This practical guide covers the preparation of a standard solution of known concentration using a volumetric flask, and the subsequent titration of this solution to find the concentration of an unknown acid or base. These techniques are fundamental quantitative skills in chemistry.
Core Specification Link
This practical underpins topic 3.1.2 Amount of substance and connects to 3.1.4 Energetics. It develops skills in quantitative chemistry, precision weighing, and volumetric analysis.
Aim
To prepare a standard solution of sodium carbonate of a known concentration, and to use this standard solution to determine the concentration of a solution of hydrochloric acid via titration.
Equipment List
- Electronic balance (2 decimal places, reads to 0.01 g)
- Weighing bottle
- Beaker (250 cm³)
- Glass stirring rod
- Volumetric flask (250 cm³) with stopper
- Wash bottle with distilled water
- Burette (50 cm³, graduated to 0.10 cm³)
- Burette clamp and retort stand
- Conical flask (250 cm³)
- Pipette (25.0 cm³) with safety pipette filler
- White tile
- Methyl orange indicator
- Small funnel
Experimental Method
Part A: Preparing the Standard Solution
- Calculate the exact mass of anhydrous sodium carbonate (Na₂CO₃) required to prepare 250 cm³ of a 0.100 mol dm⁻³ solution: \[ \text{Mass} = \text{concentration} \times \text{volume} \times M_r = 0.100 \times 0.250 \times 106.0 = 2.65\text{ g} \]
- Weigh the sodium carbonate. Place a clean, dry weighing bottle on the balance and weigh out approximately 2.65 g of anhydrous sodium carbonate.Record the precise mass of the bottle and its contents.
- Carefully transfer the solid from the weighing bottle into a clean 250 cm³ beaker.
- Reweigh the weighing bottle on the balance and record its mass . Calculate the mass of sodium carbonate transferred by difference: \[ \text{Mass transferred} = \text{mass of bottle + solid} - \text{mass of bottle after transfer} \]
- Add approximately 100 cm³ of distilled water to the beaker . Stir the mixture thoroughly with a glass rod until all the solid sodium carbonate has fully dissolved.
- Place the small funnel into the neck of the 250 cm³ volumetric flask . Pour the solution carefully down the glass rod into the flask to prevent splashing.
- Rinse and transfer washings. Rinse the beaker, stirring rod, and funnel with distilled water at least three times.Pour all washings into the volumetric flask to ensure complete transfer of the solute.
- Fill to the graduation mark. Remove the funnel, and add distilled water to the flask until the level is close to the mark.Use a dropping pipette to add water dropwise until the bottom of the meniscus is exactly aligned with the graduation line at eye level.
- Insert the stopper securely, hold it firmly in place, and invert the volumetric flask at least ten times to mix the solution thoroughly and ensure a uniform concentration.
Part B: Carrying Out the Titration
- Fill the burette. Rinse the burette with the standard sodium carbonate solution, then fill it using a funnel.Ensure the space below the tap is filled and contains no air bubbles. Remove the funnel, and record the initial reading to the nearest 0.05 cm³.
- Pipette the acid sample. Rinse the 25.0 cm³ pipette with the hydrochloric acid of unknown concentration.Use a pipette filler to draw exactly 25.0 cm³ of the acid, aligning the bottom of the meniscus with the line on the neck at eye level.
- Transfer the pipette content into a clean conical flask (it can be wet with distilled water). Touch the pipette tip to the inside wall of the flask to release the final drop.
- Add 2 to 3 drops of methyl orange indicator to the conical flask . Swirl the flask to mix: the solution should turn red/pink.
- Place the conical flask on a white tile directly under the burette tip.
- Perform a rough titration. Open the burette tap and add the standard sodium carbonate solution quickly while swirling the flask.Stop adding titrant at the first permanent colour change, to orange. Record the final reading.
- Refill the burette and record the initial reading . Pipette a fresh 25.0 cm³ sample of acid into a clean conical flask and add indicator.
- Perform a precise titration. Add the standard solution rapidly until within 2.0 cm³ of the rough endpoint.Add the solution dropwise (and half-dropwise) while constantly swirling the flask until the indicator changes colour.
