[DUDE778]

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

To prepare 1.10 L of a buffer solution having an ionic strength of 0.130 and a pH of 7.55 would require:

1. (mass) of Na2HPO4(anhydrous)
2. (mass) of NaH2PO4(anhydrous)

I have no clue what this question is asking or even saying

how would i start
Thank you

[Borek]

What is ionic strength, how do you calculate pH of buffer?

[AWK]

You should specify a method of calculation of activity coefficients.
The Debye-Huckel or EHT(correction - should be EDH = extended Debye-Hückel equation)  methods are insufficient for a such ionic strength.

Yor acidity constants of H3PO4 are probably for a null ionic strength

[coquim]

http://www.columbia.edu/~scb2001/tools/phosphate/phosphate.html

this link may could help you...this little program works good!

[Borek]

It completely ignores ionic strength, so it is of no use here.

[AWK]

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

To prepare 1.10 L of a buffer solution having an ionic strength of 0.130 and a pH of 7.55 would require:

1. (mass) of Na2HPO4(anhydrous)
2. (mass) of NaH2PO4(anhydrous)

I have no clue what this question is asking or even saying

how would i start
Thank you

Which Ka you should use for a mixture of NaH2HP4 and NaH2PO4 for pH calculation without using ionic strength?

[DUDE778]

For this question, assume activity coeffienct of all unity. 

Again, i still feel alittle lost 
thank you

[Borek]

What do you know about buffer pH calculation?

[DUDE778]

Um.. i do know that

HA(aq) + H2O(l) --> H3O+(aq) + A-(aq)
Ka = [H3O+][A-] / [HA]

[H3O+] = Ka[HA] / [A-]
pH = -log[H3O+]

Thats as far as i can think of.... i just can figure out how to start this question though 
I think i must be stupied if i cant do a simple pH problem

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

http://en.wikipedia.org/wiki/Henderson-Hasselbalch_equation

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-buffer-q2

[AWK]

[H3O+] = Ka[HA] / [A-]
pH = -log[H3O+]

This above is sufficient for pH calculation of your buffer without activities when you know concentrations.
You can approximately calculate ionic strenght for mixture of salt (ignoring dissociation od H₂PO_4-) as:
I =  c(NaH₂PO₄) + 3 c(NaH₂PO₄)

Can you now prepare your buffer neglecting activity coefficients?