Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    3
    ReputationRep:
    ph=1/2(pka-logM)


    what does logM stand for when im trying to find pH?
    Offline

    12
    ReputationRep:
    (Original post by asaaal)
    ph=1/2(pka-logM)


    what does logM stand for when im trying to find pH?
    Not sure what equation you are referring to, but in an acid dissociation

    HA \leftrightharpoons H^+ + A^-

    the relationship between pH and pKa is

    pH=pK_a - \log \dfrac{[HA]}{[A^-]}

    from which you can see that when the acid is exactly half dissociated (i.e. [HA]=[A-] )

    pH=pK_a - \log 1

    pH=pK_a
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by Plato's Trousers)
    Not sure what equation you are referring to, but in an acid dissociation

    HA \leftrightharpoons H^+ + A^-

    the relationship between pH and pKa is

    pH=pK_a - \log \dfrac{[HA]}{[A^-]}

    from which you can see that when the acid is exactly half dissociated (i.e. [HA]=[A-] )

    pH=pK_a - \log 1

    pH=pK_a


    so is the final equation ph=pka or ph = pka + log [A-]/[HA] ?
    Offline

    12
    ReputationRep:
    (Original post by asaaal)
    so is the final equation ph=pka or ph = pka + log [A-]/[HA] ?
    if you read my post, you will see I said when the acid is exactly half dissociated pH=pKa

    In other words, it's a special case of the general equation.

    If you think about it, pH could not possibly always be equal to pKa, because pH depends on concentration, whereas pKa is always the same (for a given acid)
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What's your favourite Christmas sweets?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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