Question

concentrated acid need not be strong.. why

Answer 1

Concentrated acid is an acid which is either pure (no solvent) or has a high concentration. Glacial acetic acid is a concentrated weak acid, and industrial sulfuric acid (about 98% by weight, the other 2% is water) is a concentrated strong acid.

Even strong acids are not completely dissociated when the concentration is too high: concentrated hydrochloric acid, even though it is only 12-13 moles per liter in water, is concentrated enough that there are irritating fumes of HCl gas above the acid. If you dilute the acid to 6 moles per liter, the fumes are no longer noticeable because the HCl is completely dissociated.