[Diamonds]

Which of the following mixtures creates a buffer solution?

a) 10 mL of 0.10 M NaOH, 10.0 mL of 0.10 M HF
b) 20 mL of 0.10 M NaOH, 15.0 mL of 0.10 M HF
c) 15 mL of 0.10 M NaOH, 20.0 mL of 0.10 M HF
d) 10 mL of 0.10 M NaOH,   5.0 mL of 0.20 M HF

So these are all of the type of a strong base and a weak acid.
The only thing I could think of is that because NaOH is strong, you need less of it than of the weak acid, so
a) 1 mmol NaOH, 1 mmol HF
b) 2 mmol NaOH, 1.5 mmol HF
c) 1.5 mmol NaOH, 2 mmol HF
d) 1 mmol NaOH, 1 mmol HF (but with a higher concentration)

So in choice (c), there are more moles of HF which would be fitting because as a weak acid it only ionizes partially, which means less H⁺ ions are present, requiring more of it to compensate (as opposed to all the NaOH which has dissociated).

Is this correct logic?

[Borek]

More or less. Buffer needs to contain both acid and its conjugate base in comparable amounts, c is the only solution in which it will happen.