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Topic: {Henderson–Hasselbalch}  (Read 2744 times)

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Offline winged17

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{Henderson–Hasselbalch}
« on: April 04, 2015, 01:02:35 PM »
Calculate the mass of a weak acid, HA,  that must be taken with 50 mL of 1.0 M NaOh to prepare 1 L buffer (pH of 4.5) The HA has a formula mass of 122.12 g/mol and Ka = 6.4 x 10^-5.
« Last Edit: April 05, 2015, 10:09:48 AM by Arkcon »

Offline Big-Daddy

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Re: HELP
« Reply #1 on: April 04, 2015, 01:29:43 PM »
Note first you are safe to work in number of moles, because you have the mass conversion factor for HA and V*c for NaOH.

What are your equations? What approximations can you make (if you need to make any)?

Offline winged17

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Re: HELP
« Reply #2 on: April 04, 2015, 01:52:39 PM »
I think I got the answer already. I just used the Henderson-Hasselbalch equation which gave me [ b]/[a]=2.024. I assumed [ b] to be 0.05 mol (from the given NaOH) and then I solved for [a] and got 0.0247 mol. Computing for mass, I arrived at 3.02 grams. Do you think my answer's correct?
« Last Edit: April 04, 2015, 05:32:40 PM by Borek »

Offline magician4

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Re: HELP
« Reply #3 on: April 04, 2015, 03:27:32 PM »
Quote
Do you think my answer's correct?
no, I don't think so

hint:
though your approach with HH-eqtn. seems sound to me, my impression is, that you forgot to ask one important question: where did those 0.05 moles [A-] originally come from?



regards

Ingo
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Offline winged17

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Re: HELP
« Reply #4 on: April 05, 2015, 08:03:01 AM »
It was from the NaOH which is the limiting reactant ....or not

Offline magician4

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Re: {Henderson–Hasselbalch}
« Reply #5 on: April 05, 2015, 02:32:15 PM »
before those 0.05 mol [A-] did - under the influence of NaOH - become [A-] , they of course had been HA , too

so, if your equilibrium calls for 0.05 mol A- and , as counterpart in this buffer, for  0.0247 mol HA, then, of course, you'd have had need for n0 = 0.05 + 0.0247 mol = 0.0747 mol HA , total ( i.e. before adding NaOH), to begin with.

... meaning m = n * M = 9.12236... g = 9.12 g HA

regards

Ingo


There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
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Offline winged17

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Re: {Henderson–Hasselbalch}
« Reply #6 on: April 06, 2015, 08:56:09 AM »
Oh okay, I get it now, thanks a lot! God bless  :)

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