Question

What is the pH of the solution made by mixing 0.2 mol NaH2PO4 and 0.5 mol NaOH with water to make 1.00 L of solution? The pKa values for H3PO4 are 2.12 7.20 and 12.

Answer 1

moles of H2PO4- = 0.2 mol

moles of NaOH = 0.5 mol

H2PO4-   +    NaOH   ------------> HPO42- +   H2O

0.2                 0.5                             0                0

0                   0.3                         0.2             

HPO4-   +   NaOH    ----------->   PO43- +   H2O

0.2          0.3                              0               0

0                0.1                             0.2

here strong base remains.

[OH-] = 0.1 M

pOH = -log (0.1) = 1

pH = 13.00