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Topic: Buffers of the polyprotic acids.  (Read 1802 times)

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Offline Wuffe_devil

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Buffers of the polyprotic acids.
« on: March 21, 2013, 02:35:10 AM »
Hi chemical forums!

I've been visiting this site for a while now, i havent really needed to post anything before, but im stuck on this homework problem for a while now, and i cant figure out where I'm wrong.

Given that the stepwise dissociation constants for phosphoric acid are:
Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13

We need to find mass of Na2HPO4 and NaH2PO4 needed to make a buffer of 1.10 L, having ionic strength of 0.090 and a pH of 7.45. Assume all activity coefs are 1

so what I did

1) Use Henderson-Hasselbalch equation to find the ratio between two conjugate bases.
pH=pKa + log[A-]/[A--])

7.45=-log(6.2*10^-8)+log([A-]/log[A--])

log([A-]/[A--])= 0.2424

2) Use ionic strength formula to find relationship between these two conjugate bases.

u = [A-] + 3[A--]
u=[NaH2PO4] + 3[Na2HPO4]=0.090

so we have

log([NaH2PO4]/[Na2HPO4])=0.2424 and
[NaH2PO4] + 3[Na2HPO4]=0.090

so our equation becomes

Take x for [NaH2PO4] and y for [Na2HPO4]

7.45=-log(6.2*10^-8)+log((0.09-3*y)/y)

from this, y=0.018958 M

mol(Na2HPO4)= 0.018958 *1.1 L= 0.020854 mol



m(Na2HPO4)=0.020854 mol* 141.95896 g/mol = 2.96 g

However, entering this result leads to an error.

For NaH2PO4 it's same.

I'm not sure where im doing wrong, any help would be appreciated.

« Last Edit: March 21, 2013, 02:56:32 AM by Wuffe_devil »

Offline AWK

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Re: Buffers of the polyprotic acids.
« Reply #1 on: March 21, 2013, 03:05:00 AM »
Quote
pH=pKa + log[A-]/[A--])
check your formula (nominator and denominator are interchanged)
AWK

Offline Wuffe_devil

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Re: Buffers of the polyprotic acids.
« Reply #2 on: March 21, 2013, 03:14:35 AM »
Quote
pH=pKa + log[A-]/[A--])
check your formula (nominator and denominator are interchanged)

Crap, forgot a - before log, thanks a lot!

Offline AWK

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Re: Buffers of the polyprotic acids.
« Reply #3 on: March 21, 2013, 03:27:12 AM »
Both are equivalent, but HH equation possess + before log.
AWK

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