Chemistry • Year 12 • Module 6 • Lesson 14
Titration: Standard Solutions, Technique & Calculations
Recall primary standard criteria, titration vocabulary, equipment technique rules, and the four-step calculation sequence.
1. Term–definition match
The twelve definitions below are shuffled. Write the matching term from this list in the right-hand column: standard solution, primary standard, titration, equivalence point, endpoint, titre, concordant results, burette, pipette, conical flask, mole ratio, parallax error. 12 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A solution of accurately known concentration, used as the reference reagent in a volumetric analysis. | |
| 1.2 | A highly pure, stable, non-hygroscopic, high-molar-mass substance used to prepare or calibrate a standard solution directly from mass. | |
| 1.3 | A quantitative analytical technique in which a solution of known concentration is added to a solution of unknown concentration until the reaction is complete. | |
| 1.4 | The point in a titration at which stoichiometrically equivalent amounts of acid and base have reacted — the theoretical completion point. | |
| 1.5 | The observed indicator colour change that signals the approximate completion of the reaction — detected experimentally rather than calculated. | |
| 1.6 | The volume of standard solution delivered from the burette to reach the endpoint; used in all concentration calculations. | |
| 1.7 | Titres that agree to within ±0.10 mL of each other; only these are averaged in HSC titration reports. | |
| 1.8 | A precision glass tube, graduated to ±0.05 mL, used to deliver controlled, variable volumes of the titrant solution. | |
| 1.9 | A glass instrument that delivers a single fixed, accurately known volume of liquid; rinsed with the analyte solution before use. | |
| 1.10 | The flask that holds the analyte solution during a titration; its tapered neck allows vigorous swirling without spilling. | |
| 1.11 | The ratio of coefficients from the balanced equation that relates moles of the known reagent to moles of the unknown reagent in Step 3 of the titration calculation. | |
| 1.12 | A reading error caused by the observer’s eye not being level with the liquid surface, introducing a systematic error in all burette readings. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 12 marks (1 T/F + 1 correction where needed)
2.1 NaOH cannot be used as a primary standard because it is a strong base. T / F
2.2 The conical flask should be rinsed with the analyte solution before adding the pipetted volume to ensure no dilution occurs. T / F
2.3 Titres of 22.40 mL, 22.45 mL, and 22.50 mL are concordant because the largest difference between any two is 0.10 mL. T / F
2.4 Adding distilled water to the walls of the conical flask during a titration changes the moles of analyte and introduces a systematic error. T / F
2.5 A high molar mass is a desirable property for a primary standard because it reduces the percentage error introduced by small uncertainties in the mass weighed. T / F
2.6 The mole ratio in Step 3 of the four-step titration calculation is always 1:1, regardless of the substances involved. T / F
3. Fill in the blanks
Complete the passage using the word bank below. Each word is used once. 10 marks
A titration involves adding a solution of known concentration, called the (3.1) ________ solution, to the analyte. The known solution is prepared from a (3.2) ________ standard, which must be highly pure, (3.3) ________, non-hygroscopic, readily soluble, and have a high (3.4) ________ mass. Anhydrous sodium carbonate (Na2CO3) is used to standardise (3.5) ________, while oxalic acid dihydrate is used to standardise (3.6) ________. During the titration, the burette must be rinsed with the (3.7) ________ solution before filling to prevent dilution. The volume of standard solution delivered is called the (3.8) ________. The calculation of the unknown concentration uses the balanced equation to determine the (3.9) ________ ratio and applies n = c × V, where volume must be converted to (3.10) ________ before substituting.
