(a) - Finds n(HCl) = 0.02500 × 0.1200 = 0.003000 mol. - Finds n(NaOH) = 0.02000 × 0.1000 = 0.002000 mol. - Finds excess H₃O⁺ = 0.001000 mol in 0.04500 L, so [H₃O⁺] = 0.02222 mol L⁻¹. - Finds pH = −log₁₀(0.02222) = 1.65.

- Finds n(HCl) = 0.02500 × 0.1200 = 0.003000 mol. - Finds n(NaOH) = 0.02000 × 0.1000 = 0.002000 mol. - Finds excess H₃O⁺ = 0.001000 mol in 0.04500 L, so [H₃O⁺] = 0.02222 mol L⁻¹. - Finds pH = −log₁₀(0.02222) = 1.65.

(b) - Uses the 1:1 HCl/NaOH neutralisation stoichiometry to explain why strong acid remains. - Explains that water lowers the concentration of remaining acid but does not consume its amount. - Rejects the claim that dilution alone makes the mixture neutral.

- Uses the 1:1 HCl/NaOH neutralisation stoichiometry to explain why strong acid remains. - Explains that water lowers the concentration of remaining acid but does not consume its amount. - Rejects the claim that dilution alone makes the mixture neutral.

(c) - Measures pH after thorough mixing with a calibrated probe. - Records delivered reagent volumes and concentrations rather than relying on added water volume. - Repeats the treatment and checks the final pH against the stated discharge specification.

- Measures pH after thorough mixing with a calibrated probe. - Records delivered reagent volumes and concentrations rather than relying on added water volume. - Repeats the treatment and checks the final pH against the stated discharge specification.

(a) - Identifies Pb²⁺ ions moving towards the negative cathode. - Identifies Br⁻ ions moving towards the positive anode. - Names lead at the cathode and bromine at the anode as the electrode products.

- Identifies Pb²⁺ ions moving towards the negative cathode. - Identifies Br⁻ ions moving towards the positive anode. - Names lead at the cathode and bromine at the anode as the electrode products.

(b) - Writes Pb²⁺ + 2e⁻ → Pb at the cathode. - Writes 2Br⁻ → Br₂ + 2e⁻ at the positive anode. - Combines the half-equations to give PbBr₂ → Pb + Br₂. - Explains that the external supply drives this non-spontaneous decomposition.

- Writes Pb²⁺ + 2e⁻ → Pb at the cathode. - Writes 2Br⁻ → Br₂ + 2e⁻ at the positive anode. - Combines the half-equations to give PbBr₂ → Pb + Br₂. - Explains that the external supply drives this non-spontaneous decomposition.

(c) - Contains and measures bromine without release to the laboratory air. - Separates collected lead from the molten salt while avoiding contact with hot equipment.

- Contains and measures bromine without release to the laboratory air. - Separates collected lead from the molten salt while avoiding contact with hot equipment.

(a) - Classifies propan-1-ol as a primary alcohol. - Identifies the product as propanal, CH₃CH₂CHO.

- Classifies propan-1-ol as a primary alcohol. - Identifies the product as propanal, CH₃CH₂CHO.

(b) - Uses an acidified oxidising agent and heating, with the aldehyde distilled as it forms. - For acidified dichromate, states the observable colour change from orange to green.

- Uses an acidified oxidising agent and heating, with the aldehyde distilled as it forms. - For acidified dichromate, states the observable colour change from orange to green.

(a) - Uses percentage yield = actual/theoretical × 100 = 5.76/8.00 × 100. - Obtains 72.0% (allow rounding consistent with the supplied masses).

- Uses percentage yield = actual/theoretical × 100 = 5.76/8.00 × 100. - Obtains 72.0% (allow rounding consistent with the supplied masses).

(b) - Identifies concentrated sulfuric acid as an acid catalyst that increases the rates of both forward and reverse reactions. - Explains that heating increases reaction rate but does not itself guarantee a greater equilibrium yield. - Explains that using excess reactant or removing water/ester shifts the equilibrium towards ester products.

- Identifies concentrated sulfuric acid as an acid catalyst that increases the rates of both forward and reverse reactions. - Explains that heating increases reaction rate but does not itself guarantee a greater equilibrium yield. - Explains that using excess reactant or removing water/ester shifts the equilibrium towards ester products.

