2.1.3

The Mole

(a)→amount of substance (n): number of particles in a substance/ the mole [mol]

→ Avogadro constant, N_A: number of particles equivalent to the Ar/Mr of a substance [value: 6.02 x 10²³ g mol^-1 ]

→molar mass: mass/mol same as Mr

→ n= mass/molar mass

→molar gas volume: vol occupied by one mole of any gas at room temp and pressure [24dm³]

Determination of formulae

(b)→empirica formula: simplest whole number ratio of atoms of each element present in a compound

→ molecular formula: shows the number and type of each atom in a molecule

(c)→how to find empirical formula: calculated from ratio of masses of each element in a compound (mass or % / Mr = molar ratio)

→ how to find molecular formula: [Mr / relative empirical mass] x number of each element present

(d)→water of crystallisation: when some compounds form crystals which have water as part of their structure

→hydrated compound: a compound that contains water of crystallisation e.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O

→a compound that doesn’t contain water of crystallisation

→how to calculate degree of hydration: measure mass of hydrated salt before heating, then heat salt/note mass of each componenent, divide by respective Mr and obtain mole ratio

Calculation of reacting masses, gas volume and concentrations

(e)→moles=mass[g] / mr [g mol-1]

→why calculating reacting masses is useful: to determine how much of the reactants exactly react with each other to prevent waste; needs mass and Mr of reactants and balanced equation [for moles]

→avagadro’s hypothesis: equal volumes of gases contain same number of moles; so at room temp and pressure one mole of any gas has volume 24dm³

→volume of gas dm³ = mol x 24 dm³mol-1

→concentration: the amount of solute dissolved in a solvent to make 1dm³ of solution

→concentration= mol/ volume [dm³]

→A concentrated solution is a solution that has a high concentration of solute; A dilute solution is a solution with a low concentration of solute

→how to calculate concentration: change mass to moles using Mr, change vol cm³ to dm³ by /1000

(f) →how vol of a gas relates to its pressure and temp: when a gas is heated [constant pressure] particles gain more Ek so more frequent collisions w container wall; to keep pressure constant the particles move further apart and vol increases [directly prop to temp at constant pressure]

→ pv = nrt: [Pa] [m³] [mol] [8.314] [K: +273]

(g)→stoichiometry of a reaction shows the numerical relationships between the reactants and products e.g. an equation showing the ratio of moles

Percentage yields and atom economy

(h)→not all reactants used up as: other reactions take place simultaneously; reaction not go completion; products lost during separation/purification

→percentage yield shows how much a product you get from reactants compared to theoretical amount: actual yield/theoretical yield x 100

→calculating % yield: find theoretical/actual yield by using mass, Mr, moles; plug into equation

→atom economy shows how many of the atoms used in the reaction become the desired product: Mr desired product/ Mr all reactants x 100

→sometimes atom economy is 100% as in addition reactions all atoms are used to make one desired product

(j)→high percentage yield isnt an accurate measure of how sustainable something as it doesnt account for waste products; a reaction w high % yield but low atom econ means more waste products made

→benefits of high atom economy: less waste produced, so environmentally friendly and reactions more sustainable as often use less natural/finite resources