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What weights of NaH2PO4 and NaHPO4 would be required to prepare a buffer solution of pH 7.45 that has an ionic strength of 0.1? pKa2=7.12 , mwt NaH2Po4=120g/mol NaHPO4=142 g/mol

using the henderson-hasselbalch equation i got that the ratio of the concentration of NaH2PO4 to NaHPO4 is 2.14 but after that i'm stuck
Answered by Anonymous
read the textbook
Answered by Devron
I am going to assume that you did that correctly. and go from there.


***The problem I believe is that you didn't realize what ionic strength is equal to molarity.



Let NaHPO4=A- and let NaH2PO4=HA

2.14=A-/HA

and HA +A-=0.1


Solving for A-,

A-=0.1-HA

Substitute one equation into the other to cancel out variables.

2.14=0.1-HA/HA solving for HA,


3.14HA=0.1

HA=0.318 moles


0.1-0.318 moles =moles of A-


Use molecular weights to solve for the mass needed for each one.
Answered by Anonymous
thanks!
Answered by Devron
Just remember that the masses that you solve for are only accurate for 1 liter.
Answered by ld
no the answer above is wrong , u cannot assume[HA]+[A-]=0.1
recall back the formula of ionic strength ,then formed two simultaneous equation and solve it
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