The next 6 questions involve the calculation of concentrations of the molecular

Question

The next 6 questions involve the calculation of concentrations of the molecular and ionic species in an aqueous solution that has an analytical composition of 0.260 M KH2PO4+ 0.160 M K2HPO4.
NOTE: The Ka's for the three acidic protons of phosphoric acid H3PO4are: 7.11 x 10-3, 6.32 x 10-8, and 4.5 x 10-13. It may be helpful to write out the three dissociation reactions along with their Ka's and pKa's before starting these problems.

What is the pH of this solution?

What is the concentration of the molecular species H3PO4?
What is the concentration of K+in molarity?
What is the concentration of OH-?
What is the concentration of PO43-in molarity?
What is [H2PO4-]?

Explanation / Answer

Use the Henderson-Hasselbalch equation:
pH = pKa + log ([base]/[acid])

Use the pKa of the stronger acid, H2PO4-
to find this you need the Ka of H2PO4- which is 6.2x10^-8...
pKa = -log(Ka)
pKa = -log(6.2x10^-8)
pKa = 7.21

So the equation is:
pH = 7.21 + log(0.3550/0.1550)
pH = 7.57

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