pH – Acids & Buffers (OCR A Level Chemistry A): Revision Note

PAG 11.3: pH – Acids & Buffers

• PAG 11.3 comes in three parts:

  1. Preparing hydrochloric acid solutions of a range of concentrations by dilution and measuring their pH values

  2. Preparing a range of buffer solutions and measuring their pH values

  3. Using both types of solutions, investigating how the pH changes with the addition of sodium hydroxide solution.

Preparing hydrochloric acid solutions with different concentrations

• This can be done by serial dilution

  • Serial dilution is any dilution in which the concentration decreases by the same factor in each successive step

Serial dilution method 

• Example 1

  • Adding 1.0 cm³ of HCl (aq) to 9.0 cm³ of distilled water is a 1 : 10 dilution

  • This means that the concentration will decrease by a factor of 10

  • Example 2

    • Adding 1.0 cm³ of HCl (aq) to 4.0 cm³ of distilled water is a 1 : 5 dilution

    • This means that the concentration will decrease by a factor of 5

Method

1. Label 7 test tubes 1 - 7

2. Add 10.0 cm³ of 0.1 mol dm^-3 hydrochloric acid to test tube 1  

   • Test tube 1 should not be diluted with distilled water

   • Test tube 1 will, therefore, only contain 0.1 mol dm^-3 HCl (aq)

3. Transfer 1.0 cm³ of this hydrochloric acid, from test tube 1, into a 10.0 cm³ measuring cylinder

4. Add 9.0 cm³ of distilled water to the measuring cylinder

   • The measuring cylinder should now contain 10.0 cm³ of liquid

5. Transfer the resulting diluted solution from the measuring cylinder into test tube 2

   • This is a 1 : 10 dilution

   • Theoretically, the concentration of test tube 2 will be = 0.01 mol dm^-3  

6. This process now repeats

7. Transfer 1.0 cm³ of this hydrochloric acid, from test tube 2, into a 10.0 cm³ measuring cylinder

8. Add 9.0 cm³ of distilled water to the measuring cylinder

9. Transfer the resulting diluted solution from the measuring cylinder into test tube 3

   • Theoretically, the concentration of test tube 3 will be = 0.001 mol dm^-3

10. Transfer 1.0 cm³ of this hydrochloric acid, from test tube 3, into a 10.0 cm³ measuring cylinder

11. Add 9.0 cm³ of distilled water to the measuring cylinder

12. Transfer the resulting diluted solution from the measuring cylinder into test tube 4

13. Continue this process until test tubes 1 - 6 are prepared

14. Test tube 7 should only contain distilled water

15. The pH of test tubes 1 - 7 can then be tested in the usual ways

    • Use of a calibrated pH probe

    • Addition of universal indicator solution and comparison to its colour chart

Specimen results

• The resulting solutions can be reacted with sodium hydroxide solution 

  • This will cause a change in pH

  • It can be monitored by titrimetric methods - using a pH probe / meter to track the pH as the titration proceeds

    • This should produce results / graphs similar to those seen in 5.3.5 Neutralisation

Preparing a buffer solution

• A buffer solution is a solution that is resistant to small changes in pH

• Eventually enough acid or base can be added to overcome that capacity

• The amount of acid or base needed to change the pH of a buffer is known as the "buffer capacity"

Method 

• The volume and concentration of ethanoic acid used in this preparation is 5 cm³ and 0.30 mol dm^-3 respectively

• First, calculate the concentration of ethanoic acid that will be present in 100 cm³ 

  • Number of moles of ethanoic acid in 5 cm³ =  = 0.0015 moles

  • Therefore, this is the number of moles in 100 cm³

  • Therefore the concentration = = 0.015 mol dm^-3

• Add 50 cm³ of distilled water to a 100 cm³ beaker

• Use a pipette to add 5 cm³ of 0.30 mol dm^-3 of ethanoic acid to the beaker

• Calculate the mass of sodium ethanoate required by determining the concentration of [A^–] using:

  • pH = pK_a + log₁₀ 

  • The pK_a of ethanoic acid is 4.76

  • 5.00 = 4.76 + log₁₀ 

  • 10^0.24 = 

  • [A^–] = 0.0261 mol dm^-3

• Therefore the number of moles of A^– needed in 100 cm³ is

  • = 0.00261 moles

• Therefore the mass of sodium ethanoate required is

  • 0.00261 x 82.04 = 0.214 g 

• Measure 0.22 g of sodium ethanoate into a weighing boat

  • Slightly more sodium ethanoate than required should be measured because some will be left behind when transferring in the next steps

• Calibrate a pH meter and add it to the beaker

• Slowly add the sodium ethanoate to the beaker, stirring with a glass rod to dissolve the solid

  • Do not stir using the pH probe

• Continue until the pH reaches 5.00

• Measure the weighing boat that contained the sodium ethanoate 

• Transfer your made buffer solution to a 100 cm³ volumetric flask 

• Add distilled water up to the mark, cap and mix

Preparing a buffer solution with a higher buffer capacity

• A buffer solution with a pH of 5.0 with a higher "buffer capacity" can be prepared using 0.50 mol dm^-3 

• The method to make this buffer solution will be the same as above but the calculations will change:

  • Number of moles of ethanoic acid in 5 cm³ = = 0.0025 moles  

  • Therefore this is the number of moles in 100 cm³

  • Therefore the concentration  = 0.025 mol dm^-3

• Calculate the mass of sodium ethanoate required by determining the concentration of [A^–] using:

  • pH = pK_a + log₁₀ 

  • The pK_a of ethanoic acid is 4.76

  • 5.00 = 4.76 + log₁₀ 

  • 10^0.24 = 

  • [A^-] = 0.0434 mol dm^-3

• Therefore the number of moles of A^- needed in 100 cm³ is

  • = 0.00434 moles

• Therefore the mass of sodium ethanoate required is

  • 0.0434 x 82.04 = 0.356 g 

Testing the buffer capacity

• Calibrate a pH probe

  • Rinse the pH probe with distilled water and shaken to remove excess water

  • Place the tip of the pH probe in pH 4 buffer solution ensuring bulb is fully immersed 

  • Allow the probe to sit until the pH stabilises 

  • Adjust reading to the pH of the buffer

  • Repeat this process with a pH 9 buffer solution

    • Ensure that the first step, washing with distilled water, is not missed

• Using a volumetric pipette, transfer 25.0 cm³ of the first buffer solution into a conical flask 

• Load a 50 cm³ burette with a standardised NaOH solution

• Use the pH probe to monitor the titration of the buffer until the pH changes 1 unit

• Repeat with the second buffer solution