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 at different concentrations by dilution and measuring their pH

  2. Preparing buffer solutions and measuring their pH

  3. Investigating how pH changes with the addition of sodium hydroxide solution to both types of solution

Preparing hydrochloric acid solutions

• This can be done by serial dilution:

  • A serial dilution decreases the concentration by the same factor in each step

• Example 1:

  • Adding 1.0 cm³ of HCl to 9.0 cm³ of water

  • This is a 1:10 dilution

• Example 2:

  • Adding 1.0 cm³ of HCl to 4.0 cm³ of water

  • This is a 1:5 dilution

Method

1. Label 7 test tubes 1 to 7

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

   • Test tube 1 is undiluted

   • Therefore, test tube 1 contains 0.1 mol dm^-3 HCl

3. Transfer 1.0 cm³ 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 this to test tube 2

   • This is a 1:10 dilution

   • Therefore, test tube 2 contains 0.01 mol dm^-3 HCl

6. Repeat steps 2 - 5

   • Use test tube 2 as the source for test tube 3, and so on

   • Continue until test tube 6 is prepared

7. Test tube 7 should only contain distilled water

8. Measure the pH of each solution using:

   • A calibrated pH probe, or

   • Universal indicator and colour comparison chart

Specimen results

• The following data shows the relationship between decreasing concentration and increasing pH for hydrochloric acid solutions

• These results illustrate the expected pattern for a strong acid undergoing serial dilution

• These solutions can be titrated with NaOH to monitor pH change

  • Use a pH meter to track pH throughout the titration

  • Graphs should resemble those in the Neutralisation topic

Preparing a buffer solution

• A buffer resists changes in pH when small amounts of acid or base are added

• This resistance has a limit, known as buffer capacity

Method 

• Use 5 cm³ of 0.30 mol dm^-3 ethanoic acid

• Calculate concentration in 100 cm³:

moles of ethanoic acid in 5 cm³ =  x 5 = 0.0015 moles

• Therefore, the concentration in mol dm^-3 is:

concentration = = 0.015 mol dm^-3

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

• Add 5 cm³ of the ethanoic acid using a pipette

• Use pH = pK_a + log₁₀  to calculate the required mass of sodium ethanoate:

  • 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

  • This higher mass allows for transfer loss

  • Some sodium ethanoate will be left behind when transferring in the next steps

• Calibrate a pH meter and place it in the beaker

• Slowly stir in the sodium ethanoate with a glass rod

  • Do not stir using the pH probe

• Stop when the pH reaches 5.00

• Weight the sodium ethanoate boat ethanoate 

• Transfer the buffer solution to a 100 cm³ volumetric flask 

• Dilute to the mark with distilled water, cap and mix

Preparing a buffer solution with a higher buffer capacity

• Use 5 cm³ of 0.50 mol dm^-3 ethanoic acid, instead of 0.30 mol dm^-3 ethanoic acid

• The method to make this buffer solution:

  • Is the same as the previous method

  • But the calculations will change:

moles of ethanoic acid in 5 cm³ = = 0.0025 moles  

• Therefore, the concentration in mol dm^-3 is:

= 0.025 mol dm^-3

• Again, use pH = pK_a + log₁₀  to calculate the required mass of sodium ethanoate:

  • 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

• To compare buffer capacities:

  • Calibrate the pH probe using pH 4 and 9 buffer standards

  • Rinse with distilled water before and between uses

  • Allow the reading to stabilise before adjusting

• Transfer 25.0 cm³ of each buffer to a conical flask using a volumetric pipette

• Titrate with standardised NaOH from a burette

  • Monitor the pH using the calibrated probe

  • Record the volume of NaOH needed for a 1-unit pH change

• Repeat for the higher-concentration buffer

  • Compare the results to evaluate buffer capacity

Practical skills reminder

• This practical develops key techniques in preparing and testing acid and buffer solutions.

• It also supports:

  • Performing accurate serial dilutions and using pH meters or indicators

  • Preparing buffer solutions

  • Calculating required masses using pH = pK_a + log₁₀ 

  • Monitoring pH change during titration and comparing buffer capacities