing

  • solid is blown away with the gas


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 =

  • maximise the use of raw materials that end up as useful product

  • minimise the amount of by-product

    • so more economically viable for industrial scale manufacture

  • less waste of reactants/less pollution

    • so the reaction is more sustainable.

  • saves natural resources

    • 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 one 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


titrations

a conical flask is used instead of a beaker as there is less chance of splashing so less chance of losing any solution

use concordant results (within 0.1 cm3 of each other) to calculate the mean titre

you can reduce % uncertainty by using a lower concentration of the solution in the burette, as a greater volume will need to be added, giving a larger titre which decreases % uncertainty

if you use a funnel to fill a burette and forget to remove the funnel after filling, additional drops of solution from the funnel could drip into the burette, making the value on the burette lower

improving titration methods:

  • wash burette with NaOH or whatever alkali you are using, as rinsing with water dilutes the NaOH giving a larger titre

  • use a pipette as it is more precise than a measuring cylinder so there will be a lower uncertainty

  • don’t add too much indicator, add just a few drops, as the indicator may react and affect the end point

  • keep adding alkali until a permenant colour change is seen: adding the alkali until the indicator ‘just’ changes colour is a mistake as the acid may not have fully reacted as the mixture is not swirled


when making up a standard solution:

  • measure the mass of the weighing boat and solid, then add the solid to flask and reweigh the weighing boat and subtract to find the mass of solid added

  • wash the beaker and transfer the washings into the flask/wash the stirring rod into the flask after use

  • wash the solid from the weighing boat into the beaker

  • invert the flask several times to ensure all the solid has dissolved

  • make sure the bottom of the meniscus is level with the graduation mark of the flask


the limiting reactant is the reactant that is completely used up in a chemical reaction, thereby determining or "limiting" the amount of product that can be formed.

the moles of the limiting reagent should always be used in calculations, as it indicates the maximum possible amount of product that can form.

the reactant that is left over is ‘in excess’