Word bank: litres • standard • HCl • NaOH • mole • primary • stable • titre • molar • titrant
Stuck? Revisit Cards 1 and 3 of the lesson.4. Explain the reason for each technique requirement
For each titration technique listed, write one sentence explaining why it is done. Use the word “because” to make the causal link explicit. 8 marks (2 each)
4.1 The burette is rinsed twice with the titrant solution before filling.
4.2 Air bubbles in the burette tip are removed before starting the titration.
4.3 The burette is read at the bottom of the meniscus with the eye level with the liquid surface.
4.4 The last drop is not blown out of the pipette tip after draining.
5. Sequence the steps — preparing a standard solution
The steps for preparing a standard solution from a primary standard are listed below in shuffled order. Write the correct order number (1–6) in the “Order” column. 6 marks
| Order | Step (shuffled) |
|---|---|
| Make up to the calibration mark of the volumetric flask with distilled water; invert repeatedly to mix. | |
| Weigh the primary standard (e.g. anhydrous Na2CO3) on an analytical balance to four decimal places. | |
| Quantitatively transfer the dissolved primary standard into the volumetric flask, using a wash bottle to rinse all traces from the beaker. | |
| Calculate the mass required for the target concentration: mass = c × V × M. | |
| Dissolve the primary standard in a small volume of distilled water in a beaker, stirring until fully dissolved. | |
| Record the exact mass weighed and use it (not the target mass) in all subsequent calculations. |
Q1 — Term–definition matches
1.1 standard solution • 1.2 primary standard • 1.3 titration • 1.4 equivalence point • 1.5 endpoint • 1.6 titre • 1.7 concordant results • 1.8 burette • 1.9 pipette • 1.10 conical flask • 1.11 mole ratio • 1.12 parallax error.
Q2 — True / false
2.1 False. NaOH cannot be a primary standard because it absorbs CO2 and water vapour from air, altering its effective concentration — the reason is chemical and physical instability, not its acid/base strength.
2.2 False. The conical flask is rinsed with distilled water only before adding the pipetted volume. Rinsing with the analyte leaves extra moles of analyte on the flask walls, increasing the titre and overestimating concentration.
2.3 True. The maximum difference is 22.50 − 22.40 = 0.10 mL, which exactly meets the concordance criterion of ≤ 0.10 mL.
2.4 False. Rinsing the conical flask walls with distilled water does not change the moles of analyte already present — it is safe and does not introduce an error.
2.5 True. A high molar mass means that a larger mass is required for the same number of moles, so the absolute weighing uncertainty (±0.0001 g) is a smaller percentage of the total mass, reducing the percentage error in the moles calculated.
2.6 False. The mole ratio comes from the balanced equation and is only 1:1 when the stoichiometric coefficients of the acid and base are equal (e.g. HCl + NaOH). For H2SO4 + 2NaOH the ratio is 1:2; for H2C2O4 + 2NaOH it is also 1:2.
Q3 — Cloze answers (in order)
3.1 standard • 3.2 primary • 3.3 stable • 3.4 molar • 3.5 HCl • 3.6 NaOH • 3.7 titrant • 3.8 titre • 3.9 mole • 3.10 litres.
Q4 — Technique reasons
4.1 The burette is rinsed with the titrant before filling because residual water would dilute the titrant below its nominal concentration, causing the titre to be too large and overestimating the concentration of the unknown.
4.2 Air bubbles are removed from the burette tip before starting because a bubble that is released during the titration adds to the apparent volume delivered, making the titre too large and overestimating the unknown concentration.
4.3 The burette is read at the bottom of the meniscus with the eye level with the liquid surface because reading from above or below introduces parallax error, causing a systematic over- or underestimate of the volume reading.
4.4 The last drop is not blown out of the pipette because the pipette is calibrated assuming this small volume remains in the tip; blowing it out would deliver slightly more than the stated volume, increasing the moles of analyte and making the titre too large.
Q5 — Sequence: preparing a standard solution
Correct order: 4, 2, 6, 5, 3, 1
- Calculate the mass required for the target concentration.
- Weigh the primary standard to four decimal places.
- Record the exact mass weighed.
- Dissolve in a small volume of distilled water in a beaker.
- Quantitatively transfer to the volumetric flask using a wash bottle.
- Make up to the calibration mark; invert to mix.