For an EDTA titration you need to prepare an ammonia buffer

For an EDTA titration you need to prepare an ammonia buffer with pH 10.00. How much 0.00775 M NHO₃ would you add to 250.00 mL of 0.0099 M NH₃ to prepare this buffer?

Solution

millimoles of NH3 = 250 x 0.0099 = 2.48

millimoles of HNO3 =0.0075 V

HNO3   + NH3 ------------------------> NH4+  

0.0075 V     2.48                                 0

0                2.48-0.0075 V                             0.0075 V

pH = pKa + log [base / acid]

10 = 9.26 + log (2.48-V / 0.0075 V)

5.50 = 2.48-0.0075 V / 0.0075 V

0.0412 V = 2.48-0.0075 V

V = 50.9 mL

volume of HNO3 needed = 50.9 mL