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Module 1: Practical chemistry for exams›PAG 11.3 - pH – acids and buffers

PAG 11.3 - pH – acids and buffers

An overview of PAG 11.3 - pH – acids and buffers

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In OCR Chemistry Required Practical 11 (PAG 11.3), hydrochloric acid solutions with a range of concentrations are prepared by dilution and pH values are determined. Then a range of buffer solutions are prepared and their pH values are measured.

For both types of solutions, pH changes on addition of sodium hydroxide solution are measured.

In OCR PAG 11.3: pH – acids and buffers, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Dispose of chemicals as directed by your teacher.

Experimental method for OCR Chemistry Required Practical 11 (PAG 11.3) part 1 – preparing solutions of by serial dilution.

Experimental method diagram titled 'OCR PAG 11.3 Part 1' with six numbered steps. Step 1: Prepare a results table with columns labeled '[HCl (aq)] mol dm⁻³', 'pH value', and 'pH value after NaOH added'. Concentrations listed are 0.1, 0.01, 0.001, and 0.0001 mol dm⁻³. Step 2: Calibrate the pH meter and probe using a pH 7 buffer solution and rinse the probe with distilled water. Step 3: Measure the pH of 0.1 mol dm⁻³ HCl by placing 10 cm³ of the solution in a boiling tube and recording the pH value, shown as 0.98. Step 4: Prepare other concentrations of HCl by diluting 10 cm³ of 0.1 mol dm⁻³ HCl with 90 cm³ of distilled water to create 0.01 mol dm⁻³ HCl, continuing for 0.001 and 0.0001 mol dm⁻³ dilutions. Step 5: Measure the pH of other dilutions using the pH meter and probe. Step 6: Add NaOH (aq) by mixing 40 cm³ of 0.0001 mol dm⁻³ HCl with 1 cm³ of 0.4 mol dm⁻³ NaOH, recording the pH value as 11.24.

Experimental method for OCR Chemistry Required Practical 11 (PAG 11.3) part 2 – Action of buffer solutions

An experimental method diagram titled 'EXPERIMENTAL METHOD OCR PAG 11.3 PART 2'. It outlines steps for preparing and testing buffer solutions. Step 1: Prepare buffer solutions by mixing volumes of 0.4 mol dm⁻³ ethanolic acid and 0.4 mol dm⁻³ sodium ethanoate in separate beakers labeled A, B, and C. A table shows: Buffer solution A with 20 cm³ CH₃COOH(aq) and 20 cm³ CH₃COONa(aq), B with 30 cm³ CH₃COOH(aq) and 10 cm³ CH₃COONa(aq), C with 10 cm³ CH₃COOH(aq) and 30 cm³ CH₃COONa(aq). Step 2: Prepare a results table to record pH values of buffer solutions initially and after NaOH addition. A table shows columns for pH of buffer solution and pH after addition of NaOH, with rows for solutions A, B, and C. Step 3: Measure pH of buffers using a pH meter and probe, starting with buffer A, shown as 4.53 pH. Repeat for other solutions. Step 4: Effect of NaOH on buffers involves adding 1 cm³ of 0.4 mol dm⁻³ NaOH, stirring, and measuring pH again. Another pH meter shows 4.48 pH after NaOH addition to a solution labeled 0.0001 mol dm⁻³.

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.

Calculate the expected pH of the solutions then compare with measured values, using:

For each buffer solution calculate using the relationship:

Then calculate the expected pH and compare this with the measured value, using:

Compare the change in pH when is added to the dilute and buffer solutions. The buffer shows a much smaller change in pH because the ethanoic acid reacts with the added