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pH calculations question

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    Hello! I've got some Chemistry homework but I think I've done one of the questions wrong.

    The question:
    What mass of sodium methanoate should be dissolved in 250cm3 of 0.100M methanoic acid to form a buffer solution with a pH of 5.20? (ka of methanoic acid = 1.74x10^-4.)

    My workings:
    [H+] = 10^-5.2 = 6.31x10^-6
    [H+] = ka x [HA]/[A-]
    6.31x10^-6 = 1.78x10^-4 = 0.1/[A-]
    [A-] = 2.82 mol dm^-3
    moles = vol x conc
    moles of A- = 2.82 x 0.25 = 0.705

    This is where I get confused.
    Are the moles of A- the same as the moles of HCOONa???

    What I did:
    I worked out the Mr of HCOONa to be 68.
    mass = moles x Mr
    mass = 0.705 x 68 = 47.94

    final answer = 47.9 g.

    I'm pretty sure I did the last part wrong. To summarise, my actual question is: for a weak acid, are the moles of [A-] the same as the moles of [HA]?

    Sorry for the confusing post. Thanks!
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    (Original post by JustJusty)
    Hello! I've got some Chemistry homework but I think I've done one of the questions wrong.

    The question:
    What mass of sodium methanoate should be dissolved in 250cm3 of 0.100M methanoic acid to form a buffer solution with a pH of 5.20? (ka of methanoic acid = 1.74x10^-4.)

    My workings:
    [H+] = 10^-5.2 = 6.31x10^-6
    [H+] = ka x [HA]/[A-]
    6.31x10^-6 = 1.78x10^-4 = 0.1/[A-]
    [A-] = 2.82 mol dm^-3
    moles = vol x conc
    moles of A- = 2.82 x 0.25 = 0.705

    This is where I get confused.
    Are the moles of A- the same as the moles of HCOONa???

    What I did:
    I worked out the Mr of HCOONa to be 68.
    mass = moles x Mr
    mass = 0.705 x 68 = 47.94

    final answer = 47.9 g.

    I'm pretty sure I did the last part wrong. To summarise, my actual question is: for a weak acid, are the moles of [A-] the same as the moles of [HA]?

    Sorry for the confusing post. Thanks!
    No, the moles of HA is not the same as the moles of A-.

    You seem to have the concepts correct:

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

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

    10-5.2 = 1.74 x 10-4 x [HA]/[A-]

    6.31 x 10-6/1.74 x 10-4 = [HA]/[A-]

    0.0363 = [HA]/[A-]

    and as it's a buffer solution the volumes is the same so we can work in moles.

    Hence A- mol = HA mol / 0.0363

    moles of HA = 0.25 x 0.1 = 0.025 mol

    A- mol =0.025 / 0.0363 = 0.689 mol

    mass of 1 mol HCOONa = 68

    0.689 mol = 46.9g
 
 
 
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