Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Wuffe_devil 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.
-
pH=pKa + log[A-]/[A--])
check your formula (nominator and denominator are interchanged)
-
pH=pKa + log[A-]/[A--])
check your formula (nominator and denominator are interchanged)
Crap, forgot a - before log, thanks a lot!
-
Both are equivalent, but HH equation possess + before log.