[boats_go]

Hello,

I'm having problems with the following question.

I need to prepare a 0.1M buffersolution of H3PO4 at pH 7 starting from a concentrated H3PO4 solution (84% w/w and a density of 1,15 g/ml) and NaOH pellets. And the answer should be 5,55g NaOH and 6,7ml H3PO4.

This is how I am thinking:

pH=pKa+log base/acid
with pKa the dissociation constant for the conversion H2PO4- --> HPO42- and this is 7.12
So this gives us the ration base/acid

1L 0.1M = 100mmol

At a pH of 7 the H3PO4 will be completely dissociated to H2PO4- which will be partly dissociated to HPO42-

Assuming that y mmol HPO42- will be formed at the end (also resulting in a consumption of y mmol NaOH ) and substituting this in the pH-equation this gives us -0.12=log(y/100-y)
This gives me 43 mmol NaOH. Adding the 100 mmol NaOH needed for the conversion of the H3PO4 to H2PO4- this gives a total of 143 mmol NaOH.

Comparing this with the answers I can only conclude that I'm way out of the correct path. Where do I make a mistake, where does my mind get messed up?

[Borek]

Your logic seems correct, could be some math error. What volume of the buffer do you need to prepare?

Hmm, if you will use amounts of acid and base given as an answer pH of your buffer will be a way off. And 84% phosphoric acid has a density of over 1.4 g/mL, no idea about exact value as my tables end at 60% w/w.

[AWK]

Write down  balanced neutralization reactions for NaH₂PO₄ and Na₂HPO₄ and all will be clear

[boats_go]

Ok, I have solved the problem.

As dissociation constants are not realy fixed numbers, I used the pKa2-value that was given in my notes (7.12), but more often 7.21 is used. Doing the mats with the 7.21 value, I get the 5.5 g of NaOH that is needed.

And Borek, your tip concerning the densities made my check the density of a 85% H3PO4 solution in the lab, and it should be 1.7 g/cm³ instead of tha 1.17 g/cm³ that was given in my notes. Doing it with that number I got 6.7ml of the solution.

So thank you, but unfortunately my course notes are full of this kind of (typing)mistakes.