Unit 5: Practical and Synoptic Capstone Flashcards

Weigh the weighing bottle containing the solid, transfer the solid into a beaker, and reweigh the empty bottle. The accurate mass transferred equals initial mass minus final mass.

To thoroughly mix and homogenise the solution, ensuring the solute concentration is completely uniform throughout the entire volume.

Because two independent readings are taken (initial reading and final reading), so the total absolute uncertainty is 2 × individual instrument uncertainty.

Water does not react with the acid or base, so the total amount (number of moles) of reactant placed in the conical flask remains unchanged.

Titres that agree within 0.10 cm³ of each other. Only concordant titres are used to calculate the mean titre.

Ethanedioic acid is a weak acid and NaOH is a strong base, so the equivalence point lies in the basic region (pH 8-9). Phenolphthalein changes colour in this vertical pH range (pH 8.3-10.0), whereas methyl orange changes colour prematurely in the acidic region (pH 3.1-4.4).

Colourless to the first appearance of a permanent pale pink.

1. Dissolve solid in minimum volume of hot solvent.
2. Filter hot solution through fluted paper to remove insoluble impurities.
3. Cool filtrate slowly in ice to crystallise pure compound.
4. Filter under reduced pressure (Buchner funnel), wash with ice-cold solvent, and dry.

To ensure the solution is saturated at boiling temperature so that maximum crystal precipitation occurs upon cooling, preserving percentage yield.

To remove soluble impurities from crystal surfaces without dissolving the purified solid crystals.

1. The melting point is sharp (melting range ≤ 1-2 °C).
2. The measured melting point matches the accepted literature value.

The purified product displays only a single spot on the developed plate, whereas the crude material shows multiple spots.

Absorbance is proportional to the concentration of the coloured species (at low concentrations). Beyond the OxfordAQA specification: this is the Beer-Lambert law.

Choose the colour complementary to the solution colour (e.g. red/orange filter for deep-blue copper(II) solution) to maximise light absorption and measurement sensitivity.

The calibration line is only reliable over the range of the standard solutions. At high concentrations the line curves, so dilute the sample until its absorbance lies within the range of the standards.

Because rate = Δ[S] / t, and the mass of solid sulfur needed to obscure the black cross is fixed (Δ[S] is constant).

Gradient = -Ea / R. Therefore, Ea = -Gradient × R (multiply by -8.31 and convert J mol⁻¹ to kJ mol⁻¹ by dividing by 1000).

Human reaction time uncertainty (±0.2 s) and subjective judgment of when the cross is obscured introduce much larger errors than digital stopwatch resolution.

Use a light sensor and datalogger to monitor light transmission through the mixture continuously and objectively.

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O (1:5 stoichiometric ratio).

Chloride ions (Cl⁻) would be oxidised by MnO₄⁻ to toxic chlorine gas (Cl₂), consuming extra titrant and causing an overestimation of the titre.

It is self-indicating: deep purple MnO₄⁻ is reduced to colourless Mn²⁺. The endpoint is signalled by the first permanent pale pink colour from the first excess drop of MnO₄⁻.

To ensure the condenser jacket fills completely without trapping air bubbles, providing maximum cooling efficiency.

They provide nucleation sites for smooth bubble formation, preventing violent boiling, splashing, or liquid bumping into the condenser.

Opposite the side-arm opening (T-junction) to accurately measure the temperature of the vapor entering the condenser.

To compensate for heat loss to the surroundings that occurs while the reaction takes place, allowing an accurate theoretical maximum ΔT to be determined.

Add dilute HCl followed by aqueous BaCl₂. A white precipitate of BaSO₄ forms. HCl is added first to destroy carbonate or sulfite ions that would also form white precipitates.

Draw a tangent to the curve at time t = 0 and calculate its gradient: Initial Rate = Δ[concentration] / Δt.

Because rate = Δ[S] / t, and the mass of precipitate required to obscure the black cross is fixed (Δ[S] is constant).

Gradient = -Ea / R. To find Ea: multiply gradient by -8.314 J K⁻¹ mol⁻¹ and divide by 1000 to convert to kJ mol⁻¹.

Absorbance of monochromatic light is directly proportional to the concentration of absorbing solute: A = εcl.

Choose the filter whose colour is complementary to the solution colour (e.g. red/orange filter for blue copper(II) solution) to maximise light absorption.

The calibration line is only reliable over the range of the standard solutions. At high concentrations the line curves, so dilute the sample until its absorbance lies within the range of the standards.

Exactly half of the acid is converted to conjugate base, so [HA] = [A⁻], which means pH = pKa.

To compensate for cooling / heat loss to surroundings during the reaction, allowing determination of theoretical maximum temperature rise (ΔT).

To ensure the cooling jacket completely fills with water against gravity, preventing air bubbles and hot spots, which maximises condensing efficiency.

Heating a closed system causes pressure to build up as gases expand and liquids boil, creating a severe risk of an explosion.

They provide nucleation sites for smooth bubble formation, preventing superheating and violent boiling (bumping) of the liquid into the condenser.

Opposite the side-arm opening leading to the condenser (T-junction), so the thermometer measures the exact boiling temperature of the vapour being condensed.

The compound must be very soluble in the solvent at high temperature (boiling) but virtually insoluble in the solvent at low temperature (ice-cold).

Fluted paper increases surface area and filtration speed, and removes insoluble impurities before the hot solution cools and crystallises in the funnel.

Reduced pressure pulls liquid through rapidly, producing faster separation and leaving the crystals significantly drier.

An impure sample melts over a broad temperature range (greater than 2 °C) and at a temperature lower (depressed) than the pure literature melting point.

Two independent readings are taken on the scale (initial reading and final reading). Total uncertainty = 2 × 0.05 = ±0.10 cm³.

Two readings are recorded: mass of weighing bottle plus solid, and mass of empty bottle after transfer. Total uncertainty = 2 × balance uncertainty.

Temperature rise ΔT = 7.5 °C. Two readings are taken, so total uncertainty = 2 × 0.5 = 1.0 °C.
Percentage uncertainty = (1.0 / 7.5) × 100% = 13.3%.

Add the individual percentage uncertainties together: % Total = % A + % B + % C.

Increase the titre volume (e.g. by using a larger mass/volume of sample or diluting the titrant in the burette), since percentage uncertainty is inversely proportional to volume delivered.

Systematic error: Causes all readings to be shifted by a constant amount in one direction (e.g. a wrongly calibrated balance or burette).
Random error: Causes unpredictable scatter on either side of the true mean value (e.g. judgment of meniscus position).
A trapped air bubble in the burette jet is a procedural error that makes that titre too large.

Calculating a mean from concordant repeats averages out unpredictable random fluctuations. A systematic error persists identically across all repeats and cannot be averaged out.