Question

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

Answer 1

read the textbook

Answer 2

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.

Answer 3

thanks!

Answer 4

Just remember that the masses that you solve for are only accurate for 1 liter.

Answer 5

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