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 (\(\text{Na}_2\text{CO}_3\)) required to prepare 250 cm³ of a 0.100 mol dm⁻¹ solution: \[ \text{Mass} = \text{concentration} \times \text{volume} \times M_r = 0.100 \times 0.250 \times 106.0 = 2.65\text{ g} \]
- Place a clean, dry weighing bottle on the balance, tare it, 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 the beaker, stirring rod, and funnel with distilled water from the wash bottle at least three times, pouring all washings into the volumetric flask to ensure complete transfer of the solute.
- Remove the funnel. Add distilled water to the flask until the water level is close to the graduation mark, then 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
- Rinse the burette with a small volume of the standard sodium carbonate solution, then fill it using the small funnel. Ensure the space below the tap is filled with liquid and contains no air bubbles. Remove the funnel. Record the initial burette reading to the nearest 0.05 cm³.
- Rinse the 25.0 cm³ pipette with a small volume of the hydrochloric acid of unknown concentration. Use the pipette filler to draw exactly 25.0 cm³ of the acid, aligning the bottom of the meniscus with the line on the pipette neck at eye level.
- Transfer the pipette content into a clean, dry 250 cm³ conical flask. 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 as soon as the indicator changes colour permanently from red/pink to yellow. Record the final burette 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 the volume is within 2.0 cm³ of the rough endpoint, then add the solution dropwise (and half-dropwise) while constantly swirling the flask.
- 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.
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: highly flammable in concentrated form. | 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 | 23.90 | 24.00 |
| Initial burette reading / cm³ | 0.00 | 0.00 | 0.00 | 0.00 |
| Titre / cm³ | 24.50 | 24.05 | 23.90 | 24.00 |
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 (23.90 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: \( 1\text{ mol of Na}_2\text{CO}_3 \text{ reacts with } 2\text{ mol of HCl} \).
\[ n(\text{HCl}) = 2 \times n(\text{Na}_2\text{CO}_3) = 2 \times 2.4025 \times 10^{-3} = 4.805 \times 10^{-3}\text{ mol} \]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. |
Common Exam Questions
1. Why is the conical flask rinsed only with distilled water and not with the acid it will contain?
Rinsing the conical flask with acid would add an uncontrolled amount of extra moles of acid. This would require a larger titre of standard solution, resulting in an incorrect calculated concentration. Distilled water does not affect the moles of acid already transferred by the pipette.
2. Explain why a pipette is used to transfer the acid rather than a measuring cylinder.
A volumetric pipette has a much lower percentage uncertainty (typically ±0.06 cm³ for a 25 cm³ pipette) compared to a measuring cylinder (typically ±0.5 cm³). This provides a more precise and accurate volume measurement.
3. Describe how to read a burette reading accurately.
Ensure the burette is vertical, position your eyes level with the bottom of the liquid meniscus to avoid parallax error, and read the scale to the nearest 0.05 cm³.
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.
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.