(c) - Recognises that reversible esterification may not proceed to completion. - Identifies product loss during transfer, separation or purification as a further cause of reduced isolated yield.

- Recognises that reversible esterification may not proceed to completion. - Identifies product loss during transfer, separation or purification as a further cause of reduced isolated yield.

(a) - Shows that CH₃COO⁻ consumes added H₃O⁺ to form CH₃COOH and water. - Shows that CH₃COOH consumes added OH⁻ to form CH₃COO⁻ and water. - Links removal of most added acid or base to the small measured pH changes.

- Shows that CH₃COO⁻ consumes added H₃O⁺ to form CH₃COOH and water. - Shows that CH₃COOH consumes added OH⁻ to form CH₃COO⁻ and water. - Links removal of most added acid or base to the small measured pH changes.

(b) - Recognises that capacity depends on the amounts of both weak acid and conjugate base present. - Explains that a sufficiently large acid addition exhausts the conjugate-base component. - Explains that a sufficiently large base addition exhausts the weak-acid component. - States that the three plotted endpoints demonstrate resistance only for the tested additions, not unlimited buffering.

- Recognises that capacity depends on the amounts of both weak acid and conjugate base present. - Explains that a sufficiently large acid addition exhausts the conjugate-base component. - Explains that a sufficiently large base addition exhausts the weak-acid component. - States that the three plotted endpoints demonstrate resistance only for the tested additions, not unlimited buffering.

(a) - Classifies the process as formation of an addition polymer. - Gives the repeating structural section as –CH₂–CH₂– with continuation bonds and brackets understood. - States that the alkene double bond opens during chain growth and no small-molecule by-product is formed.

- Classifies the process as formation of an addition polymer. - Gives the repeating structural section as –CH₂–CH₂– with continuation bonds and brackets understood. - States that the alkene double bond opens during chain growth and no small-molecule by-product is formed.

(b) - Explains the structure-property link: flexible non-polar chains interact through dispersion forces. - Relates stronger interchain attractions or closer packing to greater energy needed for chain movement where relevant. - Makes a polymer-specific end-of-life statement, such as sorting/reprocessing constraints, persistence, recovery or controlled reuse.

- Explains the structure-property link: flexible non-polar chains interact through dispersion forces. - Relates stronger interchain attractions or closer packing to greater energy needed for chain movement where relevant. - Makes a polymer-specific end-of-life statement, such as sorting/reprocessing constraints, persistence, recovery or controlled reuse.

(a) - Rearranges PV = nRT to n = PV/(RT) and uses the stated units consistently. - Obtains n(NH₃) = (101 × 2.60)/(8.314 × 298) = 0.1060 mol. - Obtains m(NH₃) = nM = 1.805 g.

- Rearranges PV = nRT to n = PV/(RT) and uses the stated units consistently. - Obtains n(NH₃) = (101 × 2.60)/(8.314 × 298) = 0.1060 mol. - Obtains m(NH₃) = nM = 1.805 g.

(b) - Uses the initial gradient as evidence for the highest initial production rate. - Links the flattening curve to decreasing reactant concentrations and approach to equilibrium. - Links decreasing reactant or substrate concentration to fewer effective collisions per unit time and therefore a lower production rate. - Relates the changing production rate to the supplied process chemistry without inferring an unmeasured temperature change.

- Uses the initial gradient as evidence for the highest initial production rate. - Links the flattening curve to decreasing reactant concentrations and approach to equilibrium. - Links decreasing reactant or substrate concentration to fewer effective collisions per unit time and therefore a lower production rate. - Relates the changing production rate to the supplied process chemistry without inferring an unmeasured temperature change.

(c) - Keeps initial nitrogen-to-hydrogen ratio, reactor volume and catalyst mass constant. - Uses the same calibrated measurement method and compares gradients over the same defined time interval. - Performs independent repeat runs and reports a representative result together with spread or uncertainty.

- Keeps initial nitrogen-to-hydrogen ratio, reactor volume and catalyst mass constant. - Uses the same calibrated measurement method and compares gradients over the same defined time interval. - Performs independent repeat runs and reports a representative result together with spread or uncertainty.

(a) - Identifies chip size as the independent variable. - Identifies initial carbon dioxide production rate, measured in mL min⁻¹, as the dependent variable. - Identifies initial CaCO₃ mass as a relevant controlled variable.

