Question

HF + H2O <==> H3O+ + F-
By dissolving 0.03 mol HF in 1 L water , seen that [HF] at the equilibrium is 0.027 M , then :
1) If we divide 0.03 mol/ 1 L it would be 0.03 M , why that difference ?

2) How many moles HF in 1 L water is required to reach the equilibrium ?

Answer 1

1)If we dissolve 0.03 mols HF in 1L H2O, we have prepared a solution that is 0.03M in HF. However, since HF is a weak acid, it dissociates(partly) in solution to
HF + H2O ==> H3O^+ + F^- and the HF present as the molecule is decreased to 0.027M.

2) Any amount of HF in any amount of H2O will reach equilibrium on its own. I suspect you didn't post all of this problem.