What determine the strength of acid?

For normal acids, pH is most accurate.

For hydrofluoric acid, pH is NOT a good indicator--a 1-percent HF solution has a lower pH than a 50-percent solution does, and a 10-percent HF solution has the highest pH of all. The range is from 3.1 (for a 1-percent solution) to 4.5 (for a 10-percent solution). HF is considered a weak acid--not all the HF molecules dissociate when the HF gas is put into water--but it's the most corrosive acid around.