Buffer problem

[gigi9]

here is a buffer problem in my HW that I'm not sure how to do.
Plz help me. Plz show me how. Thanks a lot.
Formula: pH = pKa + log([B-]/[HB])
***A buffer is made up of 0.500 L each of .300 M KH2PO4 and .400 M K2HPO4. Calculate
a) The pH of the buffer after the addition of 0.0500 mole of HCl
b) The pH of the buffer after the addition of 0.0500 mole of NaOH.

[GCT]

Due to the common ion effect the concentrations of the buffer should be the same at equilibrium. Use the equation to calculate the pH before the addition of any strong acid or strong base.

The acid will react with the weak base. Figure out which component of the buffer is the base. After the mole to mole reaction, less base buffer component will be left and more of the conjugate acid will form. Write out an equation to see this for yourself. Use this new ratio and substitute into the equation.

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