Buffer problem find missing concentrations and volumes TOUGH QUESTION?

Question

Please compute the mass of NH4Cl (that ammonium chloride) and volume of H2O which must be added to 10.0 mL of 15.0 M NH3 to prepare 50.0mL of pH = 10 buffer solution.

Answer 1

pKa of NH4+ is 9.25; MW of NH4Cl is 53.5 g/mole.

pH = pKa + log([NH3]/[NH4+])

Substituting,

10 = 9.25 + log([15.0]/[NH4+])

Solving, [NH4] = 2.67 M.

Therefore,

(10 mL) (2.67 moles/L) (53.5 g/mole) / (1000 mL/L) = 1.43 g of NH4Cl.

So just add 1.43 g of NH4Cl and ~39 mL water to the 10 mL of 15 M NH3 solution.