Which of the following hydrohalic acids is the weakest?

Step 1: Analyze the question We are asked to find the weakest acid among the hydrohalic acids (HF, HCl, HBr, HI).

Step 2: The concept The strength of an acid depends on how easily it gives up its H⁺ ion in water. For hydrohalic acids, two factors matter:

1. Bond length: Going down Group 17 (F → Cl → Br → I), the halogen atom gets bigger, so the H–X bond gets longer and weaker.
2. Bond strength: A weaker H–X bond breaks more easily, so more H⁺ is released. This makes the acid stronger.

Bond energies (approximate):

• H–F: ~565 kJ/mol (very strong)
• H–Cl: ~431 kJ/mol
• H–Br: ~366 kJ/mol
• H–I: ~299 kJ/mol

HF also forms strong hydrogen bonds, and the small F⁻ ion is highly stabilised in its bonded state. This makes HF dissociate only slightly in water.

Step 3: Order of acid strength

HI > HBr > HCl > HF

• HI, HBr and HCl are strong acids (almost completely ionised).
• HF is a weak acid (only partially ionised).

Step 4: Identify the answer The weakest hydrohalic acid is HF (hydrofluoric acid), which is option C.

Tip: Remember "F is the Fierce bond". Fluorine holds on to hydrogen the tightest, so HF is the weakest acid. For acid strength, think "the bigger the halogen, the stronger the acid."