February 25, 2017

Homework Help: Analytical Chemistry

Posted by Sam on Friday, February 22, 2013 at 7:05pm.

Phosphoric acid is a triprotic acid with the following pKa values:

pka1: 2.148 pka2: 7.198 pka3: 12.375

You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.45. To do this, you choose to use mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?

?? grams NaH2PO4
?? grams Na2HPO4

Hint: Use the Henderson-Hasselbalch equation to get the molar ratio of Na2HPO4 to NaH2PO4 required, then the fraction of each form from the ratio. The total moles needed will be 1.000 L × 0.0100 M = 0.0100 moles. Use the formula mass to calculate the mass needed. (FM NaH2PO4 = 119.98; FM Na2HPO4 = 141.96).

I am completely lost on how to solve this problem.

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