Question

Determine the mass (in g) of sodium hydrogen phosphate (Na₂HPO₄) that must be added to 87.8 mL of 0.0264 M sodium dihydrogen phosphate to yield a pH of 7.78. Report your answer to 3 significant figures.

Assume the volume of the solution does not change and that the 5% approximation is valid.

Answer 1

pka of phosphate buffer = 7.2

no of mole sodium dihydrogen phosphate = M*V

                                                                          = 87.8*0.0264

                                                                         = 2.32 mmole

no of mole sodium hydrogen phosphate (Na₂HPO₄) = ?

pH of acidcbuffer = pka + log(salt/acid)

                          7.78       = 7.2 + log(x/2.32)

              x = no of mol of (Na₂HPO₄)   = 8.82 mmole

   amount of sodium hydrogen phosphate (Na₂HPO₄) = n*Mwt

                          = 8.82*10^-3*142

                           = 1.25 g