pH curves, titrations and indicators (A-level only) — AQA A-Level Chemistry
Test yourself on pH curves, titrations and indicators (A-level only) with AQA A-Level practice questions.
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pH curves, titrations and indicators (A-level only) explained
A titration measures the exact volume of one solution that reacts with a fixed volume of another.
Read the full explanation
A standard solution is made by weighing the solid, dissolving it, transferring the solution and all the washings to a volumetric flask, making up to the graduation mark with distilled water, and inverting the stoppered flask repeatedly to mix. A pipette delivers 25.0 cm³ into a conical flask and the burette delivers the other solution. Rinse the pipette and the burette with the solutions they will contain, so that residual water cannot dilute them, but rinse the conical flask with distilled water only, because rinsing it with acid adds extra moles and raises the titre. Remove the funnel from the burette before starting, since drops falling from it decrease the titre. Repeat until titres agree within 0.10 cm³.
Students should be able to: perform calculations for these titrations based on experimental results.
Every titration calculation follows the same three steps. Convert the known solution to moles with n = cV/1000, using the volume in cm³. Apply the mole ratio from the balanced equation; this is where most marks are lost, because 2HCl + Ba(OH)₂ → BaCl₂ + 2H₂O is a 2 : 1 ratio, not 1 : 1. Then divide the moles by the volume of the unknown solution in dm³ to get its concentration. If 25.0 cm³ of hydrochloric acid needs 22.40 cm³ of 0.100 mol dm⁻³ barium hydroxide, n(Ba(OH)₂) = 2.24 × 10⁻³ mol, so n(HCl) = 4.48 × 10⁻³ mol and c(HCl) = 0.179 mol dm⁻³. Use only concordant titres in the mean, and quote answers to three significant figures, because burette readings support that precision.
Typical pH curves for acid–base titrations in all combinations of weak and strong monoprotic acids and bases.
Four shapes, and the key to all of them is which partner is weak. Strong acid with strong base starts near pH 1, has a long steep section running from about pH 3 to pH 11, and its equivalence point is at pH 7. Weak acid with strong base starts higher, near pH 3, shows a flat buffer region, and its steep section runs only from about pH 7 to pH 11, so equivalence lies above pH 7 because the salt formed is that of a weak acid. Strong acid with weak base starts near pH 1 and its steep section runs from about pH 3 to pH 7, with equivalence below pH 7. Weak acid with weak base has no steep section at all, just a gradual change through the equivalence region. Every curve levels off at the pH of whichever solution is in excess.
Students should be able to: sketch and explain the shapes of typical pH curves use pH curves to select an appropriate indicator.
A pH curve plots pH against volume of titrant added. Its shape depends on the strengths of the acid and base. A strong acid starts at a low pH (e.g. pH 1), while a weak acid starts higher (e.g. pH 3) and shows a buffer region. The equivalence point is marked by a near-vertical steep section. For strong acid-strong base, this spans pH 3-11; for weak acid-strong base, pH 7-11; for strong acid-weak base, pH 3-7. Weak acid-weak base curves lack a steep section. An indicator is a weak acid whose two forms are different colours. It is usable only if its colour change range (roughly pKIn ± 1) falls entirely within the steep section of the pH curve. Methyl orange (pH 3.1-4.4) suits strong acid-weak base; phenolphthalein (pH 8.3-10.0) suits weak acid-strong base.
Required practical 9 Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base.
Required practical 9 produces the curves rather than assuming them. Calibrate the pH meter against buffer solutions of known pH first, because the absolute readings depend on it. Pipette 25.0 cm³ of the acid into a beaker, stand the electrode in it, and add the alkali from a burette. Add in 1.0 cm³ portions at first but in 0.1 or 0.2 cm³ portions near the equivalence point, where the pH changes fastest and where too coarse a step will simply miss the steep section. Stir before every reading and rinse the probe with distilled water when moving between solutions. Plot pH against total volume added and draw a smooth curve rather than joining the points with straight lines. Read the equivalence volume at the mid-point of the steepest part, and read the pKa at half that volume.
Your focus
- Describe how to make 250 cm³ of a standard solution from a weighed solid, including the washings and final mixing.
- Explain why the funnel is removed from the burette before the titration, referring to the effect on the titre.
- Calculate a mean titre from a set of readings, selecting only the concordant ones.
Show all 15 objectives
- Calculate the concentration of hydrochloric acid neutralised by 22.40 cm³ of 0.100 mol dm⁻³ barium hydroxide, using the 2 : 1 ratio.
- Convert a titre and a concentration into the mass of solid present in an original sample.
- Estimate the percentage uncertainty in a titre of 22.40 cm³ when each burette reading is uncertain by 0.05 cm³.
- Sketch the pH curve for 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid titrated with 0.100 mol dm⁻³ sodium hydroxide, marking equivalence above pH 7.
- Explain why the equivalence point for ammonia titrated with hydrochloric acid lies below pH 7.
- Explain why the curve for butanoic acid titrated with ethylamine has no steep section.
- Sketch and explain the shape of the pH curve for the titration of a weak acid with a strong base.
