Rank the following acids according to acid strength, strongest to weakest: HF, HCl, HBr. Explain your reasoning.

Answer: HBr, HCl, HF
Firstly, we must understand what is meant by "acid strength". Simply put, it's the concentration of "free" hydrogen ions (H+) in solution. Acids dissociate (liberate "free" hydrogen ions) when the X-H bonds break (X can be F, Cl, Br). It makes sense to assume that the weaker the X-H bond is, the more likely it's going to break and therefore liberate more hydrogen ions.
When we look at the periodic table, we can make two observations. 1) Going down the period, the electronegativity of the halogens (F, Cl, Br) in group VII decreases. The more electronegative the halogen, the bigger is the "pull"on the partially positively charged hydrogen. So, less electronegative halogens will have a weaker "pull" on the hydrogen (weaker bond).2) Going down the period, the atomic size of the halogens increase. The bigger the halogen atom, the longer the X-H bond must be. Longer bonds are weaker.