Question #9c4f9

1 Answer
Stefan V.
Jan 31, 2015

The pH of the phosphate buffer will be 6.39.

The reactions that concern you are

NaH2PO4(aq)⇌Na+(aq)+H2PO−4(aq), and
Na2HPO4(aq)⇌2Na+(aq)+HPO2−4(aq)

So, adding monosodium phosphate will increase the concentration of H2PO−4, while adding dosodium phosphate will increase the concentration of HPO2−4.

The first thing you need to do is figure out the concentrations of the two species in the new solution. Starting with monosodium phosphate

nmono=C⋅V=80.0 mmol/L⋅250⋅10−3L=20.0 mmol

The volume of the new solution will be

Vsol=250 mL+250 mL=500 mL, which means that the concentration of monosodium phosphate (and of H2PO−4) will be

Cmono=nmonoVsol=20.0 mmol500⋅10−3L=40.0 mmol/L

Now for the disodium phosphate

ndi=C⋅V=30.0 mmol/L⋅250⋅10−3L=7.50 mmol

The concentration of disodium phosphate (and of HPO2−4) in solution will be

Cdi=ndiVsol=7.50 mmol500⋅10−3L=15.0 mmol/L

Now use the Hendesron-Hasselbalch equation to determine the pH

pHsol=pKa+log([HPO2−4][H2PO−4])

pHsol=6.82+log(15.0 mmol/L40.0 mmol/L)

pHsol=6.82−0.426=6.39

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