# PH problem weak acid and weak base

1. Nov 14, 2007

### hsgnfgkid

Hi

I have been trying several days to solve this problem. But i can't find the procedure to solve this weak acid and weak base problem. I will really appreciate some help.

What is the pH of a soft drink in which the major buffer ingredients are 6.90 g of NaH2PO4 and 5.70 g of Na2HPO4 per 355 mL of solution?

Thanks

2. Nov 15, 2007

### staf9

To find the pH of a buffer solution, you use the Henderson-Hasselbach Equation:

pH = pKa + log ([$$A^{-}$$] / [HA])

[$$A^{-}$$] is the molarity of your conjugate base
[HA] is the molarity of your acid

So you want to find the concentration of acid and base in your buffer solution. Keep in mind that NaH2PO4 is your acid and Na2HPO4 is your base.

Also, you need the Ka (acidity constant) of phosphoric acid (I don't know this off the top of my head).

Before anything else, I would find the molarity of each solution. I'm sure you know how to do this, but if not, find the molar mass of each compound and plug into the following equations:

# moles = grams / molar mass (in g/mol) and Molarity = Moles / Liters

Once you have the molarity of each compound and the Ka of phosphoric acid, find the pKa of phosphoric acid by doing:

pKa = -log(ka)

So, plug these values into the Henderson-Hasselbach equation and you'll be all set.

3. Nov 15, 2007

### hsgnfgkid

Hi Staf9

I would really want to thank you for th answer. Youre the best, really i appreciate your work. THANKS ONCE AGAIN