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Module 1: Practical chemistry for exams›PAG 11.1 - pH – problem solving

PAG 11.1 - pH – problem solving

An overview of PAG 11.1 - pH – problem solving

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In OCR Chemistry Required Practical 11 (PAG 11.1), learners are provided with a range of colourless solutions including strong and weak acids, alkalis, and a buffer solution. Equipment provided includes pH meters and probes, indicator paper and colour chart, and an indicator solution.

The aim is to plan a procedure, then to identify the solutions in practice, involving different methods of pH measurement.

In OCR PAG 11.1: pH – problem solving, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Unknown solutions should always be treated as irritants.
Ammonia vapour is a severe respiratory irritant – ensure the room is well ventilated.
Phenolphthalein is flammable – keep away from flames or heat sources.
Dispose of chemicals as directed by your teacher.

Experimental method for OCR Chemistry Required Practical 11 (PAG 11.1)

The image is a step-by-step guide titled 'Experimental Method OCR PAG 11.1' for testing solutions A to G using phenolphthalein, universal indicator paper, and a pH meter. Step 1: Prepare a results table with columns labeled 'Phenolphthalein Test,' 'UI Test,' 'pH meter and probe,' 'Identity,' and 'Explanation,' and rows labeled A to G. Step 2: Test with phenolphthalein by adding two drops of each solution to a labeled spotting tile and one drop of phenolphthalein. Record results. Step 3: Test with universal indicator paper by adding two drops of each solution to a labeled spotting tile, adding a piece of indicator paper to each well, and comparing to a color chart with pH values from 0 to 14. Record results. Step 4: Calibrate pH meter and probe using a buffer solution with a pH of 7 and rinse the probe with distilled water. Step 5: Measure pH of solutions by pouring each into a boiling tube, placing the probe into solution A, and recording results. Rinse probe and repeat for other solutions. Solutions A to G are shown in separate boiling tubes with a pH meter displaying 4.70 for solution A.

To prevent incorrect additions.

Two graduated cylinders containing blue liquids. The left cylinder is labeled '0.1 mol dm-3 CH3COOH' and the right cylinder is labeled '0.1 mol dm-3 NaOH'.

Do

Label any substance filled into clear glassware. This is especially important if similar looking materials are put into the same type of equipment.

Two graduated cylinders with blue liquid at different levels, each marked with a question mark. The cylinders are connected to a valve system at the bottom.

Don't

Assume you will remember what is in glassware based on where you left it. Not only will incorrect chemicals impact your results, poor labelling can result in inappropriate handling of chemicals and potentially dangerous reactions.

To record a clear titration curve with enough data around the equivalence point.

Two hands are holding a pipette and a stirring rod above a beaker filled with blue liquid. The pipette is dispensing a drop of liquid into the beaker.

Do

Use smaller increments () near the expected equivalence point.

Two hands are holding a blue straw and a pipette, while a beaker filled with blue liquid is placed on a surface.

Don't

Continue with large additions throughout the whole titration – you will miss the sharp pH change.

Prevent contamination on the pH probe that would affect your calibration and titration data.

A pH probe is shown inserted into a container filled with deionised water. A bottle is pouring deionised water into the container.

Do

Rinse the pH probe with deionised water between every solution.

A glass container filled with blue water, with a probe submerged in it. The probe has a black and white design and a cable extending from the top.

Don't

Insert the probe into a new solution without rinsing – it can change the composition of the solution being measured and affect the accuracy results.

To ensure accurate pH readings from the pH meter.

A laboratory setup featuring a beaker with blue liquid, a pipette, a probe, a digital display showing '11.46', and a circular clock with a red section.

Do

Stir the mixture and wait for the pH reading to stabilise before recording it.

A laboratory setup featuring a beaker filled with blue water, a drop of liquid falling from a pipette into the beaker, and a probe submerged in the water. A digital meter displays the value 11.46.

Don't

Rush taking measurements – before the reactants are fully mixed there can be local regions of variable pH. Unstable values can lead to an inaccurate pH curve.

Use pH measurements, indicator colour changes and the effect of adding acid or alkali to identify each unknown solution.

Use several pieces of evidence to support each identification.