Question

if 0.4M NaOH is titrated with 0.4M HF, how do we calcualte the ph at equivalence. The book assumes each is 1L, but why do we use 1L

Answer 1

HF is a weak acid, so you need the dissociation constant. The pH at the equivalence point is found by calculating the pH of a 0.4M NaF solution (a hydrolysis problem). If you erroneously assume that HF is a strong acid, then the answer is simple. The pH at the equivalence point is 7.