- Identifies chip size as the independent variable. - Identifies initial carbon dioxide production rate, measured in mL min⁻¹, as the dependent variable. - Identifies initial CaCO₃ mass as a relevant controlled variable.

(b) - Uses the spread among replicate initial carbon dioxide production rate results at each chip size level to assess random variation. - Compares the separation between level means with within-level spread before supporting the conclusion that initial CO₂ production is faster with crushed chips under the tested conditions. - Proposes test additional size fractions with independent repeats.

- Uses the spread among replicate initial carbon dioxide production rate results at each chip size level to assess random variation. - Compares the separation between level means with within-level spread before supporting the conclusion that initial CO₂ production is faster with crushed chips under the tested conditions. - Proposes test additional size fractions with independent repeats.

(a) - Writes iodide oxidation as 2I⁻ → I₂ + 2e⁻. - Writes chlorine reduction as Cl₂ + 2e⁻ → 2Cl⁻. - Combines the half-equations with electrons cancelled to recover the supplied net ionic equation.

- Writes iodide oxidation as 2I⁻ → I₂ + 2e⁻. - Writes chlorine reduction as Cl₂ + 2e⁻ → 2Cl⁻. - Combines the half-equations with electrons cancelled to recover the supplied net ionic equation.

(b) - Identifies chlorine as the oxidising agent because it accepts electrons. - Identifies iodide as the reducing agent because it donates electrons. - Links the brown colour to iodine formed from iodide, rather than to potassium ions.

- Identifies chlorine as the oxidising agent because it accepts electrons. - Identifies iodide as the reducing agent because it donates electrons. - Links the brown colour to iodine formed from iodide, rather than to potassium ions.

(a) - Writes Kc = [FeSCN²⁺]/([Fe³⁺][SCN⁻]). - Finds Kc for mixture A = 0.00300/(0.0200 × 0.0150) = 10.0. - Finds Kc for mixture B = 0.00300/(0.0100 × 0.0300) = 10.0.

- Writes Kc = [FeSCN²⁺]/([Fe³⁺][SCN⁻]). - Finds Kc for mixture A = 0.00300/(0.0200 × 0.0150) = 10.0. - Finds Kc for mixture B = 0.00300/(0.0100 × 0.0300) = 10.0.

(b) - Rejects the claim because both equilibria are at 298 K and give the same Kc. - Distinguishes differing equilibrium component concentrations from the constant equilibrium concentration ratio. - Uses the equal [Fe³⁺][SCN⁻] products and equal [FeSCN²⁺] results to account for the matching ratios. - Identifies a temperature change, rather than a different starting composition, as a way to alter Kc.

- Rejects the claim because both equilibria are at 298 K and give the same Kc. - Distinguishes differing equilibrium component concentrations from the constant equilibrium concentration ratio. - Uses the equal [Fe³⁺][SCN⁻] products and equal [FeSCN²⁺] results to account for the matching ratios. - Identifies a temperature change, rather than a different starting composition, as a way to alter Kc.

(a) - Writes the equilibrium expression Kc = [NO₂]²/[N₂O₄]. - Substitutes the equilibrium concentrations as (0.0850)²/0.120. - Obtains Kc = 0.060 at the stated temperature. - Writes Qc with the same species and stoichiometric powers as Kc. - Substitutes the immediately perturbed concentrations as (0.110)²/0.120. - Obtains Qc = 0.101. - Predicts net reaction left towards N₂O₄ because Qc > Kc.

- Writes the equilibrium expression Kc = [NO₂]²/[N₂O₄]. - Substitutes the equilibrium concentrations as (0.0850)²/0.120. - Obtains Kc = 0.060 at the stated temperature. - Writes Qc with the same species and stoichiometric powers as Kc. - Substitutes the immediately perturbed concentrations as (0.110)²/0.120. - Obtains Qc = 0.101. - Predicts net reaction left towards N₂O₄ because Qc > Kc.

(b) - States that forward and reverse reactions continue in the closed system. - States that their rates are equal at dynamic equilibrium even when concentrations differ. - Uses the correct concentration relationship Kc = [NO₂]²/[N₂O₄]. - Explains that stoichiometric coefficients become powers and pure solids or liquids are omitted where relevant. - Uses the graph and the statement that the forward reaction is endothermic to explain the temperature trend.

