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Practical chemistry for exams›CP 3 - Finding the concentration of a solution of hydrochloric acid

CP 3 - Finding the concentration of a solution of hydrochloric acid

An overview of CP 3 - Finding the concentration of a solution of hydrochloric acid

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In Edexcel Core Practical 3: Finding the concentration of a solution of hydrochloric acid a known concentration of sodium hydroxide is used to find the concentration of a solution of hydrochloric acid through a titration reaction.

In Edexcel core practical 3: Find the concentration of a solution of hydrochloric acid, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Phenolphthalein solution is both flammable and toxic.
Hydrochloric acid is an irritant.
Ensure burettes are filled when the top of the burette is below eye level.

Experimental method for Edexcel Chemistry Core Practical 3

The image is a step-by-step guide titled 'Experimental Method Edexcel Core Practical 2 Part 2.' It contains six steps illustrated with diagrams: 1. Use a pipette to transfer 25.0 cm³ of hydrochloric acid to a volumetric flask, filling to the mark with deionised water. A stopper is added, and the flask is inverted to mix. Diagram shows a pipette transferring hydrochloric acid into a flask. 2. Pipette 25 cm³ of dilute hydrochloric acid into a 250 cm³ conical flask, adding two drops of phenolphthalein. The phenolphthalein remains colorless in the acidic solution. Diagram shows a pipette and phenolphthalein added to a flask. 3. Set up a titration apparatus with a burette containing sodium hydroxide solution and a conical flask containing the dilute hydrochloric solution. Diagram shows a burette with sodium hydroxide, a stopcock, and a flask. 4. Carry out a rough titration, swirling the conical flask contents until the solution turns pale pink, recording the burette reading to the nearest 0.05 cm³. Diagram shows a swirling flask. 5. Conduct accurate titrations for concordant results within 0.1 cm³, adding titrant drop by drop near the endpoint. Diagram shows burette with labels 'Final reading,' 'Release,' and 'Hold.' 6. Record results in a table with columns for rough, trial 1, trial 2, and trial 3, and rows for final burette reading, initial burette reading, and titre, all in cm³.

Avoid unintentional dilution of solutions.

A laboratory pipette with a graduated scale on the side, positioned next to a container labeled NaOH, indicating the presence of sodium hydroxide.

Do

Rinse all glassware used for accurately measuring solution volumes with the solution itself.

A pipette is shown with a container above it labeled 'Distilled water'.

Don't

Rinse precision glassware with water immediately before use – leftover water will dilute the solution.

Ensure your standard solution is fully mixed.

Hands holding a stopper flask, with the instruction 'Stopper flask and invert several times to mix' displayed beside it.

Do

Invert the volumetric flask several times after filling to the graduation mark.

A volumetric flask filled with blue liquid, labeled 'Volumetric flask: still and not inverted.'

Don't

Use the standard solution without inversion – The top of the solution may be less concentrated than the bottom. Even small concentration differences can affect the accuracy of your results.

Ensure the burette is free from air bubbles.

A laboratory apparatus showing a graduated pipette filled with liquid, indicating 'No air bubbles' next to the pipette.

Do

Check the burette for air bubbles and remove them by running some solution through the tap before starting the titration.

A diagram of a pipette filled with blue liquid, showing air bubbles at the tip. The pipette has measurement markings on the side and is connected to a gray stopcock.

Don't

Start titrating with air bubbles in the burette – The volume occupied by the bubble will be taken into your volume reading. Less solution will actually be transferred than is recorded.

Keep the titration mixture well mixed.

A hand holding a conical flask containing a pink liquid, with arrows indicating swirling motion, and a dropper above the flask.

Do

Swirl the conical flask gently and consistently during titration to ensure complete contact between reactants.

A laboratory setup featuring a conical flask with a pink liquid inside, topped with a pipette holder and a blue pipette.

Don't

Leave the flask still during titration – without swirling, the acid and alkali will not mix properly, leading to an inaccurate endpoint.

Prevent parallax errors when reading volumes.

An illustration showing a measuring cylinder with a scale marked from 20 to 25 ml. An eye is depicted on the left side, and a green arrow points to the measurement of 22.14 ml.

Do

Read the burette at eye level and record to

A measurement scale showing values from 20 to 25 ml. Two eyes are illustrated, one pointing to 22.26 ml and the other to 22.02 ml, with red arrows indicating the measurements.

Don't

Read from above or below – this leads to incorrect readings due to parallax error.

Only use reliable titres in calculations.

A table displaying titration data with three columns labeled 'Titre 1 (rough) /cm³', 'Titre 2 /cm³', and 'Titre 3 /cm³'. The rows include 'Initial' with values 0.10, 0.00, and 0.10; 'Final' with values 24.95 (in red), 25.10, and 25.10; and 'Titre' with values 25.85, 25.10, and 25.00 (in green).

Do

Use concordant titres (within 0.10 ) to calculate your mean.

A table displaying data with three columns labeled 'Titre 1 (rough) /cm³', 'Titre 2 /cm³', and 'Titre 3 /cm³'. The rows are labeled 'Initial', 'Final', and 'Titre'. The values in the table are: Initial - 0.10, 0.00, 0.10; Final - 24.95, 25.10, 25.10; Titre - 25.85, 25.10, 25.00.

Don't

Include your rough titre or anomalous results – they’ll make your mean calculation less accurate.

The mean titre should be calculated using only concordant results.

Then calculate the number of moles of sodium hydroxide using the equation:

Using the balanced equation and stoichiometry, the ratio of hydrochloric acid:sodium hydroxide is 1:1.

A 25 cm³ sample was taken from a 250 cm³ solution, therefore the number of moles in the original solution must be calculated by multiplying by 10.

The concentration of hydrochloric acid can then be calculated using: