100 mL of a weak electrolyte solution is mixed with 100 mL of a strong electrolyte solution. Which of the following combinations give a buffer? (Acetic acid and ammonia are weak; HCl and NaOH are strong.)

A buffer needs both members of a weak conjugate pair after reaction. (a) 20 mmol acid + 10 mmol NaOH leaves 10 mmol acid and 10 mmol acetate: buffer. (b) 10 mmol each: only acetate remains. (c) 20 mmol NH3 + 10 mmol HCl leaves 10 mmol each of NH3 and NH4+: buffer. (d) 10 mmol NH3 + 15 mmol HCl: all NH3 consumed with excess HCl: no buffer.