- Select a suitable indicator for the titration of ethanoic acid with sodium hydroxide and justify the choice from the curve.
- Explain, in terms of the steepness of the pH curve, why an indicator cannot be used for butanoic acid titrated with ethylamine.
- Describe how to obtain a pH curve for ethanoic acid titrated with sodium hydroxide, including calibration of the meter.
- Explain why the volume increments are reduced close to the equivalence point.
- Use a plotted curve to determine the equivalence volume and the pKa of the weak acid.
pH curves, titrations and indicators (A-level only) exam tips
Marking Points
- one mark for rinsing the pipette with the solution it will deliver, so water cannot dilute it
- one mark for removing the funnel from the burette before titrating, because drops falling in would decrease the titre
- one mark for transferring the solution and its washings to a volumetric flask and making up to the graduation mark
- one mark for inverting the stoppered flask several times to mix
- one mark for using concordant titres, agreeing within 0.10 cm³, to calculate the mean
- one mark for converting a volume in cm³ to dm³ before multiplying by the concentration
- one mark for applying the mole ratio from the balanced equation, such as 2 : 1 for HCl with Ba(OH)₂
- one mark for dividing the moles by the volume of the unknown solution to obtain its concentration
- one mark for the answer to three significant figures with the unit mol dm⁻³
- one mark for carrying an earlier incorrect value forward consistently, credited as error carried forward
- one mark for a starting pH of about 1 for a strong acid and about 3 for a weak acid of the same concentration
- one mark for a steep section spanning roughly pH 3 to pH 11 for a strong acid with a strong base
- one mark for an equivalence point above pH 7 for a weak acid with a strong base
- one mark for an equivalence point below pH 7 for a strong acid with a weak base
- one mark for showing no steep section for a weak acid with a weak base
- Sketch a pH curve with the correct initial pH and final pH for the given acid and base.
- Draw the steep section at the correct equivalence volume and pH range, noting that strong acid-strong base spans pH 3-11.
- State that the indicator's pH range must lie entirely within the steep section of the curve.
- Choose phenolphthalein for a weak acid with a strong base, as its colour change range falls within the steep section of pH 7-11.
- one mark for calibrating the pH meter with buffer solutions of known pH before starting
- one mark for adding the alkali in smaller portions near the equivalence point, where the pH changes fastest
- one mark for stirring the mixture before each pH reading
- one mark for plotting pH against total volume added and drawing a smooth curve
- one mark for identifying the equivalence point at the mid-point of the steepest part of the curve
Examiner Tips
- 💡Distilled water in the conical flask changes nothing, because it does not change the number of moles present; say so explicitly if asked.
- 💡State titres to two decimal places, ending in 0 or 5, because a burette is read to the nearest 0.05 cm³.
- 💡Write the balanced equation at the top of your working, even when the question does not ask for it; the ratio mark depends on it.
- 💡Percentage uncertainty in a titre uses two readings, so double the burette uncertainty before dividing by the titre.
- 💡Decide the starting pH and the final pH first, then join them with the correct shape; the two ends fix most of the marks.
- 💡Mark the equivalence volume on the axis before you draw, so the steep section ends up in the right place.
- 💡When sketching a pH curve, clearly mark the initial pH, the equivalence point volume, the steep section range, and the final pH.
- 💡Quote the indicator range and the span of the steep section side by side; the comparison is the explanation.
- 💡If asked to improve the method, 'add smaller volumes near the equivalence point' is almost always one of the marks.
- 💡A pH meter is the answer whenever a titration has no steep section, because no indicator will work.
Common Mistakes
- rinsing the conical flask with the acid, which adds extra moles and raises the titre
- writing that leaving the funnel in the burette increases the titre, when examiners require 'decreases'
- making the standard solution up to the mark in a beaker or a measuring cylinder
- including the rough titre in the mean
- reading the burette from the top of the meniscus instead of the bottom
- dividing by 1000 twice, or not at all
- using a 1 : 1 ratio with a diprotic acid or with a Group 2 hydroxide
- including the rough titre in the mean before calculating
- quoting the answer to one significant figure when the burette readings justify three
- calculating the concentration of the solution in the burette when the question asks for the one in the flask
- drawing every curve passing through pH 7 at the equivalence point
- starting a weak acid curve at pH 1
- drawing a steep section on the weak acid with weak base curve
- letting the curve keep rising instead of levelling off once the alkali is in excess
- drawing the buffer region as a flat horizontal line rather than a shallow rise
- Drawing the steep section of a weak acid-strong base curve extending down to pH 3 instead of stopping around pH 7.
- Choosing an indicator because its range straddles pH 7 rather than because it lies entirely within the steep section.
- Using phenolphthalein for ammonia titrated with hydrochloric acid, where it changes long before the equivalence point.
- adding the alkali in 5 cm³ portions throughout, so the steep region is missed entirely
- joining the plotted points with straight line segments instead of a smooth curve
- rinsing the pH probe with the acid rather than with distilled water between readings
- leaving the meter uncalibrated and then treating the absolute pH values as accurate
- reading the equivalence point at pH 7 rather than at the mid-point of the steep section