- States that forward and reverse reactions continue in the closed system. - States that their rates are equal at dynamic equilibrium even when concentrations differ. - Uses the correct concentration relationship Kc = [NO₂]²/[N₂O₄]. - Explains that stoichiometric coefficients become powers and pure solids or liquids are omitted where relevant. - Uses the graph and the statement that the forward reaction is endothermic to explain the temperature trend.

(c) - Requires temperature to be measured and held constant before comparing Kc values. - Requires the vessel to remain sealed except for the stated controlled perturbation. - Requires sufficient time for composition readings to stabilise before an equilibrium value is recorded. - Uses calibrated composition measurements rather than colour or pressure alone when several species contribute. - Recognises that sampling or injection changes volume and composition unless explicitly controlled.

- Requires temperature to be measured and held constant before comparing Kc values. - Requires the vessel to remain sealed except for the stated controlled perturbation. - Requires sufficient time for composition readings to stabilise before an equilibrium value is recorded. - Uses calibrated composition measurements rather than colour or pressure alone when several species contribute. - Recognises that sampling or injection changes volume and composition unless explicitly controlled.

(a) - Selects +0.34 V as E°cathode and -0.76 V as E°anode for the proposed cell. - Substitutes E°cell = +0.34 − (-0.76). - Obtains E°cell = 1.10 V for the proposed spontaneous direction. - Obtains E°cell = -1.10 V when the proposed overall reaction is reversed. - Substitutes E°cell = +0.80 − (-0.76) for the alternative pairing. - Obtains E°cell = 1.56 V for the alternative pairing.

- Selects +0.34 V as E°cathode and -0.76 V as E°anode for the proposed cell. - Substitutes E°cell = +0.34 − (-0.76). - Obtains E°cell = 1.10 V for the proposed spontaneous direction. - Obtains E°cell = -1.10 V when the proposed overall reaction is reversed. - Substitutes E°cell = +0.80 − (-0.76) for the alternative pairing. - Obtains E°cell = 1.56 V for the alternative pairing.

(b) - Anode and cathode assignment: identifies Zn as the anode and Cu as the cathode in the proposed galvanic cell. - Oxidation half-equation: writes Zn(s) → Zn²⁺(aq) + 2e⁻. - Reduction half-equation: writes Cu²⁺(aq) + 2e⁻ → Cu(s). - Electron balance: balances electrons before adding the two half-equations. - External electron flow: states that electrons move from the Zn electrode to the Cu electrode.

- Anode and cathode assignment: identifies Zn as the anode and Cu as the cathode in the proposed galvanic cell. - Oxidation half-equation: writes Zn(s) → Zn²⁺(aq) + 2e⁻. - Reduction half-equation: writes Cu²⁺(aq) + 2e⁻ → Cu(s). - Electron balance: balances electrons before adding the two half-equations. - External electron flow: states that electrons move from the Zn electrode to the Cu electrode.

(c) - Spontaneity from E°cell: uses the positive calculated potential to support the proposed direction under standard conditions. - Stronger oxidising agent: identifies the ion with the more positive reduction potential in the compared pairings. - Voltage comparison: uses both calculated cell potentials rather than comparing electrode potentials in isolation. - Standard-condition limitation: recognises that practical concentrations and temperature can change measured voltage. - Salt-bridge fault diagnosis: predicts rapid voltage loss if ion migration cannot maintain charge balance. The overall judgement connects relevant chemistry to the supplied evidence in a coherent explanation.

- Spontaneity from E°cell: uses the positive calculated potential to support the proposed direction under standard conditions. - Stronger oxidising agent: identifies the ion with the more positive reduction potential in the compared pairings. - Voltage comparison: uses both calculated cell potentials rather than comparing electrode potentials in isolation. - Standard-condition limitation: recognises that practical concentrations and temperature can change measured voltage. - Salt-bridge fault diagnosis: predicts rapid voltage loss if ion migration cannot maintain charge balance. The overall judgement connects relevant chemistry to the supplied evidence in a coherent explanation.

(a) - Converts the mean titre to 0.02120 L. - Finds n(NaOH) = 0.110 × 0.02120 = 0.002332 mol. - Uses the stated 1:1 neutralisation ratio to obtain the amount of acid in the aliquot. - Finds c(CH₃COOH) = 0.002332/0.02000 = 0.1166 mol L⁻¹.