- The endpoint is reached when the solution changes permanently to orange . Record the final burette reading to the nearest 0.05 cm³.
- Repeat the titration until you obtain at least two concordant titres: results that agree within 0.10 cm³ of each other.
Try it: Put the titration in order. Read the steps, then check the order below.
Show the correct order
- Rinse the burette with the standard sodium carbonate solution, fill it, remove the funnel and record the initial reading to the nearest 0.05 cm³.
- Rinse the pipette with the hydrochloric acid, then use a pipette filler to transfer 25.0 cm³ of acid into a clean conical flask.
- Add 2 to 3 drops of methyl orange and stand the flask on a white tile under the burette.
- Rough titration: add the sodium carbonate solution quickly with swirling until the colour change is permanent, and record the rough titre.
- Accurate titration: add quickly to about 2 cm³ before the rough titre, then drop by drop with swirling until the colour change is permanent.
- Repeat until two titres agree within 0.10 cm³, then calculate the mean of the concordant titres only.
Try it: Label the apparatus. Name each part in the diagram, then check the list below.
Show the parts
- Retort stand
- Clamp
- Burette
- Tap
- Conical flask
- White tile
Try it: Run the titration. This mini practical needs JavaScript. Open the full virtual lab.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Dilute hydrochloric acid | Skin and eye irritation from spills or splashes. | Wear safety goggles and a lab coat. Clean up any spills immediately. |
| Sodium carbonate solution | Skin irritation and potential eye damage. | Wear safety goggles. Wash any splashes off the skin immediately. |
| Glassware (burette, pipette) | Cuts and puncture wounds from broken glass. | Handle glassware with care. Do not force pipettes into pipette fillers. Keep glassware away from desk edges. |
| Indicator solution (methyl orange) | Staining of skin and clothes. The solid is toxic if swallowed. | Handle indicator dropper bottles with care: use only 2 to 3 drops. Keep away from naked flames. |
Results & Calculations
Data Table Format
All burette readings must be recorded to 2 decimal places, ending in 0.00 or 0.05 cm³.
| Titration | Rough | 1 | 2 | 3 |
|---|---|---|---|---|
| Final burette reading / cm³ | 24.50 | 24.05 | 23.80 | 24.00 |
| Initial burette reading / cm³ | 0.00 | 0.00 | 0.00 | 0.00 |
| Titre / cm³ | 24.50 | 24.05 | 23.80 | 24.00 |
Worked Example: Calculation of Unknown Concentration
Using the data table above, calculate the concentration of the hydrochloric acid solution if the standard sodium carbonate solution has a concentration of 0.100 mol dm⁻³.
Step 1: Identify and average the concordant titres
Concordant titres are within 0.10 cm³ of each other. Here, titres 1 (24.05 cm³) and 3 (24.00 cm³) are concordant. Titre 2 (23.80 cm³) is not concordant with titre 1 and is excluded. The rough titre (24.50 cm³) is also excluded.
\[ \text{Mean titre} = \frac{24.05 + 24.00}{2} = 24.025\text{ cm}^3 \]Step 2: Calculate the moles of sodium carbonate used
\[ n(\text{Na}_2\text{CO}_3) = \text{concentration} \times \text{volume (dm}^3) = 0.100 \times \frac{24.025}{1000} = 2.4025 \times 10^{-3}\text{ mol} \]Step 3: Write the balanced chemical equation and determine the mole ratio
\[ \text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \]The mole ratio is: 1 mol of Na₂CO₃ reacts with 2 mol of HCl.
\[ n(\text{HCl}) = 2 \times n(\text{Na}_2\text{CO}_3) = 2 \times 2.4025 \times 10^{-3} = 4.805 \times 10^{-3}\text{ mol} \]Step 4: Calculate the concentration of the hydrochloric acid
\[ \text{Concentration of HCl} = \frac{\text{moles}}{\text{volume (dm}^3)} = \frac{4.805 \times 10^{-3}}{0.0250} = 0.192\text{ mol dm}^{-3} \]The concentration of the hydrochloric acid is 0.192 mol dm⁻³ (3 significant figures).
