pH – Acids & Buffers (OCR A Level Chemistry)

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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

1-4-9-serial-dilution-method

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 begin mathsize 14px style fraction numerator 0.1 over denominator 10 end fraction end style = 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 fraction numerator 0.01 over denominator 10 end fraction= 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 17 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

Test Tube Concentration (mol dm-3) pH 
1 0.1 1.00
2 0.01 2.00
3 0.001 3.00
4 0.0001 4.00
5 0.00001 5.00
6 0.000001 6.00
7 0.0000001 7.00

  • 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

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 = fraction numerator 0.3 over denominator 1000 end fraction cross times 5 = 0.0015 moles
    • Therefore, this is the number of moles in 100 cm3
    • Therefore the concentration = fraction numerator 0.0015 over denominator 100 end fraction cross times 1000 = 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 = pKa + log10 begin mathsize 14px style fraction numerator left square bracket straight A to the power of minus right square bracket over denominator left square bracket HA right square bracket end fraction end style
    • The pKa of ethanoic acid is 4.76
    • 5.00 = 4.76 + log10 begin mathsize 14px style fraction numerator left square bracket straight A to the power of minus right square bracket over denominator 0.015 end fraction end style
    • 100.24fraction numerator left square bracket straight A to the power of minus right square bracket over denominator 0.015 end fraction
    • [A] = 0.0261 mol dm-3
  • Therefore the number of moles of A needed in 100 cm3 is
    • begin mathsize 14px style fraction numerator 0.0261 over denominator 10 end fraction end style= 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 = fraction numerator 0.5 over denominator 1000 end fraction cross times 5 = 0.0025 moles  
    • Therefore this is the number of moles in 100 cm3
    • Therefore the concentration fraction numerator 0.0025 over denominator 100 end fraction cross times 1000 = 0.025 mol dm-3
  • Calculate the mass of sodium ethanoate required by determining the concentration of [A] using:
    • pH = pKa + log10 begin mathsize 14px style fraction numerator left square bracket straight A to the power of minus right square bracket over denominator left square bracket HA right square bracket end fraction end style
    • The pKa of ethanoic acid is 4.76
    • 5.00 = 4.76 + log10 begin mathsize 14px style fraction numerator left square bracket straight A to the power of minus right square bracket over denominator 0.025 end fraction end style
    • 100.24 
    • [A-] = 0.0434 mol dm-3
  • Therefore the number of moles of Aneeded in 100 cm3 is
    • begin mathsize 14px style fraction numerator 0.0434 over denominator 10 end fraction end style= 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

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.