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MOD 2
Quantitative Chemistry
9 formulas
Mole–Mass Relationship
n = mM
also: m = n × M | M = m ÷ n
nnumber of moles unit: mol
mmass unit: g
Mmolar mass unit: g mol⁻¹ (sum of atomic masses from periodic table)
HSC tip: Always convert mass to grams before substituting. For compounds, add up all atomic masses: M(H₂O) = 2(1.008) + 16.00 = 18.016 g mol⁻¹.
Mole–Particles Relationship
n = NNA
also: N = n × NA
Nnumber of particles (atoms, molecules, ions, formula units), no unit
NAAvogadro's number = 6.022 × 10²³ mol⁻¹
HSC tip: Specify what is being counted. 1 mol of H₂ contains 6.022 × 10²³ molecules, but 2 × 6.022 × 10²³ atoms.
Mole–Gas Volume Relationship
n = VVm
also: V = n × Vm
Vvolume of gas unit: L
Vmmolar volume depends on conditions:
STP (0°C, 100 kPa): Vm = 22.7 L mol⁻¹
SLC (25°C, 100 kPa): Vm = 24.8 L mol⁻¹
STP (0°C, 100 kPa): Vm = 22.7 L mol⁻¹
SLC (25°C, 100 kPa): Vm = 24.8 L mol⁻¹
Common mistake: NSW HSC uses STP = 0°C and 100 kPa (not 1 atm). Always use 22.7 L mol⁻¹ unless the question specifies SLC. Check your data sheet.
Concentration
c = nV
also: n = c × V | V = n ÷ c
cconcentration unit: mol L⁻¹ (also written M or mol/L)
nnumber of moles unit: mol
Vvolume of solution unit: L (convert mL ÷ 1000)
Unit trap: Volume must be in litres (L), not mL. 250 mL = 0.250 L. Write the conversion explicitly in your working.
Dilution Formula
c1V1 = c2V2
c1initial concentration unit: mol L⁻¹
V1volume of stock solution taken unit: L (or both in mL, units must match)
c2final (diluted) concentration unit: mol L⁻¹
V2final total volume unit: L (or both in mL)
Why it works: Dilution doesn't change the number of moles, only the volume increases. Since n = cV, n stays constant: c₁V₁ = c₂V₂.
Percentage Composition by Mass
% = n × ArMr × 100
nnumber of that atom in the formula (subscript)
Arrelative atomic mass of the element, from periodic table
Mrrelative molecular mass (molar mass) of the compound
Example: % of O in H₂SO₄ (M = 98): % = (4 × 16.00 / 98.09) × 100 = 65.3%
Percentage Yield
% yield = actual yieldtheoretical yield × 100
actualmass (or moles) actually obtained in the experiment, unit: g or mol
theoreticalmass (or moles) calculated from stoichiometry (assuming 100% conversion), unit: g or mol
HSC note: Both values must be in the same unit (both grams or both moles). Always calculate theoretical yield first using the limiting reagent.
Percentage Purity
% purity = mpuremsample × 100
mpuremass of pure substance in the sample, unit: g
msampletotal mass of impure sample unit: g
Common use: Used in gravimetric analysis and back-calculations. If a sample is 96.2% pure, then mpure = 0.962 × msample.
Stoichiometric Mole Ratio
nAnB = coeffAcoeffB
nAmoles of substance A in the reaction, unit: mol
nBmoles of substance B unit: mol
coeffstoichiometric coefficients from the balanced equation
4-step method: (1) Write balanced equation. (2) Find moles of known. (3) Apply mole ratio. (4) Convert moles of unknown to required unit.
MOD 4
Drivers of Reactions, Calorimetry
2 formulas
Heat Energy (Calorimetry)
Q = mcΔT
Qheat energy absorbed or released, unit: J (divide by 1000 for kJ)
mmass of solution unit: g (assume 1 mL water ≈ 1 g unless stated)
cspecific heat capacity of water = 4.18 J g⁻¹ °C⁻¹ (given on HSC data sheet)
ΔTchange in temperature = Tfinal − Tinitialunit: °C
Sign convention: If temperature rises, the reaction is exothermic (heat released to solution, Q is positive here, but ΔHrxn is negative). Always state whether energy is released or absorbed.
Molar Enthalpy Change
ΔH = −Qn
ΔHenthalpy change unit: kJ mol⁻¹ (negative = exothermic, positive = endothermic)
Qheat energy from calorimetry (Q = mcΔT), unit: kJ
nmoles of substance reacted (use limiting reagent), unit: mol
The negative sign: If Q is positive (solution warmed, heat released by reaction), ΔH must be negative (exothermic). The sign flip ensures the system's perspective is correct. Always convert Q to kJ before dividing.
DATA
Constants & Reference Values
4 values
HSC Chemistry Data Sheet, Key Values
NA
Avogadro's number = 6.022 × 10²³ mol⁻¹
Vm
Molar volume at STP (0°C, 100 kPa) = 22.7 L mol⁻¹
Vm
Molar volume at SLC (25°C, 100 kPa) = 24.8 L mol⁻¹
cw
Specific heat capacity of water = 4.18 J g⁻¹ °C⁻¹
Exam advice: These values are provided on the NESA HSC Chemistry reference sheet. You don't need to memorise them, but you do need to know when and how to use them.