- Converts the mean titre to 0.02120 L. - Finds n(NaOH) = 0.110 × 0.02120 = 0.002332 mol. - Uses the stated 1:1 neutralisation ratio to obtain the amount of acid in the aliquot. - Finds c(CH₃COOH) = 0.002332/0.02000 = 0.1166 mol L⁻¹.

(b) - Weak-acid equilibrium equation: writes CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq). - Meaning of Ka: explains the equilibrium tendency of the weak acid to transfer a proton to water. - Initial partial ionisation: links the initial pH to hydronium ions formed without assuming complete dissociation. - Buffer region: identifies a mixture containing appreciable weak acid and conjugate base. - Response to added acid: explains that CH₃COO⁻ consumes added H₃O⁺. - Response to added base: explains that CH₃COOH consumes added OH⁻.

- Weak-acid equilibrium equation: writes CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq). - Meaning of Ka: explains the equilibrium tendency of the weak acid to transfer a proton to water. - Initial partial ionisation: links the initial pH to hydronium ions formed without assuming complete dissociation. - Buffer region: identifies a mixture containing appreciable weak acid and conjugate base. - Response to added acid: explains that CH₃COO⁻ consumes added H₃O⁺. - Response to added base: explains that CH₃COOH consumes added OH⁻.

(c) - Concordant-titre mean: excludes the rough titre and averages concordant titres. - Volumetric glassware rinsing: explains conditioning the pipette and burette with their working solutions. - Meniscus reading: explains viewing at eye level to reduce parallax error. - Endpoint approach: specifies dropwise titrant addition and swirling near the colour change. - Titrant standardisation: checks the NaOH concentration against a suitable primary standard. The overall judgement connects relevant chemistry to the supplied evidence in a coherent explanation.

- Concordant-titre mean: excludes the rough titre and averages concordant titres. - Volumetric glassware rinsing: explains conditioning the pipette and burette with their working solutions. - Meniscus reading: explains viewing at eye level to reduce parallax error. - Endpoint approach: specifies dropwise titrant addition and swirling near the colour change. - Titrant standardisation: checks the NaOH concentration against a suitable primary standard. The overall judgement connects relevant chemistry to the supplied evidence in a coherent explanation.

(a) - Identifies the 1:1 feed requirement from the supplied repeat-unit stoichiometry. - Identifies 0.100 mol hexanedioic acid as limiting. - Finds theoretical dry polymer mass = 0.100 × 226.32 = 22.632 g. - Finds isolated yield = 18.10/22.632 × 100 = 80.0%.

- Identifies the 1:1 feed requirement from the supplied repeat-unit stoichiometry. - Identifies 0.100 mol hexanedioic acid as limiting. - Finds theoretical dry polymer mass = 0.100 × 226.32 = 22.632 g. - Finds isolated yield = 18.10/22.632 × 100 = 80.0%.

(b) - Identifies the two bifunctional monomers as a diamine and a dicarboxylic acid. - Identifies amide links in the polymer backbone. - Uses the supplied stoichiometry to account for eliminated water. - Recognises 0.020 mol diamine remains in excess initially in the first pilot. - Explains why matching reactive-group amounts can support longer chains. - Distinguishes isolated dry mass from chemical conversion.

- Identifies the two bifunctional monomers as a diamine and a dicarboxylic acid. - Identifies amide links in the polymer backbone. - Uses the supplied stoichiometry to account for eliminated water. - Recognises 0.020 mol diamine remains in excess initially in the first pilot. - Explains why matching reactive-group amounts can support longer chains. - Distinguishes isolated dry mass from chemical conversion.

(c) - Compares 18.80 g with 18.10 g dry recovery. - Avoids assigning the recovery difference solely to feed ratio from single trials. - Repeats both feed conditions independently. - Keeps temperature, time and separation procedure fixed. - Dries products to constant mass before comparing yield.

- Compares 18.80 g with 18.10 g dry recovery. - Avoids assigning the recovery difference solely to feed ratio from single trials. - Repeats both feed conditions independently. - Keeps temperature, time and separation procedure fixed. - Dries products to constant mass before comparing yield.

(a) - Finds recycled feed = 1000 × 0.25 = 250 kg. - Finds virgin feed = 1000 − 250 = 750 kg. - Finds virgin-feed energy = 750 × 0.075 = 56.25 GJ. - Finds recycled-feed energy = 250 × 0.030 = 7.50 GJ.

