[H^+] = 10^(-PH)
[H^+]of PH = 3 HF solution = 10^-3 mol/L
HF is a weak acid and its dissociation in water is a partial reaction.
`HF harr H^++F^-`
So it is clear that [H^+] is not equal to the initial [HF] because of partial dissociation.
We need the dissociation constant to get the actual amount of [H^+] of a 4.0 mol/L HF solution. It is clear that the [H^+] will be lower than 4 mol/L. But we cannot say whether it is greater or equal or lower than 10^-3 mol/L as found by PH.
So we cannot directly say this solution is diluted or concentrated.
Just so we are clear - the question is asking for "concentrated" or "diluted" NOT strong or weak. I am not certain what the definition is universally but in the chemistry text that I have it clearly states that any compound under 0.500 mol/L is considered dilute. Failing that anything above 0.500 mol/L is considered concentrated. Now my text is full of errors that I run into quite frequently which is no picnic let me tell you. However to me whatever is the set bar for concentrated vs. diluted the only part of the question that even matters is the "4.0 mol/L" part. No where are you asked to determine the strength. So the first answer with no explanation is wrong according to my text and the second answer scares me as this is apparently a College Teacher -- hopefully NOT a chemistry teacher! Incidently there are more errors on here from this person AND he also answered this same question completely differently (and wrong) another time.