Sources of Error & Improvements
| Error Source | Classification | Consequence & Mitigation |
|---|---|---|
| Not rinsing burette with standard solution | Systematic | Water left in the burette dilutes the standard solution, requiring a larger volume of titrant. Mitigation: Always rinse burette with the titrant before filling. |
| Leaving funnel in the top of the burette | Systematic | Drops of solution may fall from the funnel during the titration, leading to an artificially low titre reading. Mitigation: Always remove the funnel after filling the burette. |
| Overshooting the endpoint | Systematic | Adding too much standard solution yields an artificially large titre. Mitigation: Add solution dropwise and swirl continuously near the endpoint. |
| Loss of solid during transfer to beaker | Systematic | Lower concentration of the standard solution, leading to larger titres. Mitigation: Use the weighing by difference method to calculate actual mass transferred. |
Try it: Which way does the titre go? Each mistake makes the titre too large, too small, or leaves it unchanged. Sort them. Then check the table below.
Show the answers
| Mistake | Effect on the titre |
|---|---|
| Burette rinsed with distilled water only, not with the solution it will hold | Titre too large. Water left in the burette dilutes the solution, so a larger volume is needed to deliver the same number of moles. |
| Funnel left in the top of the burette during the titration | Titre too small. Drops fall from the funnel into the burette after the initial reading, so the final reading is lower than the volume really delivered. |
| End point overshot by adding too much from the burette | Titre too large. Extra solution is delivered after the end point, so the recorded titre is bigger than the volume that reacted. |
| Air bubble in the burette jet that fills with solution during the titration | Titre too large. Solution that fills the bubble leaves the burette but never reaches the flask, so the reading falls by more than the volume delivered. |
| Pipette rinsed with distilled water only, not with the solution it will measure | Titre too small. Water in the pipette dilutes the 25.0 cm³ sample, so fewer moles are in the flask and less solution is needed to react with them. |
| Conical flask rinsed with distilled water and still wet | No effect on the titre. Water changes the volume in the flask but not the number of moles delivered by the pipette, so the titre is unchanged. |
| Distilled water squirted down the inside of the flask during the titration | No effect on the titre. The number of moles in the flask is unchanged, so the same volume from the burette is needed. |
| Some of the weighed solid lost while making the standard solution | Titre too large. The standard solution is more dilute than calculated, so a larger volume is needed to supply the moles that react. |
Now explain it in writing
Exam-style question
2 marks
[2 marks]
During a titration a student leaves the funnel in the top of the burette. Explain how this affects the titre.
Show Mark Scheme
- Drops of solution can fall from the funnel into the burette after the initial reading has been taken [1]
- So the final reading is lower than it should be and the titre is smaller than the volume actually delivered [1]
Practise this practical
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
2 marks
[2 marks]
Explain why a volumetric pipette is used, rather than a measuring cylinder, to transfer 25.0 cm³ of acid into the conical flask.
Show Mark Scheme
- A pipette has a much lower (percentage) uncertainty, e.g. ±0.06 cm³ compared with about ±0.5 cm³ for a measuring cylinder [1]
- So the volume, and therefore the moles, transferred are known more precisely, giving a more accurate concentration [1]
Exam-style question
3 marks
[3 marks]
Describe how to take an accurate burette reading.
Show Mark Scheme
- The burette is clamped vertically [1]
- Read with the eye level with the bottom of the meniscus, to avoid parallax error [1]
- Record the reading to the nearest 0.05 cm³ [1]
Practical questions appear in your exam papers. Practise them under timed conditions in Exam Mode, with unlimited AI marking and model answers.
See Exam Pass
CPAC Skills Assessed
- CPAC 1: Follows written instructions to prepare solutions and carry out titrations.
- CPAC 2: Applies investigative approaches to determine unknown concentrations.
- CPAC 3: Safely and competently uses key volumetric apparatus (burette, volumetric pipette, volumetric flask).
- CPAC 4: Accurately records experimental observations and volume measurements to the correct precision.
AQA Examiner Tip
When recording titrations, ensure all burette readings are written down to two decimal places (ending in .00 or .05). You must show clear calculations for the mass transferred by difference, and use only concordant titres (agreeing within 0.10 cm³) to calculate the mean titre.