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24 exam-style questions with model answers, plus 32 quick multi-choice questions — every question on the site for this standard, grouped by the 8 pages of notes they come from.
Write a full answer before you reveal the model one — that comparison is where the marks come from. Every block links back to the notes that teach it.
State the two relationships that must be used in this investigation, define each symbol with its unit, and state what each relationship is used for.
A student plans to determine the concentration of hydrochloric acid in a descaling product by titrating it against sodium hydroxide solution. Explain why this plan does not meet all the requirements of the standard, and describe a change that would fix it.
Two students investigate the concentration of ethanoic acid in vinegar. Student A titrates the vinegar directly. Student B dilutes the vinegar tenfold first. Both obtain the same final answer. Evaluate which investigation is the better piece of work for this standard, and justify your judgement.
Describe two problems a consumer product might present that would require the sample to be modified before titration, and state the modification for each.
Explain why a back titration is used to find the amount of calcium carbonate in an antacid tablet, rather than titrating the tablet directly with acid.
A student determining the ethanoic acid content of a dark balsamic vinegar dilutes the sample 20-fold so the phenolphthalein endpoint is visible. Justify how this modification affects the validity and the accuracy of the investigation, and evaluate one drawback.
State what each of the burette, the pipette and the conical flask should be rinsed with before a titration.
Explain the effect on the calculated concentration if a student rinses the conical flask with the sample solution instead of distilled water.
A student obtains three titres agreeing to within 0.05 mL but the final concentration is 8% higher than the manufacturer's stated value. Analyse what this tells you about the errors in the investigation, and justify a set of checks to locate the problem.
State three properties a substance must have to be used as a primary standard, and give one example of a substance that is a primary standard and one that is not.
Explain why phenolphthalein rather than methyl orange must be used when titrating ethanoic acid in vinegar with sodium hydroxide.
A student standardises their sodium hydroxide solution against sodium carbonate, then uses it to titrate vinegar. A classmate argues the standardisation is a waste of time because the NaOH bottle is labelled 0.100 mol L−1. Evaluate this argument and justify whether the standardisation is necessary.
Describe how three significant variables were controlled in a titration investigation.
Explain the difference between a random error and a systematic error in a titration, giving one example of each, and explain why repeating a titration helps with only one of them.
A student writes in their evaluation: "My results were accurate because my three titres agreed to within 0.05 mL." Justify why this statement is chemically unsound, and describe what evidence would be needed to support a claim of accuracy.
A 20.0 mL aliquot of a solution of sodium carbonate required 18.60 mL of 0.1004 mol L−1 hydrochloric acid to reach the endpoint. The equation is 2HCl + Na2CO3 → 2NaCl + H2O + CO2. Calculate the concentration of the sodium carbonate solution.
A vitamin C tablet is dissolved and made up to 250.0 mL. A 25.0 mL aliquot required 14.85 mL of 0.01012 mol L−1 iodine solution. The reaction is C6H8O6 + I2 → C6H6O6 + 2HI. Calculate the mass of vitamin C in the tablet and compare it with a label claim of 500 mg. (M(C6H8O6) = 176.14 g mol−1)
A student calculating the concentration of ethanoic acid in vinegar obtains 8.25 mol L−1. The label states 4% acidity. Evaluate this result, identify the most likely error, and justify your reasoning.
State the three quantities of reagent that must be calculated in a back titration, and the relationship between them.
1.000 g of impure calcium carbonate was added to 40.0 mL of 0.500 mol L−1 HCl. The excess acid required 22.50 mL of 0.200 mol L−1 NaOH. Calculate the percentage purity of the calcium carbonate. (M = 100.09 g mol−1)
A student performing a back titration on an antacid tablet does not warm the mixture and titrates it after only two minutes. Some undissolved solid is still visible. Analyse the effect this has on the calculated mass of calcium carbonate, and justify your reasoning.
A calculation uses a titre of 18.65 mL, a standard solution of 0.1004 mol L−1 and a 25.0 mL pipetted aliquot. State how many significant figures the final answer should have and explain why.
Explain why rounding intermediate values in a titration calculation is poor practice, and illustrate with a numerical example.
A student determines a household bleach to contain sodium hypochlorite at 0.732 mol L−1, against a label claim of "4.2% available chlorine". Evaluate this outcome in relation to the consumer product. (M(NaOCl) = 74.44 g mol−1; % available chlorine is conventionally expressed as the equivalent mass of Cl2, M = 70.90 g mol−1)