- Finds recycled feed = 1000 × 0.25 = 250 kg. - Finds virgin feed = 1000 − 250 = 750 kg. - Finds virgin-feed energy = 750 × 0.075 = 56.25 GJ. - Finds recycled-feed energy = 250 × 0.030 = 7.50 GJ.

(b) - Classifies polyethene as an addition polymer. - Represents the repeat unit as –CH₂–CH₂– with continuation bonds and brackets understood. - Explains that alkene double bonds open to form a saturated covalent backbone without a small-molecule by-product. - Explains the supplied structural claim that flexible non-polar chains interact through dispersion forces. - Compares 520.0 with 465.0 % and states that the recycled blend retains the required ductility.

- Classifies polyethene as an addition polymer. - Represents the repeat unit as –CH₂–CH₂– with continuation bonds and brackets understood. - Explains that alkene double bonds open to form a saturated covalent backbone without a small-molecule by-product. - Explains the supplied structural claim that flexible non-polar chains interact through dispersion forces. - Compares 520.0 with 465.0 % and states that the recycled blend retains the required ductility.

(c) - Uses total energy per 1000 kg functional unit and the calculated 15.0% reduction in the comparison. - Uses the measured elongation at break and the at least 420% specification when judging fitness for service. - Recognises that recycled content can reduce virgin feed demand and divert material from disposal. - Recognises that collection, sorting, contamination and chain degradation can constrain recycling. - Recommends a recycled-content strategy conditional on both measured performance and lifecycle burden.

- Uses total energy per 1000 kg functional unit and the calculated 15.0% reduction in the comparison. - Uses the measured elongation at break and the at least 420% specification when judging fitness for service. - Recognises that recycled content can reduce virgin feed demand and divert material from disposal. - Recognises that collection, sorting, contamination and chain degradation can constrain recycling. - Recommends a recycled-content strategy conditional on both measured performance and lifecycle burden.

(a) - Identifies CH₃COO⁻:CH₃COOH mole ratio as the deliberately changed variable. - Identifies pH decrease after HCl addition, measured in pH units, as the dependent variable. - Explains why controlling total buffer concentration supports a valid comparison. - Explains why controlling sample volume supports a valid comparison. - Explains why controlling HCl amount supports a valid comparison. - Recognises that three independent results at each of 3 levels permit reliability to be assessed.

- Identifies CH₃COO⁻:CH₃COOH mole ratio as the deliberately changed variable. - Identifies pH decrease after HCl addition, measured in pH units, as the dependent variable. - Explains why controlling total buffer concentration supports a valid comparison. - Explains why controlling sample volume supports a valid comparison. - Explains why controlling HCl amount supports a valid comparison. - Recognises that three independent results at each of 3 levels permit reliability to be assessed.

(b) - Uses the three results at 0.50 to describe within-level spread rather than selecting one trial. - Uses the three results at 2.00 to describe within-level spread rather than selecting one trial. - Compares the mean response of 0.35 pH units at 0.50 with 0.11 pH units at 2.00. - Uses all 3 level means to describe the observed pattern in pH decrease after HCl addition. - Compares the size of the between-level change with the within-level variation.

- Uses the three results at 0.50 to describe within-level spread rather than selecting one trial. - Uses the three results at 2.00 to describe within-level spread rather than selecting one trial. - Compares the mean response of 0.35 pH units at 0.50 with 0.11 pH units at 2.00. - Uses all 3 level means to describe the observed pattern in pH decrease after HCl addition. - Compares the size of the between-level change with the within-level variation.

(c) - Explains why ratios are prepared in ascending order using one pipette without an intermediate check restricts the original conclusion. - Proposes randomise preparation order and independently prepare every ratio. - Retains total buffer concentration, sample volume and HCl amount in the follow-up. - Includes calibrating the pH probe with two certified buffers before and after the run in the follow-up method. - Requires independent repeats at every tested level and reports means with spread or uncertainty. - Uses randomised or counterbalanced run order where time-related drift could affect measurements.

- Explains why ratios are prepared in ascending order using one pipette without an intermediate check restricts the original conclusion. - Proposes randomise preparation order and independently prepare every ratio. - Retains total buffer concentration, sample volume and HCl amount in the follow-up. - Includes calibrating the pH probe with two certified buffers before and after the run in the follow-up method. - Requires independent repeats at every tested level and reports means with spread or uncertainty. - Uses randomised or counterbalanced run order where time-related drift could affect measurements.