Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    Calculate the pH of the buffer solution formed when 10.00 cm3 of
    0.100 mol dm–3 potassium hydroxide are added to 25.00 cm3 of 0.410 mol dm–3 ethanoic acid. Ka, for ethanoic acid in aqueous solution is 1.74 × 10–5 mol dm–3

    Mol OH– = (10.0×10-3) ×0.10 =1.0×10-3
    Orig mol HA = (25.0 × 10-3) × 0.41 = 0.01025

    mol HA in buffer = orig mol HA–mol OH– = 0.00925

    Why, in this question do we not convert 1.0x10-3 and 0.00925 into concentrations when we put it in the Ka expression, since they're [ ]
    Really confused :confused:
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by xDesertRose)
    Calculate the pH of the buffer solution formed when 10.00 cm3 of
    0.100 mol dm–3 potassium hydroxide are added to 25.00 cm3 of 0.410 mol dm–3 ethanoic acid. Ka, for ethanoic acid in aqueous solution is 1.74 × 10–5 mol dm–3

    Mol OH– = (10.0×10-3) ×0.10 =1.0×10-3
    Orig mol HA = (25.0 × 10-3) × 0.41 = 0.01025

    mol HA in buffer = orig mol HA–mol OH– = 0.00925

    Why, in this question do we not convert 1.0x10-3 and 0.00925 into concentrations when we put it in the Ka expression, since they're [ ]
    Really confused :confused:
    You can if you want, but it makes no difference.

    The definition of concentration is moles/volume

    So if you have:

    (moles salt/volume)/(moles acid/volume)

    and both of the volumes are the same (as they have to be as its the same solution)

    Then the volumes can be cancelled out to give:

    (moles salt/volume)/(moles acid/volume) = (moles salt)/(moles acid)
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by charco)
    You can if you want, but it makes no difference.

    The definition of concentration is moles/volume

    So if you have:

    (moles salt/volume)/(moles acid/volume)

    and both of the volumes are the same (as they have to be as its the same solution)

    Then the volumes can be cancelled out to give:

    (moles salt/volume)/(moles acid/volume) = (moles salt)/(moles acid)
    Ohh ok, but you see the 0.00925 I converted it into conc. by using the overall volume; 0.00925x1000/35 = 0.264

    So then 1.74x10-5 x 0.264 / 0.1 = 4.59x10-5 = [H+]

    Therefore pH = 4.59x10-5 which is not the answer :/
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by xDesertRose)
    Ohh ok, but you see the 0.00925 I converted it into conc. by using the overall volume; 0.00925x1000/35 = 0.264

    So then 1.74x10-5 x 0.264 / 0.1 = 4.59x10-5 = [H+]

    Therefore pH = 4.59x10-5 which is not the answer :/
    The first stage is always to calculate the excess and limiting reagent to find the moles of each component in the mixture

    Calculate the pH of the buffer solution formed when 10.00 cm3 of
    0.100 mol dm–3 potassium hydroxide are added to 25.00 cm3 of 0.410 mol dm–3 ethanoic acid. Ka, for ethanoic acid in aqueous solution is 1.74 × 10–5 mol dm–3
    mol KOH = 0.01 * 0.1 = 0.001 mol
    mol ethanoic acid = 0.025 * 0.41 = 0.01025

    As it's a 1:1 reaction the KOH is the limiting reagent and all reacts forming the same number of moles of salt = 0.001

    Moles of ethanoic acid remaining = 0.01025 - 0.001 = 0.00925

    ka = [H+][A-]/[HA]

    [H+] = ka[HA]/[A-] = 1.74 x 10-5 * 0.00925/0.001

    [H+] = 1.74 x 10-5 * 9.25 = 0.000161

    pH = 3.79
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by charco)
    The first stage is always to calculate the excess and limiting reagent to find the moles of each component in the mixture



    mol KOH = 0.01 * 0.1 = 0.001 mol
    mol ethanoic acid = 0.025 * 0.41 = 0.01025

    As it's a 1:1 reaction the KOH is the limiting reagent and all reacts forming the same number of moles of salt = 0.001

    Moles of ethanoic acid remaining = 0.01025 - 0.001 = 0.00925

    ka = [H+][A-]/[HA]

    [H+] = ka[HA]/[A-] = 1.74 x 10-5 * 0.00925/0.001

    [H+] = 1.74 x 10-5 * 9.25 = 0.000161

    pH = 3.79
    Thanks, makes sense
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 2, 2013

University open days

  • Southampton Solent University
    All faculties Undergraduate
    Sun, 18 Nov '18
  • University of Bradford
    All faculties Undergraduate
    Wed, 21 Nov '18
  • Buckinghamshire New University
    All Faculties Postgraduate
    Wed, 21 Nov '18
Poll
Black Friday: Yay or Nay?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.